Fire Safety in Timber Frame Buildings

What This Requirement Covers

Timber frame construction is widely used in UK housebuilding and is increasingly used for multi-storey residential buildings. Approved Document B does not prohibit timber frame at any height, but the fire safety requirements become more demanding as building height increases, particularly following amendments to the combustible materials ban.

Key Requirements

Combustible Materials Ban and Timber Frame

1. The ban on combustible materials in external walls (Regulation 7(2)) applies to buildings over 18 metres in height
2. This effectively prevents the use of exposed timber frame in the external wall of buildings over 18 metres
3. For buildings 11-18 metres, timber frame external walls may be used if the complete wall system passes a BS 8414 large-scale fire test assessed to BR 135 criteria, or if the timber is within a wall system where the external materials are A2-s1,d0
4. For buildings under 11 metres, timber frame construction is widely used without restriction on the frame itself (subject to the external surface achieving the required surface spread of flame classification)

Compartmentation in Timber Frame

1. Compartment walls and floors in timber frame buildings must achieve the fire resistance periods required by Approved Document B Table A1
2. Timber frame compartment walls typically achieve 30 or 60 minutes fire resistance using combinations of plasterboard linings, mineral wool insulation, and timber studs
3. Proprietary tested systems (e.g., from timber frame manufacturers) provide documented fire resistance ratings
4. Compartment walls must be carried through any cavity, including the external wall cavity, to prevent fire spreading behind cladding

Cavity Barriers

1. Cavity barriers are critical in timber frame buildings because the cavities within the wall and between the frame and the cladding can act as routes for fire spread
2. Cavity barriers must be provided at:

- Every compartment wall/floor junction - Around window and door openings - At maximum 20 metre intervals horizontally and vertically in cavities - At the junction of the wall cavity with the roof void

1. Cavity barriers must achieve 30 minutes fire integrity and 15 minutes insulation, or be constructed of materials such as timber at least 38 mm thick, steel, or calcium silicate board

Fire During Construction

1. Timber frame buildings are particularly vulnerable to fire during the construction phase before fire protection layers are installed
2. The Structural Timber Association (STA) publishes guidance on fire safety during construction
3. Recommended measures include 24-hour site security, hot works permits, separation of materials storage, and early installation of fire barriers

Practical Compliance Tips

1. Obtain the fire resistance test evidence for all compartment wall and floor details from the timber frame manufacturer before construction
2. Install cavity barriers at the same time as the frame is erected; do not leave them until after cladding
3. Ensure fire-stopping around service penetrations uses products tested in timber frame assemblies
4. Programme independent fire barrier inspections before plasterboard installation
5. For buildings over 11 metres, commission a fire engineer to prepare the external wall strategy
6. Consider the insurance implications of timber frame construction; insurers may require specific fire safety measures during and after construction
7. Keep detailed records of all fire barrier and cavity barrier installations, including photographs, for the building safety case