Timber frame: Re-engineering
for affordable housing
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ENVIRONMENT
TRANSPORT
REGIONS
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Timber frame: Re-engineering
for affordable housing
G C Pitts
aN.
TRADA Technology Report 2/2000
TRADA
TECHNOLOGY
© TRADA Technology Ltd 2000
ISBN
| 900510 25 |
Stocking Lane
| Hughenden Valley
High Wycombe
Buckinghamshire
HPI44ND
UK
Tel: +44 (0) 1494 563091
Fax: +44 (0) 1494 565487
email: information@trada.co.uk
web site: www.trada.co.uk
Contents
IQOQASHIO
-.
Leaming Resources
Centre
Introduction
3
Core principles of a re-engineering strategy
5
Re-engineering for affordable housing: Concept ideas
ff
The re-engineered design solution
8
Details
10
References
16
Cover illustration: Drawing courtesy PCKO Architects
TRADA
TECHNOLOGY
Whilst 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.
The author of this report is employed by TRADA Technology. The work
reported was carried out under a contract jointly funded by the Timber
Research and Development Association (TRADA) and the Secretary of
State for Environment, Transport and the Regions (DETR). Any views
expressed are not necessarily those of the Secretary of State for Environment, Transport and the Regions.
YDETR
ENVIRONMENT
TRANSPORT
REGIONS
TRADA Technology gratefully acknowledges the sponsorship and thanks
both organisations for their support. Thanks are also due to the individuals
and organisations who participated in the advisory group for the project
and companies which supplied materials for the models produced for
demonstration:
Sir Michael Latham
Liquid Plastics
Anchor Trust Housing Association
Louisiana-Pacific (Europe)
British Gypsum
Marley Building Materials Ltd
Peter Brown & Co
PCKO Architects
Bryant Homes
The Proctor Group
Caradon - Plumbing
Redrow Homes
Caradon (Mk) - Electrical
Sodra AB
Hanson Brick
Velux/ Velfac
HRV Design
Hyde Housing Association Group
Timber frame: Re-engineering for affordable housing:
Introduction
This document is aimed at affordable housing providers whether in the
social and/or the private sector. In this context, the housing provider is
termed ‘the client’ with the end user being ‘the customer’. The intention of
the document is to raise awareness and interest in new ways to manufacture, procure and erect timber frame for the provision of affordable
housing.
“Increased housing need and building construction skill shortages
changed much of the volume housing industry from craft-based brick-built
homes to a wide variety of off-site prefabricated systems”.
This quotation, dating from the 1960s is equally valid today.
It is significant that the expectations surrounding most of the building
systems developed in the 1960s were not realised and timber frame construction is the only mainstream survivor.
Now, there is again an increased housing need and skill shortages but the
expectations are even higher:
¢
Demand for consumer choice
¢
Reduced costs
¢
More predictable delivery
¢
Reduced defects and accidents
¢
Increased productivity.
The government is also driving changes through initiatives arising from
the Latham and Egan Reports.
The traditional craft-based approach to construction cannot meet these
expectations. Timber frame, as an already established prefabricated
system, goes some way towards a solution. The timber frame construction
method generally accepted in the UK comprises small prefabricated wall
panels with trussed rafter roofs and floors assembled on site. Details are
included in the Wood Information Sheet Introduction to timber frame
construction and in the book Timber Frame Construction. However, a
re-appraisal of the whole construction process, known as re-engineering,
can deliver significant benefits.
This document outlines the background and results of a TRADA Technology demonstration project to “Re-engineer affordable housing using a
timber frame approach”. The project was jointly supported by the Department of the Environment, Transport and the Regions (DETR) and TRADA.
The key aims of the project were:
*
To investigate existing timber frame benchmarks and best
¢
To set new benchmarks and improve on existing best practice
through re-engineering timber frame. The research project (in
addition to defining detailed manufacturing benchmarks) set
general targets for completed housing as:
practice.
Reduce costs by 30%
Reduce time to completion by 20%
Reduce reported site defects by 50%
Meet or exceed all existing performance criteria
¢
To meet the stated expectations above through Partnering
(housing provider, designer, consultants and supply chain),
Whole-Housing Thinking (not just a collection of parts eg frame,
cladding etc) and Lean Production Techniques.
¢
To produce a practical, innovative timber frame system to deliver
homes which can meet the demands of consumer choice.
The re-engineered timber frame solution proposed in this document is
intended to meet these aims and targets for clients. However, this can only
be proven when the solution is adopted with significant numbers of units
built.
