Timber frame

The term 'timber frame' typically describes a system of panelised structural walls and floors constructed from small section timber studs, clad with board products, in which the timber frame transmits vertical and horizontal loads to the foundations. It is generally not used to refer to timber post and beam structures or to timber engineered structural frames.

Timber frames can be the most suitable choice if the structural shell is required quickly, if the ground conditions are particularly poor, or if the design does not include very large structural spans. For more information, see Advantages and disadvantages of timber frame buildings.

There are a number of ways of developing timber frame structures:

• A commonly-used method for constructing timber frames is the platform frame, in which each storey is formed by floor-to-ceiling timber panels and a floor deck which then becomes the platform for constructing the next storey.
• The engineered stud method, with the installation of insulation between the load-bearing timber studs.
• The twin stud method involves two timber frame stud walls, positioned parallel to one another (sometimes with only one carrying the vertical load), with an insulated cavity separating them.
• Structural insulated panels (SIPs) take the form of an insulating core sandwiched between two structural facings. In the UK, the mainstream suppliers typically use the same structural facing - oriented strand board (OSB). For more information see Structural insulated panels.
• Cross-laminated timber (CLT) building systems. See Cross-laminated timber for more information.

Fire resistance can be enhanced by internal linings, such as plasterboard, and cavity barriers which prevent fire from entering the cavity. For more information, see Timber framed buildings and fire.

Timber elements can also be oversized to aid fire resistance. For more information, see Sacrificial timber.

Following the government's review of the ban on combustible materials in and on the external walls of certain buildings, structural timber has, since 1 December 2022, been permitted in the external walls of relevant residential buildings up to 18 metres in height, subject to compliance with the relevant Building Regulations requirements. The government has not extended this permission to buildings above 18 metres, citing insufficient evidence to justify an exemption for the highest-risk buildings. Cladding and other materials on the external walls of relevant buildings 11 metres and above must also generally achieve a European fire classification of A1 or A2-s1, d0, which in practice restricts the use of combustible materials, including many timber cladding products, on such buildings. For more information, see Approved Document B and Timber framed buildings and fire.

NB: The role of Standards in offsite construction, A review of existing practice and future need, published by BSI in 2019, states: 'Timber frame building can consist of wall panels, alongside floor and roof panels - often referred to as cassettes. These can also be open panel or closed panel. Open panels are timber frame wall panels, comprising studs, rails and sheathing on one face and a breather membrane. Closed panels also include linings on the faces of the panel, a vapour barrier and breather membrane. Closed panels may also include fitted windows, openings for doors and service routes. Manufactured in factory conditions, these cassettes and panels are brought to site and fixed together to form a rigid load-bearing superstructure. These consist of timber studs and beams, stiffened on one side with oriented strand board (OSB) and plasterboard.'

Fire safety in construction, written by HSE and published by TSO (The Stationery Office), Third Edition, 2022, suggests that timber frame is: 'A build method that relies on a timber frame for structural support. The construction method uses wall panels and floor decks, often factory-manufactured. The main systems used are classified as either open-panel, insulated or closed-panel. These panels can include the wall insulation pre-fitted and sometimes include the pre-fitting of doors, windows and service zones for on-site installation of mechanical and electrical works.'

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