ROOF CONSTRUCTION AND LOFT CONVERSION FOURTH EDITION C.N. Mindham © C.N. Mindham 1986, 1988, 1994, 1999, 2006 Blackwell Publishing Ltd editorial offi ces: Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK Tel: +44 (0)1865 776868 Blackwell Publishing Inc., 350 Main Street, Malden, MA 02148-5020, USA Tel: +1 781 388 8250 Blackwell Publishing Asia Pty Ltd, 550 Swanston Street, Carlton, Victoria 3053, Australia Tel: +61 (0)3 8359 1011 The right of the Author to be identifi ed as the Author of this Work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopy- ing, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. First edition published under the title Roof Construction for Dwellings by Collins Professional and Technical Books 1986 First edition revised and published by BSP Professional Books 1988 Second edition published by Blackwell Science 1994 Third edition published 1999 Fourth edition published by Blackwell Publishing Ltd 2006 ISBN-10: 1-4051-3963-3 ISBN-13: 978-1-4051-3963-2 Library of Congress Cataloging-in-Publication Data Mindham, C. N. (Chris N.) Roof construction and loft conversion / C.N. Mindham. – 4th ed. p. cm. Includes bibliographical references and index. ISBN-13: 978-1-4051-3963-2 (alk. paper) ISBN-10: 1-4051-3963-3 (alk. paper) 1. Roofs–Design and construction. 2. Framing (Building) 3. Lofts–Remodeling. I. Title. TH2393.M63 2006 690′.15–dc22 2006040783 A catalogue record for this title is available from the British Library Set in 10 on 12.5 pt Sabon by SNP Best-set Typesetter Ltd., Hong Kong Printed and bound in Singapore by COS Printers Pte Ltd The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp processed using acid-free and elementary chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. For further information, visit our subject website: www.thatconstructionsite.com Contents Preface vii Acknowledgements ix 1 The Development of the Pitched Roof 1 Primitive roof forms 1 The coupled roof 1 Stability 3 Ceilings 6 Trusses 7 Design for economy 9 Standard design roofs 9 Bolt and connector joints 10 Trussed rafters 11 Cost advantages 11 Legislation 13 2 Roof Shapes and Terminology 18 Terminology 23 3 The ‘Traditional’ or ‘Cut’ Roof 26 Design 26 The common rafter and purlin roof 28 The hip roof 32 The mono pitch roof truss 35 The valley structure 36 Attic roofs 37 Roof lights and roof windows 48 Additional design considerations 48 iv Contents 4 Attic Construction Using Modern Engineered Timber Components 52 The fl oor 52 Attic construction using engineered timber components 57 5 Bolted Truss Roof Construction 71 The joints 71 Truss assembly 72 The split-ring connector 74 Accuracy 75 Standard designs 75 The roof construction 75 Bolted truss hips 77 Valleys 77 Structural openings 80 Roof stability 80 6 The Construction of Trussed Rafter Roofs 82 Performance in use 82 Design 85 Design information 86 Quality control 88 Inspection and quality assurance 94 The construction of a trussed rafter roof 96 Hip end roofs 103 Valleys and intersecting roofs 103 Trimmings for openings 109 Attic and loft roofs 109 Openings for dormers, roof windows and stairwells in attic trussed rafter roofs 117 Bracing the attic trussed rafter roof 119 Trussed rafter shapes 122 7 Truss Plate Systems 123 Systems available 123 Computer programs 125 Training 128 Technical data 130 Metalwork 131 Hip roof construction 131 Valleys 138 Attic trusses 141 Raised tie trusses 143 Punched nail plate joists, rafters and purlins 145 Future developments 148 Contents v 8 Roof Construction Detailing 150 General 150 Storage and handling of timber and timber components 150 Preservative treatment 157 Wall plates and fi xings 160 Gable ends, ladders, gable restraints and separating walls 166 Water tank platforms 174 Ventilation of roof voids 178 Bracing 180 Eaves details 186 Trimming small openings 192 Infi ll 195 9 Loft Conversions 198 Development of the loft 198 The early attic 199 Floors and ceilings 201 Cottages 202 Wasted space 202 The fi nal blow 203 Examining conversion possibilities 204 Decision making considerations 204 Typical roof types 205 Letting in light 210 Can we stand up? 219 Valley 219 Access to the attic 220 The last resort 221 10 Obligations – Visual Impact on Your Home 222 The planning application 224 The structural impact on the home 225 11 The Conversion 227 Making a start 227 The survey 227 Plans and specifi cation 228 Writing the specifi cation 229 Impact on occupants during conversion 230 Builders’ stores and materials 231 12 Solutions 232 Basic accommodation 232 The common roof 233 Bolted truss and trussed rafter roofs 235 vi Contents Roof replacement 236 Ventilating the roof void 237 Openings for dormers and roof windows 240 Conclusions 240 Bibliography, Standards and Technical References 241 Index 245 Preface First published 20 years ago, this book fi lled a gap in the literature being the fi rst reference manual to include the then rapidly growing trussed rafter methods of roof construction. The second edition was expanded to include attic roofs and loft conver- sion, followed by the third edition which updated all text and drawings to the current standards of 1999. This fourth edition details the rapid growth in engineered timber components and in particular their use in room-in-the-roof construction, dealing both with the fl oor and the rafter diaphragms. These components and construction methods have been dealt with by an additional chapter dealing with I beams, prefabricated roof panels and roof cassettes. The constructions are explained with the aid of numerous drawings. Like many other building topics the roof is one of the those subjects with which everyone is familiar until it comes to actually detailing or cutting the timber com- ponents concerned, and then the lack of knowledge becomes apparent. Furthermore, research soon confi rmed the total lack of in-depth text on the construction of trussed rafter roofs, a method of construction now used on over 90% of house construction in the United Kingdom. The book aims to describe with the aid of many drawings, not the structural design analysis of the roof structure, but the design of the roof assembly as a whole entity rather than individual elements in isolation. Recognising