Core principles of a re-engineering strategy
Any construction-related re-engineering exercise aims to:
Eliminate:
Simplify:
Over production
Waiting Times
Integrate:
Automate work which is:
Technology
Customers
Difficult
Problem Areas
Suppliers
Dirty
Inventory/ Storage
Production Flows
Jobs
Dangerous
Waste
Processes
Contractors
Dull
Areas which typically offer scope for improvement and the changes which
are needed to bring about such improvements are outlined below:
Product design
Aim to “Supply solutions; rather than address parts of a problem”.
¢
Improve IT links between designer, supply chain and clients to
simplify and speed up data transfer
¢
Maximise off-site production
¢
Increase in-factory services integration
¢
Explore opportunities for “single skin” external wall construction
¢
Explore optimum standardisation of components but also
¢
Design only “open-system” homes (those with flexible internal
layout)
¢
Always utilise the roof space as part of the house
¢
Eliminate waste.
maintain design flexibility for consumer choice
Client/Customer
Provide
“Service; not lip-service”
Development
¢
Increase emphasis on partnering agreements between designers,
clients and supply chain
¢
Explore “up-chain” opportunities for earlier involvement in the
design process (eg, through Construction Management
procurement) with designers and supply chain
¢
Expand customer choice through “open system” thinking for
flexible layout
¢
Explore with key clients provision of more materials and
components within the frame package to minimise site activities
(higher quality/less defects)
¢
Explore with key clients the possibilities of forming groups or
clubs to provide level “pull” (ie consistent production week on
week) from the factory
¢
Production and Order
Fulfilment
Set benchmarks with regular improvement targets.
Provide true “Customer choice; not a customer must adapt attitude”
¢
Minimise the number of supply chain partners
°
Shift toward smaller supply chain packages for just-in-time
¢
Reduce space available for storage (if it’s there it gets filled)
delivery
*
Decrease throughput time in the factory using “multi-skilled”
personnel
¢
Do not accept waste
¢
Minimise stock levels of completed “house kits”
¢
Simplify erection processes to reduce site operations, improving
quality and reducing defects — drive toward zero rework and
recalls to site
¢
Set benchmarks with regular improvement targets.
The details and proposals outlined above are essentially a list of opportunities and potentials for introducing improvements and efficiencies in the
off-site production and procurement processes related to housing.
Re-engineering for affordable housing: Concept ideas
Four key concepts were used to focus the thinking for the re-engineering
process relating the to provision of timber frame for affordable housing.
These were:
Standardisation versus
Customisation
*
standardisation versus customisation
*
consumer choice
*
proposed changes to the Building Regulations
*
removing opportunities to get it wrong on site.
The timber frame structural grid is normally 400 mm, 600 mm, or more radically, 1200 mm for studs, joists and rafters.
A rigid discipline based on timber panel product sizes offers standardisation with minimal cutting of sheet materials to fit these grids.
Adoption of a sensible grid size means a small number of standard wall,
floor and roof panels.
Special infill panels such as a roof window, a window/door panel or a floor
“trimmed” panel offer design variability, but compromise standardisation.
For dimensional freedom, variable-size pre-made corners can be used but
again, purist’ standardisation is compromised.
Consumer Choice
“Compromised standardisation” leads to consumer choice but an “open
system” building is also essential. “Open system” in this context is
freedom of internal layout with no internal loadbearing walls throughout
the dwelling.
Another compromise of consumer choice is that due to practical and planning reasons, windows, doors, stairs and vertical services are largely fixed.
Changes to Building
Regulations
Acoustics — Part E of the England and Wales Regulations will be revised.
Timber party walls easily meet proposed new standards and compartment
floors can improve beyond current standards.
Thermal Performance — Part L of the England and Wales Regulations will
soon have higher standards. To meet the proposed new requirements,
timber frame stud sizes can be increased or the same stud size can be
retained with a change to a more efficient insulation material.
Air sealing — timber frame already exceeds current Part L requirements
and can compete in the future, but a re-engineered system needs to do
much better.
Clients and consumers will increasingly demand higher levels of performance which will exceed minimum Building Regulation standards.
Right-first-time principle
With the open-system and maximised factory production, panels can be
produced to only fit one way. Interlocking joints on each panel can provide
a “fail-safe” mechanism on site.
These joints enable positive panel to panel fixings that improve air sealing
and the thermal performance of the building.
Pre-drilling for panel connections with a single screw type minimises the
chance for error and makes inspection easier.
The re-engineered design solution
The Frame
Timber frame panels are based on a 1200 mm module for walls and
600 mm for floors and roofs, produced in standard long panels up to 12 min
length.
Dimensional
flexibility is achieved
through
the incorporation
of non-
modular panels at junctions ie:
*
wall corner
°
roof eaves and ridge
¢
floor stairwell.
