Timber frame construction : designing for high performance [5 ed.] 9781900510820

Table of contents :
1 Timber frame construction: An Overview
1.1 Methods of construction
1.2 Modem timber frame
1.3 Performance of tímber frame construction
1.3.1 Thermal performance
1.3.2 Fire performance
1.3.3 Sound insulation
1.3.4 Durability
1.4 Dimensional discipline
1.4.1 The structural grid
1.4.2 Vertical dimensions
References

2 Foundations
2.1 Design requirements
2.2 Sequence and setting out
2.3 Strip foundations
2.4 Trench fill
2.5 Reinforced concrete ground beams
2.6 Concrete rafts
2.7 Gas-proof membranes
2.8 Sloping ground levei
2.9 Accessible thresholds
2.1 O Proximity to trees
2.11 Basements
References

3 Ground floors
3.1 Design requirements
3.2 lntegration with the timber frame superstructure
3.3 Floor insulation
3.4 Concrete ground floors
3.4.1 Floating ground floor decks
3.5 Timber suspended ground floors
3.5.1 Timber joists
3.5.2 Decking for suspended timber floors
References

4 Walls
4.1 Externai walls
4.1.1 Design requirements
4.1.2 Externai wall construction
4.2 Internai walls
4.2.1 Design requirements
4.2.2 Internai wall construction
4.3 Wall linings
4.3.1 Design requirements
4.3.2 Lining materiais
4.3.3 Framing and lining junctions
4.4 Alternative wall constructions
4.4.1 lnsulation
4.4.2 Structure
4.4.3 Fire performance
4.4.4 Cladding
4.5 Multi-storey construction
References

5 Party Walls
5.1 Design requirements
5.2 Party walls for dwellings
5.2.1 Party wall construction
5.2.2 Structural stability
5.2.3 Fire resistance
5.2.4 Sound insulation
5.2.5 Proximity of windows
5.2.6 Thermal performance
5.2.7 Air-tightness
5.2.8 Junctions with other elements
5.2.9 Penetration of linings
5.2.1 O Steps and staggers
5.2.11 Specific requirements for separating walls in Scotland
5.3 Compartment walls for buildings other than dwellings
5.3.1 Compartment wall construction
5.3.2 Openings
5.3.3 Penetration of linings
References

6 lntermediate floors
6.1 Design requirements
6.2 Design of intermedíate floors
6.3 Floor joists
6.3.1 Notching and drilling
6.3.2 Trimmers and beams
6.4 Supporting internai walls
6.5 Fire resístance
6.6 Acoustic performance
6. 7 Floor decks
6.8 Ceiling liníngs
6.9 Cantilevered floors
References

7 Party floors
7.1 Design requirements
7.1.1 Fire resistance
7.1.2 Sound insulation
7 .1 . 3 Thermal performance
7 .1 .4 Structure
7 .2 Party floors for dwellings
7.2.1 Specified constructions
7.2.2 Structure
7.2.3 Fire performance
7.2.4 Sound insulation
7.2.5 Floor to wall junctions
7.3 Compartment floors where specific sound resistance is not required
References

8 Roofs
8.1 Design requirements
8.2 Pitched roofs
8.2.1 Trussed rafter roofs
8.2.2 Attic trussed rafter roofs
8.2.3 Panei roofs
8.2.4 Site-constructed roofs
8.2.5 Constructional details
8.2.6 Covering for pitched roofs
8.3 Flat roofs
8.3.1 Cold deck roofs
8.3.2 Warm deck sandwich roofs
8.3.3 Warm deck inverted roofs
8.3.4 Materiais for flat roof construction
8.4 lnsulation in roofs
8.4.1 Ventilated pitched roofs
8.4.2 Room in the roof structures
8.4.3 Cold deck, warm deck and inverted flat roofs
8.5 Ventilation in roofs
References

9 Cladding
9.1 Design requirements
9.2 Cladding materiais
9.3 Masonry cladding
9.4 Tile or slate cladding
9.5 Render cladding
9.5.1 Cement render cladding on masonry
9.5.2. Cement render cladding on paper-backed lath
9.5.3 Proprietary render systems
9.6 Brick slips
9.7 Metal sheet cladding
9.8 Timber cladding
9.8.1 Support battens
9.8.2 Cladding boards
9.8.3 Horizontal boards
9.8.4 Vertical boards
9.8.5 Durability
9.8.6 Species
9.8.7 Quality
9.8.8 Moisture movement
9.8.9 lnstallation and fixing
9.8.1 O Detail design
9.8.11 Finishes
9.8.12 Fire performance
9.8.13 Other wood-based claddings
9.9 Cavity barriers
9.1 O Junctions between self-supporting and attached cladding
9.11 Location and fixing of externai joinery
References

10 Services
10.1 Design requirements
10.2 Notching and drilling framing members
10.3 Effect of differential movement on services
10.4 Drainage and plumbing installation
10.5 Electrical installation
10.5.1 Electricity meter boxes
10.6 Gas installations
10.6.1 Gas meter boxes
10.6.2 Gas installation pipework
10.6.3 Gas appliance installation
10.6.4 lnstallation of a room sealed appliance, for example a boiler
10.7 Chimneys
10.7.1 Chimneys on externai walls
10.7.2 Chimneys on internai walls
1O.7.3 Chimneys adjacent to party walls
10.7.4 Chimneys through party floors
References

Appendix 1 Timber and wood-based materiais
A 1.1 Structural solid timber
A1.1.1 Sizes
A 1.1.2 Strength grading and strength classes
A 1.2 Structural timber composites
A 1.2.1 Glulam
A 1.2.2 Laminated veneer lumber
A 1.2.3 Parallel strand lumber
A 1.2.4 Laminated strand lumber
A 1.2.5 Cross laminated timber
A 1.2.6 Engineered beam or joist components
A 1.2. 7 Engineered panei components
A 1.3 Wood-based panei products
A 1.3.1 Performance characteristics
A 1.3.2 Oriented strand board
A 1.3.3 Plywood
A 1.3.4 Fibreboards
A 1.3.5 Particleboard - wood chipboard
A 1.3.6 Cement-bonded particleboards
A 1.4 Moisture content
A 1.4.1 Measuring moisture content
A 1.5 Preservative treatment
A 1.6 Gare of timber and components
References

Appendix 2 Materiais data
A2.1 Densities and weights of materiais
A2.2 Vapour resistivity and vapour resistance values
A2.3 Thermal conductivity of materiais
References
229
Appendix 3 Supervisor's checklist 233
A3.1 Work typically undertaken by groundworks contractor
A3.2 Work typically undertaken by timber frame
provider/timber frame erection crew
A3.3 Work typically undertaken by other subcontractors

Citation preview

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Revised by Robin Lancashire and Lewis Taylor

Proud to be pcrrt of @ elemenl

5th edition 2011 ISBN 978-1-900510-82-0 Published in 2011 by TRADA Technology Ltd (now BM TRADA) Reprinted with corrections, 2012, 2014, 2015, 2016, Jan 2017, Oct 2017, 2018, 2019 and 2020 1st edition by R B Wainwright, TRADA, and B Keyworth, Consultant Architect, published by TRADA, 1988 2nd edition revised by TRADA Technology, published by TRADA, 1994 3rd

edition revised by TRADA Technology, published by TRADA Technology, 2001

4th edition revised by Huel Twist and Robin Lancashire, published by TRADA Technology, 2008 While every effort is made to ensure the accuracy of the advice given, the company cannot accept liability for loss or damage arising from the information supplied All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form, by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the copyright owners © 2011 Warringtonfire Testing and Certification Limited BM TRADA acknowledges the assistance of Philip Whelan of PRP Architects LLP and Robin Dodyk of Oregon Timber Frame Ltd for their review of the text and illustrations lllustrations by Julie Smith of Applejam Layout by Academic + Technical Typesetting Main cover photograph: lnstalling open paneis,© Castleoak Group lnset photographs, from left to right: lnstalling sheathed open paneis, © Avebury Projects Ltd lnstalling an entire roof frame, © Oregon Timber Frame Ltd The Black House at Prickwillow, Cambridgeshire, © Mole Architects Ltd Newhall, Harlow, PCKO Architects, ©Tim Crocker All other photographs and illustrations are© Warringtonfire Testing and Certification Limited unless otherwise credited. BM TRADA MIX Chiltern House Paperfrom se responsible sources Stocking Lane !._,, FSCº C016278 Hughenden Valley High Wycombe Buckinghamshire HP14 4ND tel: +44 (0)1494 569600 email: [email protected] or [email protected] website: www.bmtrada.com

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Increased thermal performance has dominated the evolution of timber frame construction since we published the 4th edition in 2008. This 5th edition is extensively revised to cover: • • 0 0

the timber frame construction industry's response to the significant changes in building regulations across the UK in 2010 specifically the enhanced thermal performance of external walls and party walls the industry's transition to Eurocodes for structural design alternative forms of construction enabled by engineered timber products.

Robin lancashire is a senior timber frame consultant at BM TRADA and has particular expertise in timber frame construction. Lewis Taylor is a timber frame consultant at BM TRADA and has particular expertise in thermal and acoustic performance. A series of building case studies, which includes many timber frame projects, is available on the TRADA website, www.trada.co.uk. An overview of each project is available to users who register on the site; TRADA members are able to access and download the full studies.

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TRADA, the Timber Research and Development Association, is a not-forprofit, membership-based organisation delivering key services to members in support of its two main aims of 'Building markets for timber' and 'Increasing specification'. Membership encompasses companies and individuals across the entire timber supply/use chain, from foresters and sawmillers, through merchants and manufacturers, to architects, engineers and specifiers. For further information and details of membership visit www.trada.co.uk or telephone 01494 569603. For advice on timber, telephone 01494 569600.

