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Safety first when designing timber-framed construction projects

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The HSE’s new policy on timber frame construction places responsibility for fire safety within the design phase, warns Geoff Wilkinson

As you may have read in the AJ, a firm of architects (Mario Minchella Ltd) has been fined for safety failings in the construction of a new timber frame care home (AJ: 28.11.14).  The issue arose from a routine HSE inspection that found that there was nothing in the design specification produced by the architects to alert construction workers erecting the timber frame to the additional fire risk it created, and the need to take action accordingly.

In case readers are unaware of the issues, the problem arises from the fact that the separation distance between new timber frame buildings during construction and adjacent buildings is significantly greater than that required in Approved Document B. Buildings as far as 20m away from the building being constructed have ignited in a blaze (eg at Beaufort Park, Colindale, London, in 2006).

Internal fire protection measures (eg plasterboard) are not installed until the later stages of construction. This means that there is a significant period of risk when the timber frame has no fire resistance, unlike traditional construction, where bricks and blocks provide immediate fire resistance as they are laid. In this case, had the timber frame caught fire there was a serious risk that the radiant heat would cause the fire to spread to the adjacent care home, putting the lives of residents and staff inside at risk.

In such cases in the past the HSE has taken enforcement action against contractors, but this case marks a clear change of policy, placing responsibility for consideration of construction phase fire safety clearly into the design phase.  The HSE has taken the unusual step of writing an open letter to the whole construction industry to highlight the separation distances required during timber frame construction.  

The letter states: ‘All those making design and procurement decisions that significantly affect fire risk should consider and reduce the risk and consequences of fire during the construction phase through DESIGN. Failure so to do may constitute a material breach for which HSE will apply its Fee for Intervention scheme those duty holders who have contributed to the breach [sic]. Designers and manufacturers of timber frame structures have duties under CDM Regulation 11 that cannot be passed on to the Principal Contractor. Risk should be designed out as far as is reasonably practicable and information about residual risk passed to the Principal Contractor.

‘Everyone in the supply chain has a responsibility to work towards this end. Anyone in the supply chain who makes a decision, which significantly affects fire safety during construction, should be prepared to justify that decision and recommendations in terms of risk, rather than cost. If fire safety considerations are left to a late stage in the design development of the project, there is an increased potential that the solutions will be inconsistent in approach and less effective if there is a fire, and more time consuming and expensive to implement.’

Architects should:

• Include a design assessment of the particular site and its constraints relevant to the method of construction being considered.

• For timber-framed structures, this assessment should be undertaken using the Structural Timber Association’s Design guide to separating distances during construction and reference HSG 168 Fire Safety in Construction.

•  The assessment should identify that there is sufficient separation to allow an unprotected structure or the measures required (such as fire-retardant coatings) to match the site constraints.

• The appropriate category frame should always be clearly specified to the manufacturers.

• Any specific information and instructions must be passed to the principal contractor.

Geoff Wilkinson is managing director of approved inspectors Wilkinson Construction Consultants

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