Wall panels are of storey height rather than floor to ceiling height. This provides continuity in the vapour control layer and insulation through the
floor space and reduces the amount of differential movement.
Interlocking joints at the top, bottom and sides of each panel ensure the
right-first-time principle on site — if the panels do not interlock easily they
are in the wrong place.
The Wall
The external wall uses reverse wall construction — moisture resistant plasterboard and oriented strand board (OSB) on the internal face and breather
membrane, moisture resistant plasterboard or impregnated softboard on
the outer stud face.
140 x 38 mm (minimum) studs are at 1200 mm centres — saving 40% in
structural timber in the external walls and taking away the need for most
lintels.
OSB on the internal face, behind the plasterboard provides structural
sheathing, robustness on the internal face, removes the need for noggings,
forms a continuous air and vapour control layer and contributes to fire
resistance.
With cladding, joinery and an internal face of 9 mm moisture resistant
plasterboard all fixed in the factory, immediate site weather proofing is
provided.
Interlocking panel joints ensure easy erection and right-first-time location
and fixing.
The U-value is 0.27 W/m2K for 140 mm mineral wool between studs. Other
insulation and/or stud depth options can give similar U-values.
The Floor
The floor panel system is of broadly conventional construction.
The prefabricated floor units are of closed panel, stressed skin design with
18 mm OSB deck glued and screwed to joists at 600 mm centres.
Intermediate floor panels are hung from a ledger affixed at the top of the
wall panel. Non-modular infill panels can be included if required.
Intermediate floor panels include 140 mm insulation to enhance sound
insulation with 15 mm moisture resistant plasterboard on the underside
removing the need for noggings.
Ground floor panels are similar to intermediate floors but include 200 mm
insulation with a breather membrane on the under side; with mineral wool
insulation giving a U-value of 0.2 W/m2K. Other insulation and/or joist
depth options can give similar U-values.
The Roof
Prefabricated panels are used to form a useable roof space.
Panel construction has purlins at 600 mm centres spanning to gable ends.
Non-modular panels can be used at the eaves and ridge where required.
The purlin approach takes out eaves thrust and removes the need for collar
ties.
The non-modular eaves panel gives the option of open or closed eaves; the
ridge panel allows dimensional flexibility and any roof pitch.
Each panel is factory finished with a 9 mm OSB wind bracing/vapour
control layer and 9 mm moisture resistant plasterboard on the underside
and a breathable underlay, battens and counterbattens above the purlins.
Tiles or slates are fixed on site.
With 200 mm mineral wool insulation the roof U-value is 0.2 W/m? K. Other
insulation and/or purlin depth options can give similar U-values.
Service Integration
Fully factory fitted services include water pipes, telecommunication and
electric wiring. Push-fit connectors are used to provide flexibility in design,
easy installation in the factory and reliable connections on site.
Vertical services are located in the special wall corner panels.
Easy access is available for maintenance and future change.
The Details
Six key construction details are shown to illustrate the main principles.
Pre-cast foundation - Ground floor panel - Wall panel: Section
Oo
|
wen.
Kel
O)
10
Specification
|
External wall panel: 9 mm moisture resistant plasterboard on 18 mm OSB
type 3 on 140 x 38 mm timber studs at 1200 mm centres with 140 mm
mineral wool between and 12 mm impregnated softboard sheathing.
Liquid DPC factory-applied to pre-cast concrete foundation.
Bottom rail of wall panel with 50 x 38 mm timber fixed to form inside part of
interlocking joint.
140 x 38 mm sole plate with 90 x 56 mm timber fixed to form outside part of
interlocking joint.
Vertical or horizontal timber batten to form drained
behind cladding.
and vented cavity
Factory-applied cladding eg timber boarding, cement render on lath, brick
slips
Factory-applied “skirting” panel to accommodate horizontal service runs.
Pre-cast “timber frame” concrete foundation beam to be set on pre-cast
concrete piles.
Ground floor panel: 18 mm OSB type 3 glued and screwed to 235 x 38 mm
joists at 600 mm
centres with 200 mm
insulation between and Type 1
breather membrane to the underside.
Stainless steel insect mesh to base of drained and vented cavity.
10
Storey height panel - Intermediate floor panel - Wall panel: Section
Specification
l
External wall panel: 9 mm moisture resistant plasterboard on 18 mm OSB
type 3 on 140 x 38 mm timber studs at 1200 mm centres with 140 mm
mineral wool between and 12 mm impregnated softboard sheathing.
Bottom rail of wall panel with 50 x 38 mm timber fixed to form inside part of
interlocking joint.
