Affordable Housing: Re-engineering the Timber Frame Solution
An article by Paul Newman, Head of Timber Technology, TRADA Technology, published in TRADA Timber Industry Yearbook 2000.
Increased housing need, skill shortages and demands for increased consumer choice are changing attitudes to off site production in the construction industry. Despite great expectations, off site prefabrication has remained a minor league player for the housing sector for nearly four decades. This now looks set to change as client demands for speed and quality of building rise.
Timber frame construction scores highly in difficult, tight, urban locations, with poor ground conditions – the so-called brownfield sites. The lightweight, factory manufactured structural frames, which are inherent in the timber frame system, reduce foundation requirements, cut time on site and are less demanding of skilled site labour than the alternatives – major considerations when overall cost is a primary concern.
Most timber frame housing in the UK is currently based on factory prefabricated small panel wall systems, trussed rafter roofs, joist and wood-based board intermediate floors and a range of solutions for ground floors. A TRADA Technology re-engineering study, supported by the DETR and TRADA, involved a “clean-slate re-think” of the whole approach to providing homes.
Key concepts were used to focus thinking for the re-engineering process. They included:
standardisation vs customisation
extending consumer choice
improving performance: proposed changes to Building Regulations
“right first time” approach: removing opportunities to get it wrong on site.
The apparent conflict between increasing standardisation whilst also extending customer choice can be resolved by adopting ‘open system’ design ie flexible internal layout. ‘Open system’ building separates the internal layout from the structural elements, providing complete consumer choice in layout and finish.
Proposed Design Solutions
Key aspects of the proposed design solution lie in large panel construction – up to 9.6 m long. Separate corner panels provide dimensional flexibility. The frame is storey height rather than floor to ceiling height, providing continuity in the vapour control layer, insulation through the floor space and reducing the potential for differential movement. One-way-fit interlocking joints at the top, bottom and sides of each panel ensure the right first time principle on site.
The external wall uses reverse wall construction – moisture resistant plasterboard and OSB on the internal face and breather membrane or bitumen impregnated softboard on the outer. The OSB behind the plasterboard provides structural sheathing and robustness on the internal face, removes the need for noggings, forms a continuous air and vapour control layer and contributes to fire resistance.
The roof uses prefabricated panels, forming a useable roof space. Eaves panels give the option of open or closed eaves – the ridge panel allows dimensional flexibility and any pitch. Each panel is factory finished excluding the tiles or slate finish.
Modular cassette floor panels are of broadly conventional construction. Non-modular infill panels can provide dimensional flexibility.
Services are simple easy designs that can be installed in the factory and fitted together on site. Vertical services in special corner panels allow easy access for maintenance and future change. Fully factory fitted services include water pipes and electric wiring.
Numerous other strategies of improvement were identified by the study, for example ‘single skin’ external wall construction could be explored further and implemented, ‘part-set’ walls/ floors/ services/ roofs/ foundations and claddings could be developed and integrated. Progress could also be made at the product design stage, possible areas being:
Simplification of staircase integration
Increase of in-factory service integration
Improvement of IT links with supplier and clients to simplify and speed up data transfer.
Expected changes to Building Regulations have already been taken into consideration by timber frame designers. The possible revision to improve acoustic requirements (Part E in England and Wales) poses no threat, as timber party walls easily meet new standards and compartment floors are already improving beyond current standards. Alterations regarding thermal performance (Part L) are currently being reviewed, timber frame stud sizes are undergoing improvement for a more competitive energy efficiency edge. Timber frame already exceeds current regulations for air sealing. The re-engineered panels with interlocking joints enable positive panel to panel fixings that improve air sealing and the thermal performance of the building.
Full details are available in the TRADA Technology Report 2/2000 Timber frame : Re-engineering for affordable housing.
