The risk assessment of timber buildings – seeing the good from the trees?

Whilst the 20th century was generally considered to be the concrete age, pressure to reduce the carbon footprint of any new building could mean we’re now entering the timber age.

In the past decade, the use in construction projects of cross-laminated timber, known simply as CLT, has grown enormously as more architects, contractors and developers see the sustainable and practical benefits of making use of wood in their projects. With environmental awareness becoming increasingly high profile, timber is a popular choice for the construction of ‘green’ buildings.

With CLT, small timber sections are formed into large structural panels that are light, stable and very strong, leading to it having been dubbed ‘plywood on steroids’. Timber is considered a good option, not just because it’s sustainable, but because it offers:

• shorter construction times due to the potential for off-site construction
• suitability for non-visible as well as exposed finishes
• lighter buildings that require smaller foundations
• good thermal and fire performance properties.

Assessing the risks

Wood frame construction has long been popular in the US because it is regarded as cheap, flexible and ecologically-friendly. In the US, timber frame buildings are restricted in height and area and must have sprinkler systems fitted. In the UK, however, a fire involving a CLT construction at Nottingham University in 2014 saw a building that was 70% complete suffer a total loss in a few hours.

Advocates for CLT point to one of its major advantages being its inherent fire resistance and, unlike steel frames, it remains structurally stable when subjected to high temperatures. Fire resistance testing of CLT panels is to ISO 834, the same standard currently used for door sets.

The char rate of CLT has been tested at 0.67 mm per minute, and as such panels are capable of the 30-, 60- and 90-minute resistance times required by many building regulations. Performance varies according to factors such as load and temperature distribution.

Whilst experts broadly agree that there is nothing intrinsically unsafe about timber frame buildings, they do warn that fire safety is dependent on a precise building process that leaves no gaps in the timber frame, and full adherence to fire safety practices on site.

The insurer’s perspective

Once complete, wooden superstructures, like all buildings, must meet strict fire protection requirements. However, as with all other building materials, fire can be a hazard during construction, so it is imperative that standard precautions are in place both to prevent fires and to ensure that people can escape to safety if fire breaks out. To reduce the risk from fire, the Joint Code of Practice on the Protection from Fire of Construction Sites (JCop) must be strictly followed.

Additionally, the following precautions are typically required to be in place:

• A wired fire detection system should be fitted throughout the works as they progress
• Waste must be removed from construction work areas on a daily basis, with no waste left overnight
• Combustible materials and packaging must not be stored within the buildings under construction
• Hot works must be removed from site where practicable
• All sources of ignition must be removed or reduced
• A no-smoking rule must be strictly enforced on the site, with designated smoking areas located at least 20 metres away from the buildings under construction
• The site should be hoarded with minimum 3.0-metre-high security hoarding, as well as having all the other usual security measures in place.

Most insurers will consider timber frame construction with three to four storeys; above that, they are likely to request more information about the nature of the project. They will need to see an assessment of the risks involved, the risk management plan that’s in place, and they may also want to visit the construction site to make their own assessment.