How Safe Is A Timber Building In A Fire?

Always an emotive subject and always in need of clarification. Whilst we all know that the timber buildings are not only attractive, but are also sustainable, economical and provide fast construction solutions; the assumption can be that because we use wood for fuel it must therefore burn easily! However, this is not strictly accurate. In reality, wood is selected as fuel because it burns slowly and at a predictable rate and these are the same qualities which make it a safe construction product in the event of fire and should give you confidence in your timber building.

The “charring rate” of a wood structure means the time needed for the wood to be burnt through. This can be calculated to determine the safety of a building or how long it will take to burn and weaken its structural integrity: a calculation of the “degree of redundancy” in the structure. As a result of the thermal properties of your timber building this can mean that the timber log will char on one side yet not even be warm just a few millimetres inside the log. These qualities contrasts with the unpredictable nature of other structural products such as steel; which heat up more uniformly and give rise to expansion and loss of strength over the whole section, or masonry which cracks and spalls. Fire-fighters prefer to enter a burning building made of timber because they are better able to estimate how long they will be able to remain safely inside. Timber will not fail dramatically like, for instance, reinforced concrete.

Different timbers char at different rates largely depending on their density with some denser hardwoods charring at just 15 mm per 30 minutes. Even low density timbers will take 30 minutes to burn through to a 25mm thickness which immediately indicates the slow burn rate expected of an MPL wood building: which is constructed from a minimum of 68mm solid log – this will ensure that the wooden building will retain structural integrity for over an hour of exposure to fire.

Structural timbers need to meet standards BS5268-2 which give a depletion rate of 20mm in 30 minutes.

However, it is important to note that all buildings whether constructed from timber or brick and block must comply with building regulations and this includes fire prevention and containment.

Although a timber building takes a very long time to burn through, the surface can allow a flame to travel to a more readily combustible material such as soft furnishings. For this reason it may be recommended that an additional Fire protection treatment is used in areas where there is greatest risk i.e.: close to boundaries (externally) or in hallway/vestibule areas (internally).

Treatments such as NON-COM, available from specialists as Arch Chemicals, can be applied off-site to external cladding materials and will result is a certified Class O protection. Similarly internal treatments such as Nullifire, which can be applied on site in just one day, can ensure that Class O protection is available where required. To comply; materials must have a Class 1 Surface Spread of Flame and low fire propagation index in accordance with BS 476 Part 6. Most timbers can achieve Class O where treated with a suitable fire retardant treatment.

These treatments not only keep you safe in the event of fire – but can also prevent ignition and reduce the rate of heat release, all of which helps to keep the structural integrity of wooden building for longer.

Whether these treatments are required is generally determined by the Building Regulations Officer of the Council when assessing your application for the new wooden building.