Proud to be i=t of @element

BM TRADA, part of the Element Group, provides a comprehensive range of independent testing, inspection, certification, technical and training services. We help our customers to make certain that the management systems, supply chain and product certification schemes they operate are compliant and fit for purpose. BM TRADA is TRADA's appointed provider for its research and information programmes, and for the administration of its membership services. The BM TRADA bookshop offers a wide range of technical publications for professionals: https://bookshop .bmtrada.com

BMTRADA Chiltern House Stocking Lane Hughenden Valley High Wycombe Buckinghamshire HP14 4ND tel: +44 (0)1494 569600 fax: +44 (0)1494 565487 email: [email protected] website: www.bmtrada.com

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1 1.1 Methods of construction 1.2 Modem timber frame 1.3 Performance of tímber frame construction 1.3.1 1.3.2 1.3.3 1.3.4

Thermal performance Fire performance Sound insulation Durability

1.4 Dimensional discipline 1.4.1 The structural grid 1.4.2 Vertical dimensions

References

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2 Foundations 2.1 Design requirements 2.2 Sequence and setting out 2.3 Strip foundations 2.4 Trench fill 2.5 Reinforced concrete ground beams 2.6 Concrete rafts 2.7 Gas-proof membranes 2.8 Sloping ground levei 2.9 Accessible thresholds 2.1 O Proximity to trees 2.11 Basements References

3 Ground floors 3.1 3.2 3.3 3.4

Design requirements lntegration with the timber frame superstructure Floor insulation Concrete ground floors 3.4.1 Floating ground floor decks

3.5 Timber suspended ground floors 3.5.1 Timber joists 3.5.2 Decking for suspended timber floors

References

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4 Walls 4.1 Externai walls 4.1.1 Design requirements 4.1.2 Externai wall construction

4.2 Internai walls 4.2.1 Design requirements 4.2.2 Internai wall construction

4.3 Wall linings 4.3.1 Design requirements 4.3.2 Lining materiais 4.3.3 Framing and lining junctions

4.4 Alternative wall constructions 4.4.1 lnsulation

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Timber frame construction

4.4.2 Structure 4.4.3 Fire performance 4.4.4 Cladding

4.5 Multi-storey construction References

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walls 5.1 Design requirements 5.2 Party walls for dwellings 5.2.1 Party wall construction 5.2.2 Structural stability 5.2.3 Fire resistance 5.2.4 Sound insulation 5.2.5 Proximity of windows 5.2.6 Thermal performance 5.2.7 Air-tightness 5.2.8 Junctions with other elements 5.2.9 Penetration of linings 5.2.1 O Steps and staggers 5.2.11 Specific requirements for separating walls in Scotland

5.3 Compartment walls for buildings other than dwellings 5.3.1 Compartment wall construction 5.3.2 Openings 5.3.3 Penetration of linings

References

6 lntermediate floors

113

6.1 Design requirements 6.2 Design of intermedíate floors 6.3 Floor joists 6.3.1 Notching and drilling 6.3.2 Trimmers and beams

6.4 Supporting internai walls 6.5 Fire resístance 6.6 Acoustic performance 6. 7 Floor decks 6.8 Ceiling liníngs 6.9 Cantilevered floors References

7 Party floors 7.1 Design requirements 7.1.1 Fire resistance 7.1.2 Sound insulation 7 .1 .3 Thermal performance 7 .1 .4 Structure

7 .2 Party floors for dwellings 7.2.1 7.2.2 7.2.3 7.2.4 7.2.5

Specified constructions Structure Fire performance Sound insulation Floor to wall junctions

7.3 Compartment floors where specific sound resistance is not required References

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Contents

1 8.1 Design requirements 8.2 Pitched roofs 8.2.1 Trussed rafter roofs 8.2.2 Attic trussed rafter roofs 8.2.3 Panei roofs 8.2.4 Site-constructed roofs 8.2.5 Constructional details 8.2.6 Covering for pitched roofs

8.3 Flat roofs 8.3.1 8.3.2 8.3.3 8.3.4

Cold deck roofs Warm deck sandwich roofs Warm deck inverted roofs Materiais for flat roof construction

8.4 lnsulation in roofs 8.4.1 Ventilated pitched roofs 8.4.2 Room in the roof structures 8.4.3 Cold deck, warm deck and inverted flat roofs

8.5 Ventilation in roofs References

161

9 Cladding 9.1 9.2 9.3 9.4 9.5

Design requirements Cladding materiais Masonry cladding Tile or slate cladding Render cladding 9.5.1 Cement render cladding on masonry 9.5.2. Cement render cladding on paper-backed lath 9.5.3 Proprietary render systems

9.6 Brick slips 9.7 Metal sheet cladding 9.8 Timber cladding 9.8.1 Support battens 9.8.2 Cladding boards 9.8.3 Horizontal boards 9.8.4 Vertical boards 9.8.5 Durability 9.8.6 Species 9.8.7 Quality 9.8.8 Moisture movement 9.8.9 lnstallation and fixing 9.8.1 O Detail design 9.8.11 Finishes 9.8.12 Fire performance 9.8.13 Other wood-based claddings

9.9 Cavity barriers 9.1 O Junctions between self-supporting and attached cladding 9.11 Location and fixing of externai joinery References

189

10 Services 10.1 Design requirements 10.2 Notching and drilling framing members 10.3 Effect of differential movement on services

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Timber trame construction

10.4 Drainage and plumbing installation 10.5 Electrical installation 10.5.1 Electricity meter boxes

10.6 Gas installations 10.6.1 10.6.2 10.6.3 10.6.4

Gas meter boxes Gas installation pipework Gas appliance installation lnstallation of a room sealed appliance, for example a boiler

10.7 Chimneys 10.7.1 10.7.2 1O.7.3 10.7.4

Chimneys Chimneys Chimneys Chimneys

on externai walls on internai walls adjacent to party walls through party floors

References

Appendix 1 Timber and wood-based materiais A 1.1 Structural solid timber A1.1.1 Sizes A 1.1.2 Strength grading and strength classes

A 1.2 Structural timber composites A 1.2.1 A 1.2.2 A 1.2.3 A 1.2.4 A 1.2.5 A 1.2.6 A 1.2. 7

Glulam Laminated veneer lumber Parallel strand lumber Laminated strand lumber Cross laminated timber Engineered beam or joist components Engineered panei components

A 1.3 Wood-based panei products A 1.3.1 Performance characteristics A 1.3.2 Oriented strand board A 1.3.3 Plywood A 1.3.4 Fibreboards A 1.3.5 Particleboard - wood chipboard A 1.3.6 Cement-bonded particleboards

A 1.4 Moisture content A 1.4.1 Measuring moisture content

A 1.5 Preservative treatment A 1.6 Gare of timber and components References

Appendix 2 Materiais data

229

A2.1 Densities and weights of materiais A2.2 Vapour resistivity and vapour resistance values A2.3 Thermal conductivity of materiais References

Appendix 3 Supervisor's checklist A3.1 Work typically undertaken by groundworks contractor A3.2 Work typically undertaken by timber frame provider/timber frame erection crew A3.3 Work typically undertaken by other subcontractors

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In its 2010 report Low carbon construction, the Innovation and Growth Team, appointed by the Government from the construction industry, cails on companies to: • • •

de-carbonise their own business provide people with buildings that enable them to lead more energy efficient lives provide the infrastructure which enables the supply of clean energy and sustainable practices in other areas of the economy.

This book is about designing the timber frame buildings that enable more energy-efficient living. It is grounded in technical excellence ~ the outcome of highly effective relationships between key stakeholders such as the UK Timber Frame Association (UKTFA) and TRADA There has never been a better time for the UK timber frame industry to harness its potential. Everything is working in our favour - from the sustainability agenda and the ambition to reach zero carbon targets and a very clear direction from UK Government that we must embrace sustainable and efficient offsite build methods. Clearly, all things point towards timber frame. But how do we realise this potential as an industry? It starts with the realisation that the power of the industry comes from the collective working of all those parties that have a vested interested in the success of a timber frame future. Furthermore, it is underpinned by technical brilliance anda real sense of what a timber future looks like. Indeed, the tirnber trame industry has moved on in significant terms since our former chairman, Stewart Dalgarno, contributed the Foreword for the 4th edition of Timber trame construction. Today, Iam pleased to say the industry is no longer underpinned by the supply of simple timber kits; it is driven by the intelligent and integrated construction solutions that Stewart alluded to in 2008. If that alone is a measure of success then we are heading in the right direction. In this 5th edition, TRADA guides our industry through the latest regulations and standards that drive the low-energy agenda. I commend the practical advice it offers for meeting the 2010 revisions to the building regulations, in particular energy conservation. UKTFA endorses TRADA'.s advice to embrace Eurocode 5 because it enables us to harness the potential of timber frame in ways not possible using the now withdrawn BS 5268. And UKTFA mernbers will be pleased to see the enlarged section on aiternative forms of wall construction. One thing is for sure, and no doubt we will reflect on this over the coming years, there isn't another construction method in the UK that has such a bright future. The opportunity is here and now. We need to grab it with both hands and continue to focus our efforts on less talking and more doing. Simon Orrells, Chairman, UK Timber Frame Association

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This new edition of Timber trame construction comes at an exciting and challenging time for the UK construction industry. As 'zero carbon' comes ever closer, timber frame is well placed to meet these demands. We have considered improvements in elemental U-values, air-tightness, thermal bridging and party wall thermal bypass and incorporated these in this revision. This book draws together the requirements of the building regulations (which require designers and constructors to follow prescribed Standards), other recognised guidance (such as industry publications) and BM TRADA's recommendations for 'best practice'. One of the most significant European Directives for the construction and timber industries concerns the Energy Performance of Buildings. This has been incorporated into UK law and is a major influence on revisions to the structure and content of building regulations. We notice its impact most notably in the Code for Sustainable Homes that carne into effect in England in April 2007 and the recent updates to Approved Document L: Conservation of fuel and power (England and Wales) and the Energy section of the Scottish Technical Standards.

ln this 5th edition, we have addressed the key areas of air-tightness, thermal performance and thermal bridging by introducing an insulated service zone on the inner face of the timber frame external walls. This zone allows the vapour and air control layer to be free from service penetrations and requires all laps and junctions to be detailed well, that is all membrane laps mechanically fixed and clamped behind battens. lt also allows the installation of more insulation between the battens, which improves the U-value of the wall and helps to reduce thermal bridging. Another significant change in building design is the inclusion of U-values for party walls. The 2010 editions of the Energy section of the Scottish Technical Standards and Approved Document L require designers to consider the thermal performance of cavity party wall structures. ln order to assume zero heat loss through the party wall, all cavities within a party wall must be filled with insulation. This poses a number of sequencing and construction difficulties for sheathing timber party walls that we explain in this new edition. The introduction and implementation of Eurocodes is also another important driver for this revision. Eurocode 5 and its UK National Annex will replace BS 5268-2 entirely when the building regulations no longer recognise it. When published, PD 6693: Complementary information for use with Eurocode 5 (currently in draft with BSl) will reference complementary non-contradictory information found in BS 5268. This 5th edition assumes structural design using Eurocode 5 and PD 6693, while providing margin notes for those still using BS 5268.

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lt is important not to mix Eurocodes and the British Standards that they replace. For example, if the timber frame design follows Eurocode 5, then the design of cladding should follow the relevant Eurocode (such as Eurocode 6 for masonry).

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The drawings illustrate typical solutions to the design of platform frame construction to show the principles involved. They are not intended to provide a single prescriptive solution for timber frame design, and other designs and details may be equally valid. The BM TRADA frameCHECK ,/ team can assess designs as part of the suite of consultancy services they offer to the industry UK timber frame construction has now matured to a stage where timber frame walls - consisting of loadbearing studs lined with a structural sheathing board and finished with external cladding and internal linings - are now regarded as 'conventional' timber frame. The introduction of different types of wall construction, such as open-web joists, I-joists, cross laminated timber and structural insulated panels, has further expanded the scope of what is considered possible with timber buildings. New ways of accommodating continuous insulation also challenge traditional thinking. In this 5th edition we have moved towards a more 'plain English' style. For example, instead of 'the depth of beams should be checked', we say 'check the depth of beams'. Where the word 'should' remains it means this is BM TRADA's recommendation. The word 'may' means this is permissible. The word 'can' means this is possible. Throughout this book we have used the abbreviations 'dpc' for damp proof course and 'vcl' for vapour control layer. Timber frame construction is the core of a suite of publications from BM TRADA about building in timber frame. Others include: '"

Standard details for thermal performance - a suite of details based on

our standard detail publication and the new detailing in this 5th edition of Timber frame construction. The details include thermal conductivities of m~terials, U-values and y-values •

• •

•

•

Low energy timber frame buildings: designing for high performance

this book deals with building orientation, passive and active solar technologies, and design of the building envelope TRADA Wood Information Sheets - our popular WIS series covers single subjects, including many relating to timber frame EC5 Span tables - published in two versions (BS 5268-2 and Eurocode 5) these contain section sizes and spans for solid timber members in floors, ceilings and roofs (excluding trussed rafter roofs) for dwellings EC5 Timbersizer - using timber sizes readily available in the UK market, this online application suggests appropriate cross-sections based on the timber strength classes or joist depth EC5 Timberconnections for a given timber strength class this online application calculates the load capacity of individual fasteners (nails, screws, bolts, dowels and coach screws) in two- and three-member combinations of timber and steeL

Visit http://bookshop.trada.co. uk for details of publications.

