The proportion of new build timber frame homes is growing fast in both England and Wales. For all forms of construction, there is a need to consider fire risks during the construction stage, and Milner Associates joint Managing Director Guy Lewis shares how this can be best managed during the design phase for buildings of timber frame construction.
According to the Structural Timber Association, approximately 24% of new build homes in the UK are of timber frame construction. In Scotland, this proportion has always been higher. The increase in the popularity of timber frame construction is being driven by the need for more sustainable construction methods that reduce the embodied carbon of construction materials and by the need to deliver continually improving standards of thermal performance for new build housing in the UK.
The risk of fire in completed timber frame buildings is no greater than in any other form of construction. However, during construction, while the timber frame structure is still exposed and there is the risk of construction materials and sources of ignition being present on-site, the potential risk to both those persons beyond the site boundary and to the construction workforce should a fire occur must be considered.
In our experience, less attention is being given to fire risk at the planning stage of projects, with the emphasis on arranging build routes on-site for construction logistics and sales handovers being the primary focus. In Milner Associates’ opinion, this emphasis should change, and the risk of fire spreading to neighbouring residents and premises must be assessed at the planning and design stage.
It is not the responsibility of the contractor to undertake the off-site fire risk assessment; instead, it is the responsibility of the designer or person who proposes the use of the timber frame construction in the first instance. That person might be the client, the project architect or the developer. In accordance with the principles of the Construction (Design and Management) (CDM) Regulations, the person making the decision on construction type becomes a ‘designer’ and is ultimately responsible for ensuring that the construction can be built safely without placing undue risks on those persons outside the site boundary.
The risks of fire spread beyond the site boundary or to escape routes for the occupants of neighbouring occupied properties can be brought within acceptable levels at the planning and design stage by carrying out an off-site fire risk assessment using appropriate fire risk assessment guidance, such as that produced by the Structural Timber Association – ‘Design guide to separating distances during construction for timber frame buildings’ and ‘Fire mitigation product paper 5 – Guidance for structural timber frame buildings below 250m² in total floor area’ – or by carrying out bespoke radiant heat calculations.
This assessment should take into account the specific site boundary location, which may change during the course of the works. For example, developers often hand over plots for occupation on a rolling basis, and this can present a number of changing risk points that need to be carefully assessed to ensure that residents of newly occupied homes are not placed in an unacceptable fire risk situation due to the presence of nearby incomplete timber frame structures.
Occupied properties off the site must also have a safe means of escape route in the event of a fire occurring on the site. The acceptance threshold for radiant heat is much lower than for fire spread, and this can therefore govern a fire risk assessment and the need for mitigation.
The off-site fire risk assessment will identify the key risk points (that might change through the course of the project) and should specify the categories of timber frame mitigation (category A: standard open panel timber frame, category B: reduced fire spread timber frame or category C: fire spread resistant timber frame) that can be adopted to reduce these risks and bring them within acceptable threshold levels.
These enhanced categories of timber frame construction typically use materials of either limited combustibility or non-combustible materials in place of the standard untreated timber products. Working with a fire engineer with knowledge of radiant heat calculations will enable an optimum solution to be achieved that minimises the use of these enhanced and thus more expensive products.
Too often, the off-site fire risk assessment is carried out late in the design and planning process or, in the worst cases, not considered until the principal contractor has started on-site. By this point, it is not usually possible to move a building within a site to increase the separation distances to reduce the off-site risks to existing occupied properties outside the site boundary, but it might be possible to adjust build routes and construction phasing of the development to avoid risks resulting from handovers for occupation within the site.
If fire spread risk is not considered until the principal contractor is already on-site, it can also be too late for the mitigation to be built into the frames in the factory. Timber frame prefabrication is usually undertaken on a ‘just-in-time’ basis, but timber frame suppliers still need time to fabricate wall and floor components. Trying to add mitigation retrospectively on-site to frames that have already been erected is difficult, and it may put occupants of neighbouring properties at an unacceptable risk while the mitigation is retrospectively added. The preference of the Health and Safety Executive is that all mitigation should be built into the timber frames as they arrive from the factory and not retrospectively added on-site.
To be effective, the off-site risk assessment should therefore form an integral part of the planning stage, with fire risk and build routes organised to design out fire risks as the build progresses. This is also more cost-effective for developers, as it potentially avoids the need for costly mitigation measures, which increase the costs of their timber frame kits. Designing out construction-stage fire risk is best achieved by having a linear build route or by having clearly defined handover procedures, for example, which ensure that sales teams are aware that plots cannot be handed over for occupation by customers until all neighbouring plots have their claddings and dry linings completed.
The off-site fire risk assessment should form a part of the pre-construction information that is handed to the principal contractor by the principal designer at the end of the planning and design stage. During the construction phase of the project, the responsibility for fire safety on-site then becomes the responsibility of the principal contractor through the production of a fire safety plan.
Guy Lewis is a managing director at Milner Associates, part of the RSK Group. He has over 20 years’ experience of working in the timber frame construction sector. Guy has a particular focus in the field of timber frame engineering and construction-stage fire risk assessment and has worked on many projects for timber frame manufacturers, developers and self-builders. He is also a Chartered Structural Engineer and a member of the Institution of Structural Engineers.
