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28 August, 2008
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Fire drill
Spring 2007
Published:  02 April, 2007

A typical cavity barrier

Timber frame structures will have no difficulty in meeting the new Approved Document B: Fire Safety of the Building Regulations which comes into force in April. TRADA frameCHECK engineer Rob Attwater explains

All forms of construction must comply with the fire performance requirements laid down by Building Regulations and it is the building designer’s responsibility to ensure that these are met. For timber-based structures, there is no difficulty in meeting the required levels, given correct design, standards of manufacture and workmanship.

A new two-part version of Approved Document B: Fire Safety for England & Wales comes into effect on April 6, aiming to simplify guidance on fire safety. The amended document was published in December and will affect building notices or full plans submitted after April 6. Volume 1 covers ‘Dwellinghouses’, and Volume 2 ‘Buildings other than Dwellinghouses’. Changes relating to timber frame are not major.

In considering the complex phenomenon of fire, two main stages are recognised and these are reflected particularly in the testing to determine the fire performance of buildings.

1 Ignition and fire growth - Linings. The behaviour of a material in this stage is termed its “reaction to fire” and covers aspects such as ignitability, non-combustibility, and its contribution to the development of a fire.

2 The fully developed fire - Structure. At this stage a material contributes to the fire resistance of an element of building structure (such as a wall or floor). Fire resistance can be defined as the ability of an element to carry on performing a building function in spite of being subjected to a fully developed fire. The fire resistance of a timber frame structure is achieved by a combination of the internal lining material, the timber structure and the insulation.

It is necessary to control the potential contribution of the linings of walls and ceilings to a developing fire, as well as to use their benefit in meeting the specified periods of fire resistance for the building’s structural elements. The reaction to fire performance levels required for lining systems is defined in Approved Document B and two classification systems are currently accepted. Under the European system, all lining systems, excluding floors, are classified from A1 to F (A1 being the highest) in accordance with BS EN 13501-1:2002. The national system, based on BS 476-7: 1997, defines four categories, Classes 1 to 4 (Class 1 being the highest). In some situations, Building Regulations call for a performance superior to those defined in BS 476-7. This is designated Class ‘0’ and also incorporates performance criteria in BS 476-6.

Thirty and sixty minutes’ fire resistance are the current normal requirements for low- and medium-rise buildings. The higher resistance is generally obtained by the use of a double layer of plasterboard (Class ‘0’ material) with the joints staggered. Plasterboard is the most commonly used lining because of its economy, ease of working and good fire performance. However, it cannot be considered as a uniform material; different manufacturers provide various products that all perform differently, so individual specifications for boards and fixings should be met.

Screw fixings are typically used as it is easier to avoid breaching the paper layer – vital in maintaining the integrity of the plasterboard. When two layers are specified, both should be fixed individually to the correct specification and a structural engineer should be consulted if fixings are to provide structural racking resistance.

For timber frame construction, another relevant section of the Building Regulations is that which concerns cavity barriers and fire stops. According to UK fire statistics, cavity fires (where a fire breaks out in a cavity wall construction or where cavity materials are responsible for fire growth) are rare, representing about 0.07% of all fires attended by fire services. Statistics indicate that there have been no fatalities or injuries resulting from cavity fires, and that material damage has been minimal.

Chiltern International Fire research entitled Understanding fire risks in combustible cavities concluded that, when properly installed, current commonly specified cavity barrier types meet the functional requirements of Building Regulations and there is no evidence to suggest that a rise in the number of timber frame residential buildings would result in an increase in fire casualties.

Specification of cavity barriers will differ depending on the type of external cladding used. Current requirements for cavity barrier placement are given in Approved Document B. Every cavity barrier should be constructed to provide at least 30 minutes’ fire resistance and may be formed of:

∑ Timber at least 38mm thick

∑ Polythene-sleeved mineral wool, or mineral wool slab, in either case under compression when installed in the cavity

∑ Wire-reinforced mineral wool blanket at least 50mm thick

∑ Calcium silicate, cement-based or gypsum-based boards at least 12.5mm thick

∑ Intumescent cavity barriers

∑ Steel at least 0.5mm thick.

If timber is protected from direct attack by a fire source (for example in a timber frame building with plasterboard linings) it cannot ignite and burn before a temperature in excess of 400°C is reached at the timber’s surface.

Contrary to many people’s perceptions, timber in construction performs well in fire. It burns steadily at a predictable rate and charcoal forms on the surface, insulating and protecting the core. When fire does take hold, timber reacts differently from other common structural materials, providing uniform charring at a low rate (after the protective plasterboard has fallen away), low heat conduction and no deformation at high temperatures.

Although all buildings under construction can be vulnerable to damage from fire and other hazards, Building Regulations do not cover buildings at this stage. The HSE estimates there are around 11 construction site fires every day (although the vast majority are not on housebuilding sites).

The majority of fires can be prevented by designing out risks, taking simple precautions and by adopting safe working practices. On every construction project, all members of the team are required to comply with the Construction (Design & Management) Regulations 1994. This includes producing a risk assessment for the site, its design and construction, and covering the use of any particular building material. This assessment should include the possibility of fire during construction.

The Construction Confederation and Fire Protection Association have published a ‘Joint Code of Practice on the protection from fire of construction sites and buildings undergoing renovation’. Compliance with this Code often forms a condition of insurance. It describes simple precautions and safe working practices which will ensure that adequate detection and prevention measures are incorporated during the design and planning stages, and that work on a site is to the highest standard of fire safety. There is also a checklist of things to consider. Further information on preventing fire on construction sites is given in the Fire Protection Association Joint Code and Checklist and the HSE information sheet on ‘Construction Fire Safety’.

To provide evidence supporting multi-storey timber frame dwellings, Chiltern International Fire, TRADA and the BRE carried out a series of full-scale fire tests at the six-storey TF2000 building, Cardington, during the late 1990s. Further information is given in ‘Multi-storey timber frame buildings: a design guide’ where the specific project reports are referenced.

The primary objective of the fire test was to evaluate the performance of a medium-rise six-storey timber frame building when subjected to a real compartment fire. Fire resistance issues of structural integrity and compartmentation were assessed. This research demonstrated that the performance of a complete timber frame building, subject to a real fire, is equivalent to that obtained from standard fire tests on individual elements. This ensures timber frame construction can meet the requirements of the Building Regulations for England and Wales, the Building Standards for Scotland and Building Regulations for Northern Ireland in terms of limiting internal fire spread and maintaining structural integrity.

If all factors from correct design, standards of manufacture and workmanship are met, there is no difficulty in timber-based structures meeting the required performance criteria.

• TRADA Technology has published a new Wood Information Sheet ‘Fire performance of timber frame residential dwellings’, which TRADA members and registered users may view at www.trada.co.uk.

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