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This chapter provides a broad introduction to timber frame construction. Subsequent chapters consider particular aspects in detail. Timber frame is a method of construction. It is not a system of building although there are a number of well researched systems which use timber frame as a basis. Timber frame construction uses timber studs and rails, together with a structural sheathing board, to form a structural frame which transmits all vertical and horizontal loads to the foundations (Figure 1.1).

Figure 1.1 Platform frame construction

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Timber frame construction

The exterior cladding is non-loadbearing, although it may contribute to wind resistance; it is used to assist in weatherproofing the building and to provide the desired external appearance. In the UK, timber trame construction generally uses factory-manufactured wall trames and roof trusses, or, in some cases, roof panels. Only a few specialist companies make and erect trames on site. The extent of factory prefabrication can and does vary considerably; trom the prevalent so-called 'open panels' consisting of simple sheathed stud panels with a breather membrane, to the less common 'closed panels' that include insulation and possibly internal linings, and may also include components such as joinery and sometimes also cladding. Floor and roof panels may also be factoryprefabricated as open panels with simple joists or rafters and sheathing panels, or as closed panels with insulation, linings etc fitted in the factory. Economic and functional issues typically dictate the choice between open and closed construction. The selection of an appropriate arrangement is an early decision in the design process. Timber trame developed in the UK primarily for house building, although it is now also widely used for buildings such as blocks of apartments, hotels, clinics, care homes, schools, student accommodation, offices and similar structures.

1.1 Methods of construction Platform trame is the most commonly used method in the UK. Each storey

is tramed with floor-to-ceiling height panels and the floor deck of one floor becomes the erection platform of the next (Figure 1.1). This book concentrates on design and construction principles of the platform trame method. The prefabricated wall panels can be either small units designed to be manhandled into place within health and safety guidelines (Figure 1.2a) or up to full elevation-width panels with or without ancillary components for placing with a crane (Figure 1.3). On-site stick build small panel construction is also utilised in the UK (Figure 1.2b) although it is less common.

Figure 1.2 Platform frame construction: a Small paneis; b On-site stick build

Other methods which may be used include: Floor-to-floor panel trame is an erection r:irethod whereby the wall panels

(except for the topmost storey) are floor-to-floor storey height - rather than floor-to-ceiling - and the intermediate floors are hung inside the wall panel 14

1 Timber frame construction: An overview

Figure 1.3 Platform frame construction: Large paneis

(Figure 1.4). This method is not common in the UK but does reduce crosssectional shrinkage of timber in the external wall and enables the insulation, vapour control and air barrier layers to be continuous up the wall face. Volumetric involves the factory fabrication of box units which can form individual rooms, or larger spaces, complete with finishes and services, and which require crane erection (Figure 1.5). This is best suited to repetitive units, such as hotels or nursing homes, although use in flats and houses is becoming. more common. Bathroom pods are a common use of volumetric construction methodology in the UK timber trame industry and well suited to hotels, student accommodation and other similar buildings. Pods that form complete dwellings are not widespread, due to the high tront-end cost and the requirement for a large number of repetitive units. Post and beam comprises a loadbearing system of posts and beams with lightweight timber or glazed infill panels (Figure 1.6). ln the UK, this tends

Figure 1.4 Storey height paneis (may be large or small)

to be used in the specialist 'traditional appearance' market, but there are modem timber trame systems using post and beam in the UK and elsewhere in Europe. Timber trame panels and other components are usually obtained trom a specialist manufacturer/fabricator although they can, less commonly, be

Figure 1.6 Post and beam construction

Figure 1.5 Volumetric construction

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Timber frame construction

manufactured by the contractor, either off site or in a temporary factory on the site. Most fabricators and their engineers have developed their own method of fitting together the timber frame components and are able to offer a full building kit to their own designs, or to designs produced by their clients. Some produce components to details supplied to them and leave responsibility for the structural or constructional detailing of the final building to the designer. Whatever the procurement method used, it is important that the client and supplier agree exactly what the timber frame package will comprise, even down to supply and/or fixing of loose items such as noggings, strutting and cavity barriers.

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1.2

Platform frame external wall panels are constructed from vertical studs, normally at 400mm or 600mm centres, nailed with simple butt joints to top and bottom rails. Strength graded timber must always be used; 89mm X 38mm and 140mm X 38mm are the most common sizes but 97mm X 47mm sections are occasionally used. With the increasing need for energy efficiency, even thicker walls with more insulation are being specified. This is typically achieved with the use of deeper solid timber studs, the use of engineered timber joists (I-section or open web) as studs or additional timber battens installed internally to form an insulated service void. In this 5th edition of Timber trame construction, an insulated service zone has been shown on the inner face of the timber frame external walls. Other methods of increasing insulation thickness are equally valid. However, this method of construction retains the sarne core structural wall design that the industry is used to, while also remaining non-proprietary. The size of panels dictates the method of construction, for example whether to use a crane for erection. Wind bracing is usually provided by a structural sheathing board, normally nailed to the external face of the frame (Figure 1. 7a) or in certain cases, by cross-bracing combined with internal plasterboard. Alternative wall designs, using either wood-based boards, mineral fibreboards or fibre-reinforced gypsum boards, fixed on the internal and/ or external face of the studs to provide the wind bracing, have also been developed and are discussed in more detail in Section 4.4. External claddings may be chosen from a wide range of materials, including brickwork, cement-rendered blockwork, tiling, slating, timber boarding, cement render on battens or proprietary claddings including engineered cladding boards, panelised timber or metal cladding, or render systems on carrier boards or insulation. Internal loadbearing and non-loadbearing walls may be constructed simply by using a stud frame lined on both sides with plasterboard or other sheet material. Party walls are generally constructed from two separate stud frames (twin leaf), with fibre insulation between the studs, and room linings of at least 30mm of plasterboard applied in two or more layers with staggered joints. Recent studies into the thermal performance of cavity party wall structures have highlighted the movement of air within the cavity. This movement of air can lead to convection currents transporting heat from the party wall cavity into unheated areas of the building, such as the loft space. This mechanism of heat flow is referred to as party wall thermal bypass and has been shown to have a substantial impact on the overall thermal performance of buildings. Building regulations in England,

16

1 Timber frame construction: An overview

Wales and Scotland now require the thermal performance of the party wall to be considered and controlled. This is typically achieved by fully filling all cavities within the party wall with insulation. Care must be taken in the choice of insulation so that the acoustic performance of the wall is not compromised. It is also possible to construct party walls with a brick or concrete block core between the two timber frames; this provides a method for supporting masonry cladding to walls at steps (changes of level) between buildings. Floor and roof framing are similar to other forms of construction, although the junction detailing is different. The intermediate floors and the roof provide structural diaphragms which, by means of structurally efficient connections to the walls, contribute to the overall stability of the building. Floor framing, particularly for medium-rise buildings, is taking advantage of super-dry timber. There is an increasing use of engineered timber components such as I-joists and metal web joists, and structural timber composites such as laminated veneer lumber (LVL) or parallel strand lumber (PSL) for header joists and rim beams. The structural design of all timber frame buildings needs to be approved by a structural engineer. The timber frame manufacturers will produce their own drawings prior to panel manufacture. These will have a high degree of accuracy and are often used directly via CAD/CAM systems in the manufacturing process. The platform frame method of building timber frame, favoured in the UK, is suited to both low-rise and medium-rise buildings. Medium-rise timber frame (normally up to a maximum of seven storeys) has seen significant growth in the UK over recent years. The need for a significant increase in house-building in the UK is generating interest in modem methods of construction to deliver the numbers of units required. Timber frame is a tried and tested method able to deliver the speed and quality associated with off-site construction. The sustainability and climate change agenda is encouraging clients, developers, builders and occupiers to recognise the environmental benefits of using a renewable structural material.

1.3 Performance of timber frame construction 1.3.1 Thermal performance The overall thermal resistance of the building envelope is the main factor in determining its energy requirement, although other factors such as ventilation control, the building form, building orientation and efficiency of the heating system all influence the cost of achieving comfortable conditions. Current timber frame construction incorporates high levels of insulation within the structural elements, but clients are now seeking designs which deliver improved thermal performance. There are many ways in which this can be achieved, including the use of deeper solid timber studs, deep engineered timber studs (typically timber I-joists or open web joists) or additional layers of insulation material installed internally or externally. Other options include the use of novel construction methods such as SIP (structural insulated panel) construction methods. Each of these possible solutions

17

Timber frame constructíon

,,,;,-~---------

Internai lining

r,_:;;:-y.

\ \ \ \

V'

\ \ \ \

/>

V'

/>.

450 mm min \ ~

\

V'

\ \ \ \ \ \ \ \

Raft dimensions, reinforcement, cover and concrete mix to be designed by an engineer

Note: Gaps in construction shown to illustrate membrane laps

Brickwork cladding _ _ __,__. set out from sheathing face to ensure correct cavity width '\IL~-UJ.--"""'1---

1i:i~"'-lll----

Figure 2.4 Concrete raft foundation

Internai lining vcl dpc Batten Seal between gas membrane and dpc

',,

Open perpends at 1.5 m max

-------"'"-'"-'~v----Batten

T _l_ Gro~u~n~d""'"""=:l==:;z::=

lnsulation

'--..---~dpm

150 mm min

Concrete beam and block floor to manufacturer's specifications

levei

150 mm min Timber frame structural engineer to specify fixing of timber frame to foundation

Proprietary periscope ventilators

Floor void ventilated to meet regulation requirements

,"

Figure, 3.4 Concrete beam and block floor with floating floor and insulation; at externai wall

~,_,1..,.1_ _ _ _ _ Loadbearing internai wall lnsulation dpc

+-------- Concrete

t

beam and block floor to manufacturer's specifications

150 mm min

i

Cross ventilation may be required

Floor void ventilation to meet regulation requirements

Figure 3.5 Concrete beam and block floor with screed and insulation; at internai loadbearing wall

45

Timber trame construction

+-"._,.__ _ _ _ Loadbearing internai wall lnsulation dpc Floating flooc

dpc/dpm lapped

\

-

i

Concrete beam and block floor to manufacturer's specifications

150 mm min

i

Floor void ventilation to meet regulation requirements

Cross ventilation may be required

Figure 3.6 Concrete beam and block floor with floating floor and insulation; at internai loadbearing wall

JI

Non-loadbearing internai wall

lnsulation dpc

-L'.....i'.....i""-".""-"·~~/'-'-"'-'~"'-'"'-'4-"~::::::::::::::::::::P..::~~~~-~~~~~.:.::..::..~....~""'-~""'-~"'"-~"'"-~"'-"-""-+-----dpm

+-------

t l

Concrete beam and block floor to manufacturer's specifications

150 mm min

\/\11 Concrete fill

Floor void ventilated to meet regulation requirements

Figure 3.7 Concrete beam and block floor with screed and insulation; at non-loadbearing internai wall

When floating decks are used in conjunction with an in situ concrete floor it is important that the slab has sufficient time to dry out before the deck is applied. Lay the damp-proof membrane on top of the concrete slab.

