Whilst there are significant advantages to using timber frame there can also be significant risks if not managed. A government report from 2012 (Analysis of fires in buildings of timber framed construction, England, 2009-10 to 2011-12) identified that fires in timber framed dwellings under construction had on average larger areas of damage compared to dwellings of no special construction. Out of total fires in timber framed dwellings under construction, 24% of these resulted in damage of an area of more than 100 m2 compared to 4% for dwellings of no special construction.
Why Timber Frame?
When compared to traditional masonry construction the advantages of choosing timber frame are numerous with the two biggest selling points being speed and cost. Other potential advantages include the reduction of wet trades and associated deliveries on site, reduction in labour, reduction in occupational risks including noise and dust associated with cutting and grinding and greater guarantee of build quality with most the structure being built under factory conditions. With all of this in mind you would be right in thinking that this type of construction is not only good for the client but good for health and safety. This will also undoubtedly form a significant part of the solution to the UK housing shortage. There is however one big risk that needs serious consideration in timber frame and that is fire.
History of Timber Frame
One of the most famous timber frame fires started just after midnight on the 2nd September 1666 in Pudding Lane. After burning for three days it destroyed nearly 90 percent of the inhabitants of London’s homes.
It’s not by accident that many conditions that contributed to the impact of the Great Fire of London don’t exist today.
The primary reason the fire spread so fiercely was because of the type of materials all the houses in London were constructed with at the time which was timber and thatch. Combined with the proximity of those houses to one and other its easy to see why the fire spread so easily. These factors are key considerations in design in the twenty first century. Prior to 1991, building regulations restricted the construction of timber frame buildings above four storeys but after changes to the building regulations it was possible to construct timber frame buildings up to eight storeys.
This change in the building regulations combined with advantages timber frame construction offers meant the popularity in timber frame took off especially with social housing projects. However, after several high-profile timber frame building fires, five of which happened within London in Croydon, Charlton, Hackney, Camberwell and Peckham, an investigation by the London Assembly was launched. What followed from the UKTFA (UK Timber Frame Association) was the publication of definitive guidance on the construction of timber frame buildings on high risk sites in densely populated areas. In 2014 the HSE (Health and Safety Executive) published an open letter summarising that failure by all those involved with “the design, specification, procurement and construction of timber frame structures” to consider and reduce the risk of fire during construction may be subject to a material breach. In one case in 2016 a contractor was fined £100k for failing to run a safe timber frame construction site.
What Are The Risks?
The risk of fire in timber frame buildings is far greater during construction than post construction.
Sources of Ignition
The main cause is simply because there are far more sources of ignition and the fire loading is far greater. Sources of ignition will include;
- Hot works – cutting, grinding, soldering, hot pitching
- Faulty electrical equipment – damaged sockets and equipment, service strikes, temporary supplies and halogen lighting
- Arson – works in high crime rate areas, protests and objections to the scheme, disgruntled employees or contractors
- Reactive chemicals
In the instance of timber frame, the frame should be included in the fire loading calculations in the construction fire risk assessment. In addition the construction phase will introduce additional fire loading. Sites will store materials, equipment and waste and depending upon how these are stored and where on the site they are stored these will increase the fire loading of the site. Most things will burn if they reach the required temperature. Some sources of fire loading typical of construction sites include timber pallets, cardboard boxes, insulation materials, UPVC’s and plastics, diesel, fabric membranes for roofing or groundworks and general site waste. Whether timber frame or masonry build these materials are likely to be present. Unsafe storage of materials and poor housekeeping are all factors that increase the risk of fire on Timber Frame construction sites.
Fire Spread – The Offsite Risks
Risks that exist beyond the boundary of the site are known as offsite risks. A key consideration at feasibility is the separation distances between the proposed new structure and existing surrounding structures and features that are offsite. With ever decreasing space in densely populated towns, maintaining safe separating distances is in many cases impossible and so other solutions must be used to protect adjacent buildings which can be costly. Even if surrounding buildings are made of brick construction combustible materials such as UPVC fascia’s and windows, roofs and external cladding products are still likely to be exposed to the risk of fire. Failure to take into consideration offsite risks at the design stage can result in fines for designers as was the case for a firm of architects who failed to take into consideration the proximity of a care home when specifying timber frame.
Typical garage infill sites will have an entrance of 2 to 3 meters which will impede access. This problem is further confounded when the need to have plant and site accommodation is factored in. Whilst firefighting techniques have developed significantly from gangs of people with buckets of water and levelling houses with gunpowder to create fire breaks, the fire brigade still need the room to get to the building. They also need to know that if they get to site they will have the water pressure available to extinguish the fire so their engagement is important early in design.
Contractor’s temporary accommodation is required to be a safe distance away from the structure not only to protect the accommodation from potential timber frame fire spread but to protect the structure from risk of fire spread from the temporary accommodation.
Fire risk on timber frame sites is greater during construction than post construction. According to the Fire Statistics Monitor 2009-2010 1 in 8 fires (50 out of 400) were on timber frame buildings in construction as opposed to 1 in 59 (802 out of 47,602) post construction. It is important that the client is aware during design that any work that exposes the frame or breaks compartmentation undertaken as part of maintenance needs to be fire stopped to ensure the integrity of fire compartmentation. This may be difficult to enforce in tenanted properties where access by the client for inspection can be restricted.
How Can You Reduce The Risk?
The Structural Timber Association (STA), (previously UKTFA) guidance, which is freely available from their website identifies the measures required to ensure fire risk is given the necessary consideration throughout the scheme and has aligned it with the RIBA stages, albeit the old RIBA Stages.
As with the objectives of the CDM Regulations 2015, the key to successful design risk management is to do it from the start.
Choosing the right material, even if it’s not timber, is an easier decision to make when less time and money has been invested. Put simply, from the outset, the design team should be asking what material is right for the site location given the constraints. Simple controls that should also be considered to reduce risk include;
- Using closed panel systems
- Installing sprinkler systems / fire detection systems early on in construction
- Designing out hot works
- Building compartmentation and protected fire routes in as the building is constructed
- Increase security for the site – CCTV, Full height hoarding, signage
- Engagement of local Fire Brigade – to assess water pressure and accessibility
- Proper fire risk assessment that considers fire loading and fire separation distances
Using the STA Risk Assessment Checklist, 16 Steps to Fire Safety and Design Guide to Separating Distances During Construction will ensure the right considerations are taken and the risks are suitably reduced to avoid HSE intervention and more importantly that no one is hurt.
Faithful+Gould’s national Health & Safety Services Team have experience of working on projects from various sectors that have relied on timber framed solutions. Our CDM Advisers are able to steer and influence these considerations during design stages and ensure best practice and legal compliance is applied.