140 x 38 mm top rail with 90 x 56 mm timber fixed to form outside part of
interlocking joint.
Vertical or horizontal timber batten to form drained and vented cavity
behind cladding.
Factory-applied cladding eg timber boarding, cement render on lath, brick
slips.
Factory-applied “skirting” panel to accommodate horizontal service runs.
235 x 38 mm edge beam factory fixed to wall panel with 63 x 388mm ledger
to support bearing floor joists of floor cassette.
12
Intermediate floor panel: 18 mm OSB type 3 glued and screwed to 235 x 38
mm joists at 600 mm centres with 140 mm (sound absorbent) insulation
between and 15 mm moisture resistant plasterboard below.
i
Special corner panel - Standard wall panels:
Plan
Specification
External wall panel: 9 mm moisture resistant plasterboard on 18 mm OSB
type 3 on 140 x 38 mm timber studs at 1200 mm centres with 140 mm
mineral wool between and 12 mm impregnated softboard sheathing.
140 x 388 mm end stud of wall panel with 50 x 38 mm timber fixed to form
inside part of interlocking joint.
140 x 38 mm end stud with 90 x 38 mm timber fixed to form outside part of
interlocking joint.
Vertical or horizontal timber batten to form drained and vented cavity
behind cladding.
Factory-applied cladding eg timber boarding, cement render on lath, brick
slips.
12
Upper wall panel - Roof eaves panel: Section
13
Specification
External wall panel: 9 mm moisture resistant plasterboard on 18 mm OSB
type 3 on 140 x 38 mm timber studs at 1200 mm centres with 140 mm
mineral wool between and 12 mm impregnated softboard sheathing.
Vertical or horizontal timber batten to form drained
behind cladding.
and vented
cavity
Factory-applied cladding eg timber boarding, cement render on lath, brick
slips.
Tiles or slates.
Tiling battens to suit tiles or slates.
50 x 38 mm
counter battens.
Roof panel: 9 mm moisture resistant plasterboard ceiling on 9 mm OSB
type 3 wind bracing panel with 235 x 38 mm purlins at 600 mm centres
with 200 mm insulation between and a “Roofshield” breather underlay
above.
Timber fascia.
72 x 38 mm “rafter” notched over purlins as an eaves
dimension overhang with open or closed eaves).
ladder (flexible
18
Notched eaves purlin.
19
Angled timber roof panel “stop” factory-fixed to topmost wall panel.
13
Ridge panel - Standard roof panels: Section
Specification
20
Standard roof panel edge purlin with 50 x 38 mm timber fixed to form inner
part of interlocking roof joint.
21
Standard roof panel edge purlin with 184 x 38 mm
outer part of interlocking roof joint.
13
Tiling battens to suit tiles or slates.
14
50 x 38 mm counter battens.
22
Ridge special panel: Plasterboard, OSB, insulation and underlay all as
standard panel, but with 235 x 38 mm timber as rafters between edge
purlin and ridge board. Length and pitch can vary to suit design.
14
timber fixed to form
Upper wall panel/Roof verge: Section
1
Specification
External wall panel:
9mm moisture resistant plasterboard on 18 mm OSB
type 3 on 140 x 38 mm timber studs at 1200 mm centres with 140 mm
mineral wool between and 12 mm impregnated softboard sheathing.
Vertical or horizontal timber batten to form drained
behind cladding.
and vented cavity
Factory-applied cladding eg timber boarding, cement render on lath, brick
slips.
Tiling battens to suit tiles or slates.
14
50 x 38 mm counter battens.
15
Roof panel: 9 mm moisture resistant plasterboard ceiling on 9 mm OSB
type 3 wind bracing panel with 235 x 38mm purlins at 600 mm centres
with 200 mm insulation between and a “Roofshield” breather underlay
above.
23
90 x 38 mm
joint.
timber fixed to roof panel to form outer part of interlocking
15
References
EGAN, Sir John. Rethinking construction. Department
ment, Transport and the Regions. London. 1998.
LATHAM,
TRADA
Publications
of the Environ-
Sir Michael. Constructing the team. HMSO, London. 1994
Timber Frame Construction — Low-rise buildings. TBL 58. 3rd edition in
preparation. Please contact the TRADA Information Centre for details.
Introduction to timber framed construction. Wood Information Sheet 0-3.
Timber framed building — materials specification. Wood Information Sheet
0-5.
Prices and details of all TRADA publications are available from the TRADA
Information
Centre,
Tel:
01494
563091
or from
the
website:
www.trada.co.uk
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Fax: +44 (0) 1494 565487
Website: www.trada.co.uk