46

3 Ground floors

..-.r------

Non-loadbearing internai wall

lnsulation dpc dpc/dpm lapped

+----

t

Concrete beam and block floor to manufacturer's specifications

150 mm min

\/\11 Concrete fill

t

Floor void ventilated to meet regulation requirements

Figure 3.8 Concrete beam and block floor with floating floor and insulation; at non-loadbearing internai wall

With a layer of insulation being placed between the floor deck and the slab, with no means of ventilation, there is a risk of condensation occurring on the top surface of the slab. Place a vapour control layer (vcl) over the layer of insulation to prevent vapour/moisture from the dwelling side entering the insulation. The BRE Report BR 262 Thermal insulation: avoiding risks(7) recommends a 500 gauge (0.12mm) polyethylene vcl between the floor deck and the insulation unless composite panels with plastic insulation that has a high vapour resistance are installed. It is then acceptable to locate the vcl beneath the insulation. Lap the vcl joints by 150mm and tape. Around the perimeter of the floor, tum up the edges to just below skirting depth (minimum 38mm) and fix behind the skirting board. Since it is important to prevent moisture penetrating floors of this type, consider providing a waterproof floor finish and sealed upstand skirting in potentially wet areas such as bathrooms or utility rooms. Floating floors can also be used with proprietary suspended precast concrete floors. When beam and block floors are used, follow the manufacturer's recommendations with regard to the need to keep infilling blocks dry or allowing them to dry out before the floating deck is laid. Common decking materials for floating ground floors are: '"

particleboard (chipboard) on continuous insulation support - boards should be Type P5 or P7 to BS EN 312 Particleboards. Specifications( 8 l, 18mm minimum thick, tongued and grooved with all joints glued

47

Timber frame construction

'"

plywood on continuous insulation support - boards should be type EN 636-2 or EN 636-3 S to BS EN 636 Plywood. Specifications( 9 ), 15mm minimum thick, tongued and grooved with all joints glued.

OSB is not normally recommended as the floating overlay board on a continuously supported floating floor, although it has been used successfully as part of a floor where the floating floor is supported on battens. Particleboard (chipboard) needs to be conditioned to the approximate moisture content that it will settle to during the occupancy of the building to prevent excess movement. Lay plywood and particleboard decks so that all end joints are staggered. Allow for movement of the floating floor deck by providing a gap, which will normally be covered by the skirting, between the edge of the deck and the perimeter wall or other abutment. This should be at least 10mm or not less than 2mm (divided between each end) per metre run of deck On large floor areas, manufacturers may recommend intermediate expansion gaps. Build loadbearing internal walls off a structural kerb and not off the floating deck Internal non-loadbearing walls may be supported directly off the deck but the insulation must be of sufficient density to support the wall loads. If internal walls are supported on a floating floor deck, consider both vertical and lateral movement, especially around door openings.

3.5 Timber suspended ground floors Tirnber floors may be supplied by the manufacturer of the timber frame superstructure as pre-cut joist components, as floor cassettes, or they can be site-cut to the relevant sizes and constructed by the contractor. The\ floor can be designed to forma platform at ground level (similar to intermediate floors). If ground floors are insulated with fibre insulation, take precautions to prevent the insulation from becoming wet before the building is weathertight. Floors can also be fitted within the building, allowing its installation to be delayed until the shell is erected and the floor is protected from the weather. Complying with health and safety requirements in the latter choice may require additional resources. A suspended timber ground floor is particularly appropriate for timber frame buildings since it is constructed by the sarne trades as the main structure, is of dry construction and can be readily insulated to similar high standards as the walls and roof. The floor joists may span between the foundations of the loadbearing walls or may be supported at closer centres by means of sleeper walls. The latter allows smaller section joists but increases the amount of substructure required. Table 3.2 shows the types of suspended timber ground floors and the common Type A is illustrated in Figures 3.9 to 3.12.

All building regulations require the ground beneath the floor to be covered to restrict the passage of moisture and to prevent plant growth. Specific requirements vary so check the national regulations. e-~·

Regardless of floor type it is important to maintain a uniform support for the main superstructure to avoid differential settlement.

48

3 Ground floors

Table 3.2 Types of suspended timber ground floors Points to watch

TypeA 1111 11

11

1111 11

1 :11 11 1

1 1 1 1

1111 1 11

LJ

LJ 1 1

Floor joists spanning over externai and internai foundations as platform for loadbearing walls above. Typical for manufactured floor paneis craned into position.

Joist depth as intermediate floor: may add to differential movement; increases height of floor above adjacent ground levei

Floor joists on joist hangers or ledgers spanning between externai and internai loadbearing walls.

Floor joists independent of walls in terms of sequence of construction and differential settlement.

Floor joists spanning over sleeper walls, independent from externai and internai loadbearing walls

As Type B but smaller depth joists: minimum height of floor above adjacent ground levei.

TypeB 1 1

LJ

::íl 11

íl

íli

LJ

LJ 1 1

i

TypeC 1 1

LJ

11

11nni 1 1

Cladding

n

1

líln

t

n nn::

11

Sleeper walls (spacing variable)

---------'---+

~ :·.

:·· ·: ·:·:···· · :-.:- ~-:· .. ·: :.:.~::·.::·. Ground cover

~Polyethyleoe

t"med

up at edges

Figure 3.9 Type A floor at externai wall: joists parallel with wall; floor deck continuous underwall Note: Gare must be taken to avoid insulation getting wet during construction

To prevent water collecting on the ground cover, the top surface of the ground cover (the polythene sheet when used in conjunction with sand, gravel or lean mix concrete) should be above the level of the surrounding ground or, on sloping sites, it should fall to a drainage outlet above the lowest level of the adjoining ground. To minimise risk of decay TRADA Technology recommends that the external ground level is 150mm below the lowest structural timber. This means a viable access for disabled users needs to be considered, for example ramps. See Section 2.9 for details. It is essential that the void beneath the floor is adequately ventilated and that the ventilating air has a free path across the floor void. Ventilation should be the equivalent of 1500mm2 per metre run of two opposite sides of the floor or 500mm2 per m2 of floor area, whichever is the greater. The

49

Timber frame construction

Externai wall panei ----'----'----'""-.... Cladding-----~

Treated header joist (additional holding down may be required)

Sole plate

- - - - - - 1 - - l _ jlk,l~~~:::,..,~:::,..,~:::,..,/_-

J

dpc , Air brick Min and sleeved vent to outside 150mm Figure 3.10 Type A floor at externai wall: Joists at right angles to wall; floor deck continuous under wall Note: Gare must be taken to avoid insulation getting wet during construction

Finished ground levei

l

rr:I==~~~~~: I

I

Ventilated air space 150mm minimum

J.-.!-~~~~~,.!._

~:.:_;~'..:..~::..:..:-e..

1

1

1

Ground cover

~: 1

1

1

1

Loadbearing internai wall

Full depth blocking to support internai wall and floor deck Ventilated air space 150mm min Figure 3.11 Type A floor at internai loadbearing wall: Joists at right angies to wall; floor deck continuous under wall

Ground cover

1---------

Figure 3.12 Type A floor at internai loadbearing wall: Joists parallel with wall; floor deck continuous under wall

50

Loadbearing internai wall

3 Ground floors

minimum dimensions between the top of the ground cover and the underside of the floor construction are shown in the details. Figure 3.13 illustrates various methods of supporting insulation in timber

floors. A common method is to support fibre insulation on galvanized wire, plastic mesh or breather membrane which can be stapled to the sides of the joists to form a trough to accept the insulation. Impregnated softboard can be fixed to the underside of the joists to support the insulation. Before using support materials, check whether there is a condensation risk due to the relatively high vapour resistance of the material compared with that of the decking. Rigid foam insulation may also be supported by battens or clips fixed between the joists. This method requires care to ensure a tight fit with the joists since failure to do so can result in thermal bridges at these points. This degree of accuracy is achievable in factory prefabrication but may not be possible on site where joist setting out is unlikely to be sufficiently accurate, and would require each piece of insulation to be cut to fit. The manufacturers of these materials offer guidance on suitable specifications. Until the building is weathertight, protect from the weather ground floor insulation that is installed on-site (or in the factory and delivered to site).

Galvanized wire plastic mesh or breather membrane fixed to form troughs and support insulation

Rigid insulation slabs

r

Battens fixed to joist to support insulation slabs Note: Considerable care is necessary to ensure tight joints between slabs and joists, and adjacent slabs to avoid cold bridges

Vapour control layers are not normally required in insulated timber suspended ground floors. It is preferable to allow water vapour to diffuse freely through the floor to be dispersed by the underfloor ventilation.

3.5.1 Timber joists Timber joists should be strength graded in accordance with BS EN 14081 Timber structures. Strength graded structural timber(l0),(11),(12),(13) and installed ata moisture content of not more than 20%. They should be marked with the strength grade and/or strength class and stamped DRY GRADED (see Appendix 1).

Mesh or fibreboard fixed beneath joists to support insulation

When designing with engineered timber I-section or metal web with timber flange (chord) joists, the manufacturer's third party approved guidance must be strictly followed. The ground cover and ventilation provisions incorporated into the floor construction will normally ensure that moisture content of the timber remains below the decay threshold. However, consider treatment of nondurable species as insurance. BS 8417 Preservation of wood. Code of practice(14) includes ground floor

joists in use class 2, service factor C or D (where the risk of fungal decay is low but where remedial work would be difficult and expensive). Treatment is considered desirable or essential and so the timber would commonly be treated. Treatment options for a 60-year life include water-borne copperorganic preservatives, micro-emulsions or boron (see WPA Commodity specification C905l).

51

Figure 3.13 Methods of incorporating insulation in a suspended timber ground floor

Tirnber frarne construction

Timber joist sizes may be selected from:

Variations when using BS5268

Floor joists can be sized using TRADA Technology's Span Tables 2nd edition(19) and calculatíons done in accordance with BS 5268-2 Structural use of timbeL Code of practice for permissible stress design, materiais and workmanship(20).

.. ..

TRADA Technology's Eurocode 5 span tables(16) some national building regulations documents

'"

BS 8103-3 Structural design of low-rise buildings. Code of practjce for timber floors and roofs for housing(17)

or calculated in accordance with Eurocode

5(1s).

Joints in joists should only occur over sleeper walls or walls supporting a loadbearing internal timber frame wall. Joists which overlap on sleeper or supporting walls should be nailed together and should not project more than 100mm beyond the wall. Joists which are butt-jointed over a sleeper or supporting wall should be joined mechanically to both sides of the joists using: .. • '"

solid timber plywood type EN 636-2 (to BS EN 636) OSB3 or 4 gussets (to BS EN 300 Oriented strand boards. Definitions, classification and specifications(21)

'"

galvanised or stainless steel proprietary nailed plates.

