Timber frame 101

What is timber frame construction? What are the structural limitations? And how eco friendly is it?

Houses built using prefabricated timber frames will typically consist of a system of timber frame wall panels within the external walls. The frames carry the loads from the roof and upper floors and everything is supported on the foundations.

Similar to any other type of dwelling, the foundations will typically be of concrete and designed to suit the site conditions. A complete prefabricated solution will incorporate the roof trusses or roof panels, upper floors and internal and external wall panels in one package which is transported from the factory and assembled on site.

Prior to the delivery and assembly stage, the foundations will have been constructed along with any base walls or footings required. After the timber frame structure has been erected, work then begins on the external leafs of the external cavity walls and the roof covering is fixed according to the design.

It is important to remember that although prefabricated timber frame panels are factory produced, suggesting a degree of automation, most manufacturers still manufacture their panels by hand. They use automated equipment in the form of different types of powered saws and nail guns but only a few have installed fully automated panelmanufacturing machinery.

Armed with the knowledge of the correct panel materials to use and how they should be put together, it is feasible that panels could be manufactured on a DIY basis. Such an approach would require that the panels be designed and then certified by a structural engineer, but if you had sufficient time and a suitable shed available, it is certainly achievable.

Structural considerations

The maximum permitted height of timber frame residential buildings has received a lot of attention over the past few years, but self-build homes of up to two storeys, with each storey having a typical floor to ceiling height of 2.4 metres, remain covered by standard building regulations.

Across the world, the limits which timber frame construction can attain are continually being stretched. At the time of writing, at 25 storeys and 86.6 metres high, the Ascent MKE Building in Milwaukee, Wisconsin, is the world’s tallest mass timber building. It was built using a framework of cross-laminated timber (CLT) and Glulam, with a concrete base, elevator and stair shafts.

As with all ‘tallest’ buildings, it will be surpassed by one planned for construction in Toronto at 90 metres high and then by a 100 metre tall residential building in Switzerland which is planned to complete in 2026.

In reality, if a complex design means that external walls require multiple studs at relatively close intervals, then it can quite easily reach a point where a conventional timber frame is not the answer.

The building should use efficient framing techniques that minimise the total mass of timber to be used, so in some cases a hybrid solution that uses steel or engineered timber beams and columns along with the ordinary timber frames to achieve larger open spaces, can be the most efficient option.

One effect of using too much timber in external wall panels is that their perceived thermal insulation advantages can be diminished due to a high timber to void ratio.

Environmental credentials

Which leads us to a persistent myth concerning the selling of prefabricated timber frame kits. Almost all of the timber frame manufacturers tend to cite the thermal insulation superiority of their timber frame walls over those built using traditional blockwork or brick methods.

This is misleading, as houses are all designed to the same energy efficiency standard so any properly designed and constructed thermal elements of the external structural envelope (i.e. walls, roofs and floors) will achieve the same levels of performance.

Also often quoted is the speed of construction, however any time saved on site should be compared to the time required to produce the frames in the factory and any order lead times which may apply.

The weather is frequently mentioned too, where construction of the timber frames can continue in the factory regardless of weather conditions; but when they get to site, just as for traditional builds, they should not be stacked, stored, erected or otherwise left standing incorrectly or unprotected in adverse weather conditions.

That said, timber frame construction does have the environmental upper hand over traditional bricks and mortar. A study commissioned by the Committee On Climate Change (CCC) in February 2019 concluded that a timber framed house had lower embodied carbon and also stored more sequestered carbon in the structural elements compared with a functionally equivalent masonry house.

It also should have lower levels of embodied energy, i.e. the energy required for the manufacture of the building components or materials, delivery to site and building.

Timber still has a way to go to become as truly sustainable as we would like it to be. According to an All Island study by Irish advisory body COFORD, over the entire island of Ireland the supply of timber falls short of the demand of the construction industry in volume terms. That includes sawmilled products and wood based panels.

And because timber grown in Ireland tends to be fast growing, many timber frame manufacturers import from slower growing regions, which yield stronger timber.