Steve Griffiths of Taylor Lane Timber Frame discusses why working closely with timber frame manufacturers is going to be of benefit to any architect
If I had to give just one tip for architects considering timber frame construction, it would be to get on board with your timber frame supplier as early as possible. Capitalise on their expertise and experience; by working together an economic solution can be achieved, without compromising on structural integrity. Your timber frame supplier should be aware of everything – the design details, building materials etc. It all affects the structure and price. The design stage is critical for fully realising the benefits of working with timber frame, determining the U-values, and setting the SAP calculation. By not embracing timber frame in the early stages, the impact can be felt throughout the supply chain.
Timber frame explained
A timber frame is a panellised system where large wall and floor panels are made in a factory and delivered to site, ready for erection. The roof is assembled onsite, from pre-formed timber trusses. The ground and first floor wall panels, floor joists and trussed roof are typically offered as a kit – either ‘supply only’ or ‘supply and erect’. The frame can be erected onsite by the manufacturer or a qualified contractor. It’s renowned for being a fast build method as many of the build components are fabricated in the factory, with only final assembly required onsite. The pre-fabricated panels are precision engineered in a controlled environment, ensuring accuracy and consistency. A timber frame build allows for a more predictable construction schedule.
Setting standard room sizes on similar sized plots is an easy win when designing and building with timber frame. Standardisation generates speed throughout the process, from estimation to design; manufacture to delivery; erection right through to fit-out. This doesn’t mean that all the houses must appear the same. The timber frame structure can be ‘wrapped’ in a variety of external finishes including traditional brickwork, cladding and render. It’s the size of each structure that matters, particularly on low volume developments.
When considering the exterior of the timber frame build, it’s vital that any high-level brickwork is designed out on builds with a step-in level or stepped roof. This is particularly common on semi-detached houses or when there is a single storey adjacent to a dwelling with more storeys. The timber frame cannot bear the load of the high-level brickwork, due to differential movement between the frame and masonry, so a steel frame would be required. This is a costly addition and can cause a thermal bridging issue if not designed in correctly. A lightweight cladding, slip or render system is often chosen as an alternative. Also, it’s worth remembering that larger spans demand a bigger structure, often with additional steel supports and this again, is where the price can start to creep up. Big rooms mean bigger spans and more complex roof structures. We work with the architect to realise their design, and ensure that the timber frame solution is practical, economic and structurally sound. Factors such as this are considered during the estimating and design phase. The other question to ask is: can you get it all in – the building height is set by planning so increased structural support can have a negative impact on the ceiling height. It’s worth checking with your timber frame supplier that they seek advice from structural engineers when required.
Thermal efficiency & air tightness
Cavity widths are generally different in traditional builds compared with timber frame. Nowhere is this more apparent than when pursuing the desired U-value. A standard wall constructed using timber frame can achieve a U-value of 0.19 W/m2K. A traditional build would usually need a wider wall construction to achieve 0.19 W/m2K; a bigger cavity with more insulation which would impact on room size. Furthermore, a timber frame building is more economic to run due to its ability to heat up quickly and retain that heat for longer. To comply with Building Regulations, a Standard Assessment Procedure (SAP) rating must be submitted, alongside the predicted Energy Performance Certificate (EPC) for new dwellings prior to work commencing. Thermal bridging and air tightness are key indicators in achieving an SAP rating. Timber frame construction can exceed the national standards in both instances; for example, some manufacturers can achieve an average Y-value of 5 per cent, a 3 per cent improvement on the national standard. The lower the Y-value, the less heat is lost from the structure. Airtightness can be designed into the building fabric helping to save energy and reduce household bills across the lifetime of the property.
Steve Griffiths is senior design manager at Taylor Lane Timber Frame