The gussets or nail plates should extend to at least three quarters of the joist depth and be nailed with at least four nails into each side of each joist. Where joists are required to support non-loadbearing internal walls running either parallel to or across the joists, additional joists may be required or the WALL COINCIDENT WITH JOISTS

u+------

~

WALL AT RIGHT ANGLES TO JOISTS

Non-loadbearing wall

~

-

Flooc dock

Span directions

Joists at standard spacing

J

~

J

~ Additional joist(s) if required by calculation

u----

WALL LOCATED BETWEEN JOISTS

Non-loadbearing wall

==::;::::;:::===:~:::::====:;:::;::::::== Floor deck

~ ~· ~ ,}

Floor joísts at specified centres

~~,

Stan?ard spacing

When wall runs across joists, the bottom rail can be nailed to joists at each intersection

~Extra joist(s) if required by calculation

Figure 3.14 Supporting non-loadbearing internai walls

52

u 1

~

Noggings with z-clips to floor designer's specification

3 Ground floors

joist span should be amended as required. See Figure 3.14 and Section 6.4 for further information. Check the depth:breadth ratio of joists to ensure there is no risk of buckling under design load. Eurocode 5 does not prescribe maximum depth:breadth ratios. However, TRADA Technology recommends the values (taken from BS 5268-2) shown in Table 3.3 as a starting point, subject to a structural engineer's check Table 3.3 Maximum depth:breadth ratio

Max depth:breadth ratio of joist

Degree of lateral support

Ends held in position and co~;;~~";;cig:'iield in Íine by ciir;;;\ connection of sheathing, deck or joists

5

Ends held in position and compression edge held in líne by dírect connection of sheathing, deck or joists, together with adequate bridging or blocking spaced at intervals not exceeding 6 times the joist depth

6

Edges held in position and both edges held firmly in line

7

Solid timber blocking, timber herringbone strutting or proprietary struts can be used to provide the required lateral restraint. Timber herringbone strutting should be at least 38mm X 38mm. Solid blocking should be at least 38mm thick and extend at least three quarters of the joist depth. Regardless of the depth:breadth ratio of the joists, they should be strutted using timber herringbone strutting, solid blocking or proprietary struts as shown in Table 3.4. Table 3.4 Strutting recommendations

Jolst span (m)

Rows of strutting

up to 2.5

none

2.5.to4.5

One at mid span

over4.5

Two at 1/3 points

Strutting between joists provides not only lateral restraint but stiffens floors against vibration. These strutting recommendations are the sarne as those for intermediate timber floors where the ceiling lining provides additional stiffness. Therefore, the structural engineer should consider closer spacing of the strutting in longer span suspended timber ground floors to compensate for the absence of a plasterboard ceiling. Notching and drilling of joists should comply with the limits given in Section 10.2.

For engineered timber components, the product manufacturer's third party approved guidance must be strictly followed.

3.5.2 Decking for suspended timber floors Designers must avoid supporting heavy internal wall loads by floor decking materials only. Guidance on the support of internal walls is given in Figure 3.14 and Section 6.4. Further information on timber and wood-based boards is given in Appendix 1 and in TRADA Wood Information Sheet 2/3-57 Specifying wood-based paneis for structural

Variations when using BS 5268

use(22)_

The design of the flooring should follow BS 5268-2.

The design of the flooring should follow Eurocode 5.

53

Timber frame construction

3.5.2.1 Particleboard (chipboard), oriented strand board and cement-bonded particleboard floor decking Flooring grade particleboard (chipboard) should comply with BS EN 13986 Wood-based panels for use in construction. Characteristics, evaluation of conformity and marking(23) and be of Type P5 OI P7 in accordance with BS EN 312. It should also be marked 'FloOiing' to indicate that point load and impact tests have been carried out in accOidance with BS EN 12871 Woodbased panels. Performance specifications and requirements for loadbearing boards for use in floors, walls and roofs(Z4)_ Strength and stiffness values

should be obtained from the manufacturer. Oriented strand board (OSB) should comply with BS EN 13986 and be OSB 3 or OSB 4 manufactured in accOidance with BS EN 300. It should also be marked 'FloOiing' to indicate that BS EN 12871 point load and impact tests have been carried out. Strength and stiffness values should be obtained from the manufacturer. Cement-bonded particleboards tend to be used in specialist applications and supplied direct from the manufacturers. Cement-bonded particleboard should comply with BS EN 13986 Wood-based panels for use in construction. Characteristics, evaluation of conformity and marking{25), and with BS EN 634-2 Cement-bonded particleboards. Speciflcations. Requirements for OPC bonded particleboards for use in dry, humid and externa] conditions(Z6)_ It should also be marked 'Flooring' to indicate that BS EN 12871 point load and impact tests

have been carried out. Strength and stiffness values should be obtained from the manufacturer. Follow the manufacturer's recommendations for laying. Flooring grades of particleboard (chipboard) are available with tongued and grooved edges on all four sides, or to the long edges only, OI as square edged boards. OSB flooring is available with all edges tongued and grooved or square-edged. Particleboard (chipboard) has equal spanning capacity along and across the boards; OSB is stronger along the board than across the board. These characteristics affect the way in which the boards are laid over the supporting joists. Tongued and grooved particleboard (chipboard) and OSB sheets are laid with long edges across the joists with the board ends located centrally on joists. Support is not necessary to the long edges except at perimeters of floors and to cut edges. Square-edged OSB is also laid across the joists with the long edges supported by noggings between the joists. Square-edged particleboard (chipboard) is usually laid with the long edges on joists and the short edges supported by noggings between the joists. SuppOit all edges of square-edged boards. Both tongued and grooved and square-edged boards should be laid with the end joints staggered. Any cut board shou~ld span at least three joists. Although the boards recommended for floor decks are classed as moisture resistant, they are not intended for prolonged exposure to direct wetting and

54

3 Ground floors

the building should be made weathertight as soon as possible. Where there is a risk of more prolonged exposure, provide specific protection, for example by the use of boards pre-faced with protective film, following the manufacturer's recommendations for installation. This is likely to include taping or sealing of joints. Most wood-based boards are manufactured ata low moisture content and it is normally recommended that they are conditioned on site by looselaying them in place for at least 24 hours to arrive at the desired moisture content before fixing down. Boards should not be fixed to joists or noggings which have a moisture content greater than 20% as this can cause localised swelling. Board joints should be tightly pulled together before fixing and fixed in accordance with the schedule, typically along edge and intermediate supports with either screws or nails at board corners and at 300mm centres. Fixings should be not less than 8mm from the board edges. Nails should be of improved/annular ring shank type. Nail length should be 2.5 times the board thickness and nail heads should be punched below the board surface. Unless power nailing is used, pre-drill cement-bonded particleboard before nailing. Tongued and grooved board joints should be glued with a PVAC or similar adhesive and the joints between the joists and the board should also be glued with PVAC to obviate possible creaking of the floor. Consider expansion and shrinkage of the floor deck due to change of moisture content. Conditioning of floor deck material is normally recommended and board manufacturers may recommend intermediate expansion gaps on large floor areas. Where future access is required, for example by water authorities, purposedesigned screwed access panels should be provided.

3.5.2.2 Plywood floor decking Plywood for flooring should comply with BS EN 13986 and be of BS EN 636 grade EN636-2 S or EN 636-3 S. It should also be marked 'Flooring' to indicate that BS EN 12871 point load and impact tests have been carried out. It is important to lay plywood sheets in the correct direction for optimum panel strength, which is usually with the direction of the face grain at right angles to the joists. Some plywood is manufactured with the face grain parallel to the shortest side, but for most, the face grain is parallel to the longest side. When the strongest and stiffest direction of the plywood is in the direction of the face grain and the face grain is parallel to the longest side, the face grain and the longest side are laid at right angles to the joists. If the strongest and stiffest direction is parallel to the short edges then the boards are normally laid in the other direction. Plywood for flooring is available as square-edged or tongued and grooved edged sheets. Commonly only the long edges of the sheets have the tongued and grooved profile. Support square-edged sheets by joists or noggings at all edges. Tongued and grooved boards with square-edged ends only need support at square edges and at the room perimeter if not supported on the header joist. Supported ends should occur centrally over joists or noggings. Any cut sheet should span at least three joists. 55

Timber frame construction

The type of plywood specified will depend on the use required. Plywood which will form the finished surface (with a surface coating) should be sanded and have a better visual surface appearance than that to be covered. Where carpets, thin tiles or sheets are to be used, 'touch sanded tight faced' plywood should be specified. Where wood strips, blocks or other tiled finishes are to be used, an unsanded surface with lower face grades may be acceptable. Tongued and grooved board joints should be pulled together (not cramped) before fixing and fixed with either screws o~ nails to the fixing schedule, typically at the corners and at 150mm maximum centres along edge supports and at 300mm centres along intermediate supports. Fixings should be not closer than Bmm to the board edge. Nails should be of improved/annular ring shank type. Nail length should be 2.5 times the board thickness and nail heads should be punched below the board surface. Tongued and grooved board joints should be glued with a PVAC or similar adhesive and the joints between the joists and the boards should also be glued with PVAC to reduce the risk of creaking of the floor. Where future access is required, for example by water authorities, purposedesigned screwed access panels should be provided.

3.5.2.3 Tongued and grooved boarding Softwood timber boards for flooring should comply with BS 1297 Specification for tongued and grooved softwood flooring(27). It should not be fixed until the building is weathertight unless specific protection is provided. This is to avoid excessive increase in the moisture content of the boards, resulting in expansion problems and, possibly later, a greater degree of shrinkage on drying out. The moisture content of the boarding should not exceed 19% at the time of laying. In a normally heated building, the boards will eventually attain a moisture content of aro.und 10%, so some shrinkage will be inevitable. Where the boarding is proposed as a decorative feature it should not be laid until the building is completed and has dried out. The moisture content of the boards should be 12% +/- 2%. It may be preferable to install a structural floor to allow the decorative flooring to be added later. For tongued and grooved boards, the finished face widths usually correspond to board thicknesses, for example a 16mm finished thickness board usually has a face width of 65mm; a 19mm board a face width of 90mm; anda 21mm board a face width of 113mm. Table 3. 5 gives recommended maximum joist centres for tongued and grooved floor boarding for domestic floor loadings. Table 3.5 Maximum spans for tongued and grooved floor boarding in domestic floors (1.5 kN/m2) Finíshed board thickness (mm)

16 19 21

56

Maximum span (centre to centre) (mm) (

400 600 600

3 Ground floors

Floor boards should be tightly butted together before nailing down and floor cramps are commonly used for this. Boards nailed down without cramping can result in excessive gaps between the boards when they dry to the equilibrium moisture content. Nail length is normally 2.5 times the board thickness and all nails should be punched below the level of the floor surface. Butt joints should be square and occur over joists with both boards adequately supported. Joints should be staggered so that they are at least two board widths apart and any board should span at least three joists.

