Steve Griffiths of Taylor Lane Timber Frame and the Structural Timber Association looks at the latest developments on fire test data in the timber frame industry, post-Grenfell
For many years, the construction industry – not least the timber frame sector – has relied upon fire test data provided by plasterboard manufacturers. Plasterboard is the industry standard for dry lining walls in domestic properties and provides good fire protection as it is designated a ‘material of limited combustibility.’
Plasterboard does not form part of a timber frame kit. Typically, this comprises wall panels, roof trusses and joists. Most standard wall panels are designed with reflective breather membrane, OSB, and a timber stud. Many frame manufacturers then offer a variety of panel options which include additional products such as insulation, vapour control layers, service battens and conduits that can be fitted offsite. The plasterboard is fitted onsite by dry liners once the timber frame has been erected.
Post-Grenfell, some plasterboard manufacturers withdrew their test data however, and the responsibility then rested on UK timber frame manufacturers to produce independent fire resistance test evidence for their bespoke products. These are tested in accordance with European test standards – BS EN 1365-1:2012 (walls) and BS EN 1365-2:2012 (floors).
Wall systems for example, must meet the REI 30 and 60 minute standards, with REI is defined as R (load bearing), E (integrity) and I (thermal insulation). They are tested on the internal face only.
The wall is also under load during testing to mimic the full permissible load of the structure. In the past, fire tests were conducted with either 60% or 100% load ratio. Today, there is scant guidance on load ratio, and it is down to the test agency to decide on the load applied to the panel for testing. The timber frame industry is working hard to standardise this, so that there is a commonality to loadings applied and harmonised testing of materials.
A ‘pattern book’
The testing undertaken by timber frame manufacturers and industry organisations is complemented by a research project led by the Structural Timber Association (STA) – representing the structural timber sector and associated supply chain companies.
The STA has invested in a “fire in use” research project to test commonly used timber frame wall, floor and roof make ups. This work has been done in partnership with CSIC, University of Edinburgh, and trade associations and has been reviewed by BRE Global. The pattern book of EN tested timber wall, floor and roof systems provides best practice recommendations for the design, specification and construction of buildings up to 18 metres. The intention is that it will develop as the STA collaborates with suppliers and organisations to increase the number of systems.
Fire stopping socks
One vital element in fire stopping is the fire stop sock which restricts the spread of fire and smoke within external masonry walls. These products are often the responsibility of the timber frame manufacturer, and as such can be supplied and fitted by them. However, due to potential ‘tampering’ by subsequent trades, this responsibility is on the proviso that evidence is provided when the timber frame is handed over.
Typically, an architect would specify a 50 mm clear unventilated cavity in an external masonry wall. Fire socks break up this cavity, limit the oxygen, and ultimately stop the spread of flames and smoke.
They offer flexibility, tolerance and adaptable compression. This can be particularly useful in the gable end of a single dwelling where the tolerance may have drifted with brick laying. Fire socks can also assist in closing gaps between units, for example, in the roof space over the party wall spandrel. One consideration is that because there are too many variables in a render or lightweight cladding finish, a timber frame manufacturer would not supply fire socks for these.
Steve Griffiths is technical director for Taylor Lane Timber Frame and committee member of the STA fire task team