Timber frame - where are we now?

Timber frame – where are we going?

Timber frame – where are we going?

Timber frame’s market share has risen substantially over the last eighteen months or so with large volume housebuilders making significant investments. New manufacturing facilities are coming on stream as existing companies expand and others enter and re-enter the market.

The Egan report on the construction industry highlighted prefabrication as a means of making significant improvements in construction. At present most timber frame in this country is constructed as platform frame using open panels which are then serviced and insulated on site. Volumetric units are being used in the construction of budget hotels and other similar developments where there are repetitive units and a need to bring the building into service as quickly as possible.

Innovations and trends

Changes taking place in timber frame can be divided into three broad areas

  • prefabrication and panel design

  • building taller

  • improved thermal and acoustic performance.

Prefabrication and panel design

Increasing levels of prefabrication demand higher capital investment, more time spent in design and planning and arguably can frequently result in more standardisation. On the other hand, the amount of time spent on site and site skill requirements are drastically reduced.

For walls, greater prefabrication means a move towards closed panel systems and possibly towards full volumetric units, although uniformity and size constraints are likely to limit volumetric buildings to niche markets. The use of volumetric units to produce pre-finished bathroom or kitchen ‘pods’ for housing is also gaining favour within certain sectors. Closed panel construction may not suit everyone, at least not in the short term, as it also places additional requirements on the design team and timber frame manufacturer.

SIPS – Structural Insulated Panels can be considered as a prefabricated wall or roof component, comprising timber-based board facings with an insulating foam plastics-based core. These are laminated together to produce a one-piece structural building system. First imported from North America, SIPS panels are now being manufactured on a small scale in the UK. A TRADA Technology research project is under way to examine various performance aspects including; fire, acoustic, durability and safety and health in use of SIPS in construction. Ultimately the project aims to provide guidance to manufacturers, designers, builders, developers and the checking and approval authorities on their use.

Conventional timber frame construction utilising open panels already demonstrates a relatively high level of prefabrication. It will be interesting to see just how timber frame technology develops over the next few years. Will it move to ever higher degrees of prefabrication and standardisation or will the robust, simple and flexible approach to timber frame offered by open panel systems remain the most attractive option? Only time will tell! We can be more certain that the trend towards factory-produced floor cassettes and the ground-level fabrication of complete trussed rafter roofs will accelerate as the increased use of cranes on building sites, partly driven by health and safety legislation, has made increasing the scale of prefabricated components an economic proposition.

Building taller

Changes to the England and Wales Building Regulations in the 1990’s allowed timber frame construction to be used for buildings of four and more storeys. Timber Frame 2000, a research and benchmarking demonstration project led the way in the development of multi-storey timber frame construction and there are now six-storey projects at various stages of design and construction.

Six-storey timber frame development under construction for St James Homes (North Thames Region) at Woolwich.

TF 2000 comprised 24 two-bedroom flats on six floors around a central service core which was also built using timber frame technology. Constructed with small panel components and a ground-level prefabricated trussed rafter roof, the building took 17 working days to erect using a team of six. The research programme has proved the robustness of the construction through removal of parts of the structure to demonstrate compliance with disproportionate collapse requirements. Full-scale fire tests and acoustic trials have informed recent changes and proposals to amend Building Regulation requirements.

Thermal and acoustic performance

New requirements for the thermal performance of buildings are already in place for England and Wales and Scotland. Timber frame has the advantage that there are a number of ways that the improved U values can be met and it seems likely that there will be increasing variety in wall designs with designers and fabricators developing systems and adopting calculation methods to meet their own requirements. Options include increasing timber stud sizes from the current norm of 89 mm depth to 140 mm to incorporate increased thickness of mineral wool insulation, utilising prefabricated I beams as stud components and/or incorporating other types of insulation material.

Proposals for increasing sound insulation requirements have been put forward for England and Wales. The standard form of timber frame party wall already in use has been shown in tests to meet the proposed levels. The current standard design for party floors will need to change to meet the proposals. TRADA Technology has tested a number of alternatives which meet the new requirements. These are available in the report ‘Acoustic performance of party floors and walls in timber framed buildings’.

The TRADA book Timber Frame Construction covers the technology from foundations to roof:

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