1 BR 211: Radon: guidance on protective measures for new buildings, extensions, conversions and refurbishment, ISBN 978-1848060135, BRE, 2007 2 BR 413: Radon: guidance on protective measures for new dwellings in Northern Ireland, ISBN 1860814697, BRE, 2001 3 BR 376: Radon: Guidance on protective measures for new dwellings in Scotland, ISBN 1860813348, BRE, 1999 4 BR 212: Construction of new buildings on gas-contaminated land, ISBN 0851255132, BRE, 1991 5 BS EN ISO 13370:2007 Thermal performance of buildings. Heat transfer via the ground. Calculation methods, BSI 6 Guide A. Environmental design, 7th edition, ISBN 1903287669, CIBSE, 2006 7 BR 262: Thermal insulation: avoiding risks, ISBN 1860815154, BRE, 2002 8 BS EN 312:2010 Particleboards. Specifications, BSI 9 BS EN 636:2003 Plywood. Specifications, BSI 10 BS EN 14081-1 :2005 Timber structures. Strength graded structural timber with rectangular cross section. General requirements, BSI 11 BS EN 14081-2:2010 Timber structures. Strength graded structural timber with rectangular cross section. Machine grading. Additional requirements for initial type testing, BSI

12 BS EN 14081-3:2005 Timber structures. Strength graded structural timber with rectangular cross section. Machine grading. Additional requirements for factory production control, BSI

13 BS EN 14081-4:2009 Timber structures. Strength graded structural timber with rectangular cross section. Machine grading. Grading machine settings for machine controlled systems, BSI

14 BS 8417:2011 Preservation ofwood. Code ofpractice, BSI 15 Commodity specification C9 in Industrial Wood Preservation Specification and Practice, Wood Protection Association, 2007 16 Eurocode 5 span tables: for solid timber members in floors, ceilings and roofs for dwellings, TRADA Technology, 2009 17 BS 8103-3:2009 Structural design of low-rise buildings. Code of practice for timber floors and roofs for housing, BSI 18 BS EN 1995-1-1:2004+A1:2008 Eurocode 5. Design oftimber structures. General. Common rules and rules for buildings, BSI

57

Timber frame construction

19 Span tabies: for soiid timber members in floors, ceilings and roofs for dwellings, 2nd edition, TRADA Technology, 2008 20 BS 5268-2:2002+A1:2007 Structurai use of timber. Code of practice for permissibie stress design, materiais and workmanship, BSI 21 BS EN 300:2006 Oriented strand boards (OSB). Definitions, classification and specifications, BSI 22 WIS 2/3-57 Specifying wood-based paneis for structurai use, TRADA Technology, 2005 23 BS EN 13986:2004

Wood-based paneis for use in construction. Characteristics, evaiuation of conformity and marking, BSI

24 BS EN 12871:2010 Wood-based paneis. Performance specifications and requirements for ioadbearing boards for use in floors, walls and roofs, BSI 25 BS EN 13986:2004

Wood-based paneis for use in construction. Characteristics, evaiuation of conformity and marking, BSI

26 BS EN 634-2:2007

Cement-bonded particieboards. Specifications. Requirements for OPC bonded particleboards for use in dry, humid and externai conditions, BSI

27 BS 1297:1987 Specification for tongued and grooved softwood flooring, BSI

58

lls .1

Is

Variations when using BS 5268

.1 Design requirements Timber frame external walls are required to carry the dead and imposed loads (including wind loads) acting on the structure and transmit them to the foundations. The structural design should be in accordance with Eurocode 5(1). All buildings need to be sufficiently robust to sustain a limited extent of damage or failure, depending upon the class of the building, without collapse to meet the disproportionate collapse requirements of national building regulations. Disproportionate collapse means that in the event of accidental damage to the building, the structure not directly affected by the accident must not collapse excessively in relation to the severity of the accidental event. External walls are required to have thermal performance levels (insulation, thermal bridging and air-tightness) to meet the requirements of building regulations. Durability is an essential requirement and the walls also need to provide support for the cladding materials to be used. External walls are required to have appropriate fire resistance and intemal surface spread of flame (reaction to fire) characteristics (see Section 1.3.2 Fire performance). Fire resistance requirements normally relate to resistance from within the structure. When the building is within one metre of a relevant or notional boundary, there are also requirements for external wall fire resistance and surface spread of flame (reaction to fire) characteristics from outside the structure. Full details are given in national building regulations. The periods of fire resistance required vary depending upon the building's purpose group and height. National building regulations differ in detail so check for specific requirements. Loadbearing walls must have equal fire resistance to the floors that they support. ln recent years a number of large timber frame projects under construction have experienced fire damage to large areas of the building. Solutions exist to minimise fire risk These include: '" '" ..

phasing of construction tasks, so that large areas of structural timbers are not left exposed alternative material technologies, which provide enhanced fire protection during erection tight security to prevent arson during the critical phase; and supplying closed panel walls to site.

Many timber frame wall types can be factory manufactured as closed panels, with insulation, membranes, possibly linings and sometimes services installed. This method delivers speed of site construction and the potential for better quality and improved fire resistance during the construction phase. Take care to ensure that the closed panel structure is protected from mechanical and moisture damage during delivery and erection. Closed

59

The E>tructural design should be in accordance with: '"

BS 5268-6 Section 6.1 Code oi practice for timber trame walls. Dwellih.gs not exceeding seven storeys(2) or Section 6.2 Code of practice for timber trame walls. Buildings other than dwellings not exceeding four storeys(3)_

Guidance on checking the structural design, strength and stability of tímber frame houses is given ín Timber trame housing: UK Structural recommendatíons(4).

Timber frame construction

Breather membrane cut with cavity tray tucked under by minimum 100 mm Cavity tray (with open perpends above for drainage) Steel lintel (with clips when required) ~---

Vertical dpc {lapped with vcl)

Timber lintel

/ Internai wall lining Thermal insulation

Window bottom rail Breather membrane

Flexible wall tie, nailed to studs

Vapour control layer (vcl) Bottom rail

Stud marker tape

Open perpends to / ventilate and drain / cavity, spaced at ---------~7"'0:::::::-.:::::___ max 1.5 m centres ~ /

2'9

Ventilated

cavity~

Brickwork cladding

/

/ /

.!

~

Opening located across structura 1 grid

Open ing wider than structural grid

'=

\ t

Top and bottom window jack studs to be on structural grid

l

l

above and below window opening for fixing sheathing and lining

Figure 4.9a Openings requiring lintels

Floor joist or roof truss load. Where a separate head binder is used these loads may be offset subject to calculation. Point load from beams etc should be supported by studs beneath them

With brick or block cladding additional studs may be required in dotted positions for the fixing of wall ties

l

\Additional jack studs may be required

l

''"===!flll1 1

,1

1 "

l+---1---#-----+'

:

1

:11;::::=::::;:111

....._

-

1

1

'1 1:

l~I

111==;;:=1

:

: 'I

:

i

1

1

1

1 1

1111+~-»I

1

1

~-------Additional

Maximum opening achieved with inner face of studs on structural grid lines

jack studs may be required above and below window opening for fixing sheathing and lining

73

Figure 4.9b Openings not requiring lintels

Timber frame construction

Ring beam

Plasterboard batten

Figure 4.10 Metal web joist system supported on ring beam

There are floor joist systems where the joists are supported from a ring beam, such as I-section joists on metal hangers and metal-web joists (Figure 4.10). ln these systems, lintels may not be required but the ring beam must be continuous over the opening. An additional stud is still required to each side of an opening for the fixing of cavity barriers and wall ties. Building design, size and room type influence where openings and point loads occur. These are frequently off grid so additional framing is needed and cutting and possible waste of sheathing and lining materials may result (Figure 4.11). It is ideal to select opening widths that are a multiple of the cladding and structural grid with openings located on grid but this is not always practical. Irrespective of where such additional studs occur, the main studs should still be located at the structural grid line to provide regular fixing positions for lining, sheathing, wall ties and/or cladding battens.

Floor joist or roof truss load. Where a separate head binder is used these loads may be offset, subject to calculation. Point load from beams etc should be supported by studs beneath them

l

l

l

l

l

.. Untei

l Untei

u-'

Figure 4.11 Modular openings off grid

~

t===== 74

.·

li

1

l Hl 1

4 Walls

Roof load, usually at 600 mm centres. May be offset from studs where head binder is used, subject to check by calculation

i

i

i

i

i

i

i

Untei

*

Load from lintel forms point load on lower lintel

*

1

Floor joist load; see / n o t e as roof load

t

' Untei

Number of cripple studs determined by load on lintel, may be one or two cripple studs

Figure 4.12 Staggered openings in externai wall

Staggered window openings at different storeys (Figure 4.12) or point loads from above may result in lintels to the lower floor openings being heavily loaded and structural composite beams may be needed. Structural calculation will determine the precise requirements.

4.1.2.9 Unteis Softwood lintels are sufficient for the majority of domestic loads and spans. The dimension between the base of the panel and the underside of the lintel is normally 2100mm, plus the depth of the floating floor where appropriate. Depending upon the size of the lintel, packing or framing may be required to the underside to form an opening (Figure 4.13). The design of lintels will be determined by the load (uniform or combined uniform and point load), the span, availability of the required size and strength class of timber and the deflection limit imposed (particularly over large span openings). For preferred deflection limits, consult the National Annex to Eurocode 5 and TRADA Technology's Eurocode 5: Timber design essentials for engineers(2 1l. Lintel deflection over large openings may affect the operation of, for example sliding patio doors, and should preferably be limited by a specified dimension. Tables of lintel capacities and guidance on the design of bolted flitch beams are included in Timber trame housing: UK Structural recommendations. Multi-member lintels should be fixed together in accordance with the structural design. Figure 4.14 shows typical lintel types. Independent steel or concrete lintels are required to carry external brick or block cladding vertical load, see Chapter 9. Under no circumstance should the vertical weight of masonry walls be transferred to the timber frame

75

Packing if required to compensate for lintel depth/panel height/door opening relationship Head binder Top rail Untei - 1 or 2 softwood or hardwood members Stud Cripple stud(s) to support lintel

---\-

+- Bottom

rail

This section of bottom rail cut away on site for door openings

Figure 4.13 Typical lintel arrangement for door opening in loadbearing wall

Timber frame construction

2 Usually has vertical stiffeners at 600m centres '""""-~....--"""!

..--- Top rail of wall panei

K-+--

l-"'-...-1--''"'I+-

Standard timber size Void (fill with insulation)

1;::;~~!1-----

Void (fill with insulation)

Packer thickness as required

4

3

Head binder is essential with this type of lintel

Top rail of wall panei +---

Packer thickness as required

+-------

5

• i

Figure 4.14 Typical lintels

3. 4.

5. 6.

Flitch beams will usually be made to the specific depths required Packer thickness as required

6

~.

1. 2.

Top rail of wall panei

oom~"'

Strudural

1

Standard lintel: cripple studs to extend to underside of lintel member Lightly loaded: fill void with thermal insulation; cripple studs to extend to underside of lintel member Heavy load: structural hardwoods or softwood and steel flitch, sized in depth to allow for timber shrinkage, bolts recessed into lintel members Heavy load, large span: top rail omitted; requires head binder and end fixing into wall studs to restrain lintel against overturning; can incorporate steel flitch if structurally required Glulam: refer to manufacturer's literature for span details Structural composite (for example laminated veneer lumber or laminated strand lumber): proprietary prefabricated; refer to manufacturer's literature for span information.

wall. Steel lintels should be independently assessed for lateral support of brickwork cladding to the timber frame.

4.1.2.1 O Support for point loads Point loads from beams, for example trimmers carrying joists, should be transmitted directly to the foundations by the use of additional studs to provide a continuous line of load (Figure 4.15), the exact number being determined by calculation. Deep beams require pockets to be formed within the wall panels as shown in Figure 4.16 or top hung hangers are specified. Load transfer for beams should be followed through all panels and floor framing to foundation level when specified by the structural engineer.