the growing trend to refur- bish older homes, the traditional or ‘cut’ roof is described. The bolted and connec- tored roof is dealt with in some detail, for despite the popularity of the trussed rafter this older system is still chosen by some builders. The bolt and connector truss roof is particularly popular for small extension projects where it often continues the con- struction of the original roof. Chapters 6 and 7 cover the trussed rafter roof in great detail, dealing with the often misunderstood hip construction, valleys, girder truss assemblies, and the forming of openings in roofs as well as attic constructions. Chapter 7 compares the various truss plate systems and has been made as accurate as possible, bearing in mind the many changes being introduced by these manufacturers to their engineering services and viii Contents computer programs and with the constant updating of BS 5268: Parts 2 and 3 and Eurocodes. Chapters 9 to 12 deal with all aspects of loft conversion to attic rooms of the roof structure itself. The text does not address the subject of fi re protection and escape, or the installation and alteration to services. Variations between buildings being converted in shape of roof, size, number of storeys, and intended use of attic are so great that it is impossible to cover all situations likely to be encountered. My text and illustrations will, however, cover most common constructions. It is the intention that the book be used for reference, and to this end there is a small degree of repetition between chapters, and there is frequent cross-referencing between chapters for both text and illustrations. Although some basic common knowledge of building is anticipated, most terms used are fully described, making the book equally suitable for use by both the building student and the professional. The text takes into account the latest issues of both the British Standard for timber engineering, BS 5268: Parts 2 and 3, and the Building Regulations 2000 and all subsequent amendments. However, as it was felt to be outside the scope of this book, the subject of fi re resistance and spread of fl ame has not been dealt with. Reference should be made to Building Regulation Approved Documents. For ease of reference all drawings have been given a number, the fi rst digit of which refers to the chapter, and the second and third digits being the numerical sequence in that chapter. Generally, shading has been used to highlight those elements dis- cussed in the text to which the illustration applies. Most drawings have been pro- duced in perspective form to aid quick appreciation of the three-dimensional nature of all roof structures. Chapter 2 sets out the terms used throughout the book to de- scribe roof and truss shapes, and individual roof members. The specialised terminol- ogy of the trussed rafter is given in Fig. 6.2. Finally for those involved in the design aspects of roof structures, the British Standard 5268: Parts 2 and 3 should be avail- able for ready reference. C.N. Mindham 14 Harrowden Lane Finedon Northants NN9 5NW Acknowledgements I would like to thank Martin Moore of Wolf Systems Ltd who has been of great help with the structural design aspects of this revised edition, and all who, however fl eet- ingly, have helped me with the research and the preparation of this fourth edition of the book. I would also like to thank some of the purchasers of the fi rst three editions, who have taken the time to telephone, write and discuss with me various aspects of both the text and the illustrations. Some of the changes I have made in this edition have been in response to their suggestions. The new Chapter 4 and the modifi ed Chapter 7 have been produced with the will- ing cooperation of Boise Building Products, Trus Joist, Smartroof Ltd, Kingspan Ltd, Milbank Roofs, Wolf Systems Ltd, Gang-Nail Systems Ltd, MiTeck Industries Ltd, Alpine Automation and TRADA. All of these organisations have given freely of their technical information and I am indebted to them for the use of their various illustra- tions in these chapters. Finally my thanks are due to my wife for tolerating the sometimes not inconsiderable mess of paper, literature and drawings cluttering the family home. CHAPTER 1 The Development of the Pitched Roof PRIMITIVE ROOF FORMS Man has always needed a roof for shelter. Early man used roofs formed by nature such as caves, but nomadic peoples had to be more resourceful, creating shelters of a temporary nature each time they moved. It is likely that simple tents formed with animal skins over branches were the early form of constructed roofs, with more permanent shelter being pit dwellings. These were simply a shallow excavation cov- ered with a simple roof of branches and skins. It is an easy step from this type of dwelling to a simple wall on the edge of the pit to raise the headroom and then to use shaped branches to give a slight pitch, thus improving rain run-off and therefore the quality of the environment within the shelter. The simple ‘cruck’ frame comprised two curved pieces of timber standing on the ground at one end and meeting at the top. Across several of these ‘crucks’ were tied horizontal members onto which, again, were fi xed skins or as time progressed simple thatch. THE COUPLED ROOF Moving away from early roof forms that provided both wall and roof in one unit, the next development showed a true roof built on masonry or timber walls. The simplest form of roof was a coupled roof, consisting of two lengths of timber bearing against each other at the top and resting on a wall plate at their feet. The timbers, called couples, were pegged together at the top with timber dowels and were similarly pegged or spiked to the wall plate. The term ‘couple’ was used until the fi fteenth century when the terms ‘spar’ or ‘rafter’ started to be used. The term rafter of course is still used to describe the piece of timber in a roof spanning from the ridge to the wall plate. The couples were generally spaced about 400 mm apart tied only by horizontal binders and tile battens. The simple couple was adequate for small span dwellings and steep pitches, but the outward thrusting force at the feet of the rafters caused