76

4 Walls

~----------Beam

on grid

- - - - - - Beam off grid

:;;:::==:==:;0:;;===:==;;:::::==:::::::::;;:::=;0;;;=:::;;::==:==::::;;+-- Head binder where used r1J--H-----f+---+H---++---+l---

Additional stud(s) as specified by structural engineer to transfer load from beams on grid

11-tn. '*''---""""-...__ _ _ _ __

Mastic seal Joists parallel with wall require double joist

Cavity barrier of wire reinforced mineral wool only at compartment floors in England and Wales and Northern lreland and to all floors in Scotland

•+----------------------+•

Joists at right angles to wall. Header joist and solid blocking between each joist nailed to top and bottom rails, to maintain fire resistance and sound insulation

k~~hi~~~~~~~-- Resilient bar support ~--------

lf required by structural engineer ---------++-17"'"=~""'2 light metal ties 40 mm x 3mm max thick, at 1.2m horizontal centres, one per storey height vertically

Resilient bar

,.__________ Plasterboard fixed with staggered joints, not less than 30mm thickness; eg 1 layer 19mm plus 1 layer 12.5mm

~---e-----------

Dimension not less than 200mm, 240mm preferred for improved sound insulation

Mastic seal

Breathable air barrier

Note: Double joist and joist/joist blocking over party walls should be installed tightly to block air paths

Figure 5.1 Typical party wall construction with 60-minutes fire resistance for dwellings Note: The location of lhe party wall sheathing may be varied to allow for lhe installation of insulation in lhe cavity between lhe party wall leaves.

101

Timber frame construction

between dwellings where a lightweight cladding is not acceptable above the lower roof are a and exposed brickwork is required. See Section 5.2.1 O.

Structural stability The twin party wall frames with plasterboard linings can usually provide adequate resistance to wind load on the front and rear walls. However, the temporary bracing on the party walls will need to remain in place until the plasterboard is fixed. Sheathing or bracing can be introduced in the wall if structural calculations indicate the safe racking resistance of the plasterboard lining is exceeded, or if it is not desirable for the temporary bracing to be relied on for safe racking resistance during the construction of the building. ln many cases, 1200mm wide partial sheathing of the wall can provide adequate stiffness. The structural engineer will specify the position of this additional sheathing as determined by the amount of extra stiffening required. Often party walls on the cavity side are completely sheathed for greater separation of dwellings (Figure 5.2). When diagonal bracing is fixed permanently to the cavity side of the wall frames, take care to ensure that any applied bracing does not contact the opposite wall frame. Party wall cavity insulation if required Sheathing as required structurally

Vertical cavity barriers to close cavity at party wall/external wall junction and opening in party wall cavity Fibre insulation may be specified Figure 5.2 Typical arrangement for sheathing party walls for additiona! stiffness

Wire reinforced mineral wool or approved cavity barriers

5.2.3 Fire resisfance The two leaves should forma complete barrier to fire, including in the roof spaces. This is usually achieved with plasterboard in two or more layers to contribute to the 60-minutes fire resistance required. The complete fire barrier includes not only the wall itself, but the junctions with the floor, the roof and the external walls and any penetration of the wall permitted under the regulations. Correct installation of cavity barriers and firestops to the required fire resistance is essential. To provide a positive fixing, avoid the plasterboard lining paper being broken by the fixing head. The use of powered screwdrivers with a depth stop has encouraged the increased use of screw fixing of plasterboard and allowed consistent accuracy for stopping the fixing head at the correct depth for positive fixing. Accurate fixing depth reduces the incidence of filler popping off fixing heads. Fixing centres differ between plasterboard manufacturers from 150mm to 300mm. Check specifidltions with individual manufacturers and follow their recommendations.

102

5 Party walls

5.2A Sound Airborne sound insulation is achieved in lightweight walls by combining structural discontinuity, dense wall linings and absorbent insulation in the cavity. The usual specification used for wall linings is a minimum of 30mm thickness of plasterboard on both wall faces; typically one layer of 19mm plasterboard plank and one layer of 12.5mm plasterboard, fixed with joints staggered. Consider both thickness and density of the plasterboard to provide the required sound insulation. This thickness can be reduced to two layers of 12.5mm plasterboard in the roof space when a 12.5mm plasterboard ceiling is used. Where the roof timbers are not parallel with the party wall, alternative specifications which have been used in conjunction with non-habitable roof spaces are shown in Figure 8.15. The sound absorbent insulation in the party wall is normally unfaced mineral wool quilt of not less than 10 kg/m 3 density (12 kg/m3 in Scotland); 25mm thick if suspended in the cavity between the two frames, 25mm in each frame or 50mm thick if fixed to one of the frames. Robust Details Ltd details require 60mm of absorbent insulation in each leaf of the party wall. Many designers and contractors install thicker and denser mineral wool material in the party wall to simplify installation and to rationalise the number of types of insulation needed on site. If using other than mineral wool insulation, acoustic test evidence would be necessary. The choice of acoustic insulation may also be influenced by thermal performance requirements for the party wall. See also Section 5.2. 6. The need for acoustic testing to show compliance with building regulations varies, so check national requirements. Where testing is required, take measurements in completed buildings in accordance with BS EN ISO 140-4 Acoustics. Field measurements of airborne sound insulation between rooms( 3 ) to determine the standardized level difference (DnTl for airborne sound transmission. Then recalculate the weighted standardized level difference (DnT.wl for airborne sound as defined inBS EN ISO 717-1 Acoustics. Ratingofsound insulation in buildings and of building elements. Airborne sound insulation(4).

Spectrum adaptation terms can be applied to the DnT,w values to better indicate sound insulation performance against specific types of noise. In this context, a Ctr value is added to DnT,w to give an indication of airborne sound insulation against low frequency bass music and traffic noise. There is evidence that the combined value better represents the insulation against airborne noise nuisance than DnT,w alone. Note: Ctr is normally a negative number, making the combined value numerically smaller.

5.2.5 Proximity of windows There are no restrictions in current regulations on the proximity of windows to timber frame party walls in timber frame dwellings. Test evidence shows that the dose proximity of windows (when closed) does not generally affect the overall sound insulation performance of a timber party wall.

5.2.6 Thermal performance Although national regulations differ, the thermal performance of party wall structures should now be considered. Air movement in party wall cavities 103

Timber trame construction

has been shown to lead to significant heat loss and result in what is known as party wall thermal bypass. This explained in TRADA's Construction Briefing Party wall thermal bypass(5)_ The heat loss though the cavity of a party wall can be considered to be zero when the wall cavities are fully filled with insulation and the edges of the cavity sealed to inhibit air movement. Mineral fibre insulation with densities of 10 kg/m3 to 60 kg/m3 or cellulose could be used to fully fill all stud voids, and the cavity between the timber studs. To avoid acoustic bridging of the party wall leaves, do not use rigid insulation products (for example very dense mineral wool or wood fibre batts) or rigid foam insulation boards. Check the specification of insulation materials against the requirements of any national regulation or schemes which avoid the need for pre-completion acoustic testing. With fully sheathed timber party walls, consider the type of insulation and when it is to be installed. If insulation is to be installed during erection of the frame, it must be protected from moisture and remain dry. This may not be possible in large multi-storey developments. As an altemative, fibre insulation could be blown into the cavity between the sheathing boards once the building is weathertight. Another altemative would be to move one or both sheathing boards to the room side of the party wall studs. Temporary bracing could be used during erection of the structure to provide racking resistance where required. Once the building is weathertight, the cavity and stud voids can fully insulated, temporary bracing removed and structural sheathing installed. To reduce heat loss from the party wall cavity, consider effective perimeter edge sealing. Edge sealing of the party wall cavity can be achieved in a number of ways; however, it is normally achieved with the use of flexible cavity barriers and/or lapping of membranes over the end of the party wall cavity, that is breather membranes.

5.2. 7 Air-tightness For enhanced air-tightness, breathable air barrier membranes can be installed between the wall linings and the timber frame structure. These may be joined betweén floor zones using the method shown in Section 6.2.

5.2.8 Junctions with other elements 5.2.8.1 Junction with the roof Cover the top of the party wall frames with wire reinforced mineral wool or equivalent that is tightly fitted to the underside of the roofing membrane. This provides 30-minutes fire resistance at the top opening of the party wall cavity, closing the concealed space and provides 60-minutes fire resistance between dwellings (Figure 5.3). Fill the space between the roofing membrane (which is continued over the wall) and the roofing tiles with mineral wool for the full width of the wall. This ensures continuity of the line of 60-minutes fire resistance between dwellings to the underside of the roof covering. Section 8.2.5.3 shows more details.

5.2.8.2 Junction at the eaves At each party wall position, firestop the void formed by the slope of the rafter and the horizontal soffit to the eaves to the required fire resistance. This can

104

5 Party walls

Mineral wool packed between battens and tiles

-'--J'~'--J~'--J.,.--....,,.__,,_..~ reinforced mineral wool

+---~rlt---Wire

Trussed rafter

Figure 5.3 Typicai junction of party wall with roof

Dimension of trussed rafter centres

be achieved by fixing a non-combustible board to all faces of the projecting spandrel frames or by filling the space with mineral wool. If a non-combustible board is used, cut it to suit the profile of the roof slope and butt it tightly to the soffit and fascia board. The party wall firestop should project out to the fascia line and meet the vertical cavity barrier between the two frames. ln the external wall cavity, continue the cavity barrier up to form a tight joint with the firestop (Figure 5.4). Even small gaps at these junctions could create a flue effect, inducing vertical or horizontal spread of smoke and flame in the event of a fire. Wire reinforced mineral wool between battens Wire reinforced mineral wool between spandrel panei and roof membrane

Spandrel panei plasterboard (two layers) Wire reinforced mineral wool to boxed eaves, alternatively noncombustible board fixed in place

Sheathing and breather membrane omitted for clarity

Head binder and top rail of wall frame

Maximum distance required to support roof battens (typically 600 mm centres)

Figure 5.4 Party wall junction with eaves

5.2.8.3 Junction with externai walls There are no restrictions on the type of cladding passing across the ends of party walls. Seal the cavity behind the cladding (a concealed space) with a cavity barrier at the party wall/external wall junction. This confines any cavity fire within the boundaries of one occupancy or compartment.

105

Timber trame construction

~ Partywall

7 Flexible cavity barrier of wire reinforced mineral wool or approved cavity barrier

Cavity barrier: Typically proprietary third party approved sleeved mineral wool cavity barriers

Sheathing with breather membrane over Figure 5.5 Party wall junction with externai wall

ln addition to vertical cavity barriers in the external wall cavity at the party wall/external wall junction, a vertical cavity barrier is required at the opening of the party wall cavity into the external wall cavity. This is usually a 50mm thick wire reinforced mineral wool quilt stapled or nailed to one side. Party wall cavity barriers must provide 30-minutes fire resistance. Figure 5.5 shows a typical detail with brick cladding; Figure 9.11 shows a similar detail using tile hanging.

5.2.8.4 Internai junctions The integrity of the party wall against fire spread and sound paths must be maintained. This is usually achieved by inserting additional vertical studs into the party wall to form L-type corners that fully support the party wall plasterboard joint where internal walls abut the party wall (Figure 5.6). This allows the total structure to be erected before the lining materials are fitted. Internal walls may bé nailed to horizontal noggings fixed between the studs if the plasterboard runs through before fixing the internal wall framing to Non-loadbearing walls L-type studs 19mm plasterboard carried through

Figure 5.6 Party wall junction with internai walls

L-type studs if plasterboard not carried through

Loadbearing wall

106

Both layers of plasterboard carried through and internai wall fixed to nogging in party wall

5 Party walls

Plasterboard support as required

r - - Junction of elements /

Wire reinforced mineral wool if required

should be taped and filled ~--

Fibre insulation may be specified

~~-tt---~---

Additional stud may be required

Fibre insulation may be specified

Wire reinforced mineral wool firestops if required

Figure 5.7a (left) and b (right) Junctions between party walls

ensure a sealed connection. With this method, if internal walls are loadbearing, the structural engineer will specify a panel-to-panel connection. ln some plan forms it is necessary for several party walls to abut. Figure 5. 7a illustrates a junction between four party walls and Figure 5. 7b a party wall 'T-junction' as typical examples. Where an intermediate floor abuts a party wall, maintain the fire resistance integrity of both at the junction (see also Chapter 6). To maintain the fire resistance where solid timber floor joists are parallel to the party wall, provide a double joist at the edge of the floor (Figure 5.Ba). Where solid timber floor joists are at right angles to the party wall, fix full depth blocking between the joists (Flgure 5.Bb). ln addition to blocking when using l-section joists, web fillers are used between the flanges. To meet this requirement with top hung metal web joists, a double perimeter header joist is typically used, with joists parallel or perpendicular to the party wall. Party wall with joists parallel towall lnsulation if specified

Cavity barrier if required Party wall strap if specified

Seal

Partywall with joists perpendicular to wall lnsulation if specified

~ Batten to support ceiling

Double joists

Cavity barrier if required Party wall strap if specified

Seal

Blocking Header joist

Figure 5.8 Junction of party wall and intermediate floor: a (left) Joists parallel with wall b (right) Joists perpendicular to wall

107

Timber frame construction

5.2.9 Current regulations in Scotland and Northern Ireland do not permit services in the void formed by the party wall. ln England and Wales, wall linings can be penetrated and services run in the void, subject to the integrity of the wall for fire resistance and sound insulation being maintained. When electrical socket outlets, switches or other service ducts are installed in party walls, do not fix them back to back. Protect the boxes at the back by plasterboard or other material equal in performance to that used on the wall surface (see Chapter 10). Wherever possible, however, services should be routed away from the party wall or placed in a service void. Water services should not be contained within the party wall but may be fitted in a specially designed service duct (see Chapter 10).

5.2.1 O Steps

staggers

Changes in floor level because of ground contours often occur at party walls. Adjoining buildings may also stagger in plan, with or without steps. These conditions give rise to special requirements in detailing. With all steps and staggers, especially if they are substantial in dimension, check the structural stability of both units with particular regard to wind forces acting on the party walls where they effectively become external walls. As the party wall is formed using two independent trames, it is necessary to clad the exposed wall area above the lower roof level, ideally with lightweight cladding. The cladding should have the required surface spread of flame

Fibre insulation if specified

--------+1~

L.)"4t9*-7iltt------ Party wall cavity min 50 mm

-----Vcl

Breather membranes lapped at comer Cavity barriers +-'.....-.f!-------

Movement joint

50 mm +/- 1O mm -------t---+-~i.. externai wall cavity

Figure 5.9 Plan at stagger, or step and stagger: Timber frame party wall

108

Externai wall to thermal specification

5 Party walls

1

t

225mm is practicable minimum dimension 375mm is preferable minimum

lf cladding on exposed wall (shaded on illustrations) is not fire resistant, then fire resistant externai sheathing may be required

_l_ y

~X~ a Step and stagger dimensions

b Step and stagger dimensions with common roof plane

The relationship of step and stagger to give a common roof slope may be found by the following formula: y-;- X = tangent 8 where x = dimension of stagger y

=dimension of step

8 = angle of roof pitch

For example: Assuming a roof pitch of 30º and a step of 8 brick courses (600 mm) 600 + x = tangent 30º

= 600 + tangent 30º Therefore stagger = 1040 mm x

When brickwork cladding is used, dimension y should be a brick course module so that coursing can continue across the stepped area Figure 5.1 O Step and stagger dimensions

(reaction to fire) performance and provide the appropriate fire resistance from outside to the upper building when required (Figures 5.9, 5.10 and 5.11).

5.2.10.1 Dimensions for steps and staggers The module of the external wall cladding, for example brick coursing, is likely to be the governing factor at steps between claddings. Although not essential, it is preferable to stagger buildings on plan so that the stud positions in the party wall frames are directly opposite each other. This simplifies the installation of the metal ties (if required) to connect the two wall frames together and any vertical cavity barrier required in separating walls. Dimension steps and staggers so that a change in roof plane occurs on both pitches or a common roof plane is maintained on one side of a dual pitch roof. It is important to ensure adequate roof separation for the correct detailing of verges and flashings. A small change in roof plane is difficult to weatherproof adequately and is better avoided (Figure 5.10).

5.2.11 Specific requirements for separating walls in Scotland In Scotland the void formed by the separating wall frames should be subdivided by cavíty barriers at junctions with floors, roof and at 10m maximum horizontal centres. Several types of cavity barrier are permitted but 50mm thick wire reinforced mineral wool is generally used.

109

Timber frame construction

Possible line of brickwork beyond Cladding (combustible cladding may be acceptable if fire resistant sheathing on adjacent externai timber frame surfaces)

Fire resistant sheathing may be required

Cavity between tiling battens and tile/slate profiles over wall filled with mineral wool if adjacent wall not sheathed in non-combustible board

Flashing above tiles usually easier to fabricate than secret gutters which give difficult flashing at eaves and brickwork

Wire reinforced mineral wool Mineral wool quilt to roof space required in - - - - - - - t t i r > Scotland ie above where wall ceases to be ,...,,.-...,....,,.........,,...""" 11__,,...,i,.~,...,,..-1 a party wall

!+-------

Plasterboard reduced to 2 layers of 12.5mm in roof space, fixed with staggered joints (in Scotland, full 30mm plasterboard thickness is required, with party wall insulation)

Roof insulation Mineral wool insulation fixed to the upper -------tlf-l'?~tv4.-...:::;.i of the two wall frames. -•

',I '

Figure 6.4 Maximum recommended offset of joists or rafters from loadbearing studs in wall trame with a single top rail without justification by a structural engineer

Metal web with timber flange joists that are top hung on a header joist typically have load transfer independent of stud positions (see Figure 4. 1O). Similarly, engineered timber I-section and solid timber joists that are supported by metal joist hangers would also typically transfer load independent of stud positions. In arder to maintain a constant wall height throughout the building, it is normal practice to use a constant floor depth and adjust the joist centres to compensate for varying span and load requirements. Alternatively, adjust the joist width and/or the grade of timber used for the longer spans. For the sarne depth, engineered timber joists typically achieve longer spans than the timber species and grades commonly used in timber frame construction. Joints in joists should only occur over loadbearing walls or beams. Joists which overlap over the wall or beam should be nailed together and should not project more than 100mm beyond the support. Joists which are butt jointed over the support (Figure 6.5) should be joined mechanically to both sides of the joists using a structural wood-based board or galvanised or stainless steel proprietary nail plates;foThe gussets or nail plates should extend to at least three quarters of the joist depth and be nailed with at least four nails into each side of the joist.

116

6 lntermediate floors

Solid blocking between joists

Splice plate •«--------

Loadbearing wall frame Figure 6.5 Joining joists end to end

Note: The use of sheet types of floor deck precludes lappíng joísts on supports when joíníng them, unless lhe junction occurs beneath a wall to lhe upper storey. Abut joists end to end and check bearing. Use splice plates or short lengths of plywood or OSB to give additíonal stiffness

6.3.1 Notching and drilling For notching and drilling solid timber, follow the guidance in Section 10.2. For engineered timber components, follow the manufacturer's third party approved guidance.

Variations when using BS 5268

Span tables, 2nd editíon includes

tables for sizing trimmers and trimmer joists.

BS 8103-3 includes tables giving sizes/spans of trímmers, trimming joists and fixing scheduíes.

6.3.2 Trimmers and beams Floor depth beams or trimmers can be fabricated by nailing or bolting floor joists together so that they act structurally as one unit (Figure 6.6). Eurocode 5 Span tables does not yet include tables for trimmers and trimmer joists. For engineered timber joists, follow the manufacturer's guidance when double joists are to act as one unit. When long spans and/or larger loads have to be supported, beams of greater depth may be required. Alternatively, trimmers and beams can be of a structural timber composite, hardwood or steel flitch beams. Where beams and trimmers are of greater depth than the floor thickness, both downstand and upstand arrangements can be used. These must be provided with adequate protection against fire (Figure 6.7). Steel beams can be used but can be difficult to place and fix in the timber structure and will also need fire protection. Beams and trimmers in floor construction will require additional studs or posts in the timber wall panels to support them and transfer their load to the foundations. When small panel construction is used it is often possible to locate a panel junction beneath the beam or trimmer so that the connected studs form a post to provide support. ln large panel structures (or where panel junctions do not coincide) provide additional studs or posts in the wall panels (Figure 6.8). Consider the implications of the actual deflections of long span trimmers and beams to ensure that deflections do not impose load onto non-loadbearing wall elements and that combined joist and beam 117

Figure 6.6 Double joists used to form a floor depth beam Note: Multiple joísts naíled together to a designed nailing pattern may avoid deep downstand beams. Joísts are shown fixed with proprietary joist hangers. Alternatively a tímber ledger can be nailed to the side of the double joist and the abutting joists notched over it. Ledger nailing should be calculated and notch síze approved by an engíneer Variatlons when using BS 5268

A design example for a steel flitch beam is included in TRADA Technology' s Tirnber trame housíng: UK structural recommendationsi.7l.

Timber frame construction

-I adjacent paneis to support beam or studs inserted ----+-++-.,__._._ _,__.,., into panei to support beam

Structural composite, hardwood or softwood or timber and steel flitch beam

Figure 6.8 Alternative methods of supporting floor beams in wall framing

will not add to the required differential movement gaps due to shrinkage. However, most timber frame elements will still need allowance for compression and settlement. The amount of settlement depends on how tightly the building is built. If horizontal composite structural timber or steel elements are designed into the building, engineered timber should be specified as joists to simplify the differential movement issues. If solid timber joists are specified with horizontal composite structural timber or steel elements, differential moment detailing will need careful consideration. This will include the number of plates, rails and head binders used. Top hung metal joist hangers supported on timber plates fixed to the top of the horizontal composite structural timber or steel elements may need to be specified. Careful detailing is also necessary if composite structural timber or steel columns are required in timber frame structures. See Chapters 9 and 10 for additional differential movement information.

6.4 Supporting internai walls The floor framing provides support for both loadbearing and non-loadbearing internal walls and transfers these loads to the supporting walls below (Figure 6.9). It also provides a head fixing for the lower wall frames. Where nonloadbearing walls occur above the floor, loads must be transferred to the walls below via the floor joists (Figure 6.1 O). Additional joists and/or noggings will often be required to carry non-loadbearing walls which run parallel to the joist span. Internal walls at right angles to the joist span can normally be carried by the joists, but allow for the additional load imposed when selecting or calculating joist sizes. The structural engineer should verify the reduced spanning capacity of the joist. 119

Timber frame construction

o-----

Loadbearing upper floor wall frame

--------~----- Floor deck •~-----

Full depth blockings

Joist span ~----

1+------

Nogging to support ceiling lining Loadbearing internai wall frame

Note: Detail shows joists at right angles to walls. Where joists are parallel, full depth blockings are replaced by joists (as specified by structural engineer) and noggings to support the ceiling

Figure 6.9 Typical detail where loadbearing walls are located both above and below intermediate floor

-n-

Noggings to fix ceiling

& g g i n g s to fix ceiling

11-

Wall coincident with joists

Wall on joist grid

-