Timber and Fire Safety

Timber is a versatile and sustainable building material that has been widely used in the construction industry for many years around the world.

However, the appetite for timber-framed commercial buildings and homes in the UK remains more muted than in other countries. This is due to a combination of factors that includes a lack of institutional knowledge required to work with timber at scale and fears around the combustibility of timber, dating back to the Great Fire of London, which have recently been reinforced by concerns around combustible materials triggered by the Grenfell tragedy.

Timber is no more dangerous to build with than other materials as long as its characteristics are understood, and buildings comply with the same building codes and safety standards which mitigate risks in all buildings. The biggest barrier to using timber is a lack of expertise and knowledge in the UK.

[edit] Characteristics of Fire in Timber

One of the advantages of timber as a building material is that it burns steadily at a predictable rate, and charcoal is formed on the surface of the timber, which serves to insulate and protect the core.

When fire does take hold, timber reacts differently to other common structural materials, with uniform charring at a low rate and low heat conduction. If timber is protected from direct attack by a fire source, such as in timber frame buildings with plasterboard linings, it cannot ignite and burn until the surface temperature reaches at least 400°C.

Charing can be taken into account when using timber for structural purposes. For instance, you can choose the thickness of load-bearing timber based on its strength after charring occurs to maintain structural integrity in a fire.

[edit] Reaction to Fire

To determine a building material's ability to withstand fire, it's essential to consider two main factors: reaction to fire and resistance to fire. It's crucial to differentiate between the two.

Reaction to fire refers to whether the material acts as fuel for the fire prior to the flashover. This is evaluated based on a fire classification system consisting of seven groups, from A to F, where A indicates non-combustible materials and F indicates highly flammable ones.

During the early stages, when a fire is still becoming established, the five important factors are:

• Ignitability – how readily will a material ignite and catch fire?
• Spread of flame – once ignited, how quickly will flames spread across the surface of that material?
• Heat release – once alight, how much heat energy will be generated by the burning material, which will contribute to the further growth of the fire?
• Smoke production – how much smoke will be generated by the burning material?
• Flaming droplets – will the burning material disintegrate and produce burning droplets or debris which might fall onto and ignite other surfaces?

The factors contribute to a material's reaction to fire properties, which can be evaluated, measured, and improved if needed through the use of a WPA Benchmark-approved flame retardant treatment and quality-assured application system.

Once a flashover occurs, the fire becomes uncontrollable, and all combustible materials are assumed to be burning. At this point, the focus shifts to ensuring fire containment for safety purposes. This is achieved through the performance of building structures and design elements such as walls, ceilings, floors, and doors, and not solely based on the performance of individual materials.

[edit] Fire resistance

There are two different types of fire resistance measures for timber buildings: passive and active:

Passive fire protection measures are designed to contain or slow down the spread of fire and smoke through the building structure. This includes the use of fire-resistant walls, floors, and ceilings, as well as fire doors, fire dampers, and fire-resistant glazing. These measures are intended to prevent the spread of fire and smoke, limit damage to the building, and provide time for occupants to evacuate safely. Fire retardant treatment is often applied to raw and finished wood to meet the fire resistance standards outlines in BS476-22:1987, so it can contain and delay the spread of fire travelling.

One specific example in timber construction is Cavity Barriers. Timber frames in construction must be placed within a ventilated "cavity" to avoid rotting. However, this cavity poses a high risk of fire entering it, as it can cause the frame to burn inside the walls, making it challenging to extinguish and potentially causing the building to collapse. To prevent this, cavity barriers made of mineral wool insulation are used in timber frame buildings to prevent the spread of hot gases from a fire and limit its ability to spread. These barriers must be properly installed on-site for them to work correctly.

Active fire protection measures are designed to detect and control fire when it occurs. This includes fire alarms, sprinkler systems, and other fire suppression systems. These measures are intended to extinguish the fire quickly or contain it until firefighters arrive, reducing the damage to the building and protecting occupants.

Together, passive and active fire protection measures contribute to the overall fire resistance of a building, helping to ensure the safety of occupants and limit the potential damage caused by a fire.

[edit] Risk assessment

Conducting a risk assessment is crucial for timber buildings because wood is a combustible material and more susceptible to fire hazards than other building materials. A fire risk assessment can identify potential fire hazards and evaluate the likelihood and consequences of a fire, enabling appropriate measures to be taken to prevent or minimise the risk of fire.

Such assessments can also help identify appropriate fire protection measures, such as the installation of sprinkler systems or fire-resistant materials, to enhance the safety of occupants and protect property. Ultimately, conducting a fire risk assessment for timber buildings is an essential step in ensuring the safety and well-being of occupants and the longevity of the structure.

[edit] The Timber Fire Safety Resources

To support the increased use of timber, the timber industry has launched a new website known as Timber Fire Safety.

Timber Fire Safety is a peer-reviewed live website that provides information and guidance on fire safety in timber buildings. The website is a collaboration between Timber Development UK (TDUK), Swedish Wood, and the Structural Timber Association (STA) and focuses on both UK regulations and European standards. It is a valuable resource for housebuilders and the industry to stay up to date with changing regulations and industry best practices.

Timber Fire Safety is an invaluable resource for the industry to stay up to date with changing regulations and industry best practices regarding fire safety in timber buildings.

Timber Fire Safety, gives users a single point of access to information and guidance on this multi-faceted and safety-critical subject, it is designed for all construction professionals and is available via https://timberfiresafety.org

--Timber Development UK 09:14, 05 Apr 2023 (BST)

[edit] Related articles on Designing Buildings

• Ablative material.
• A guide to the use of urban timber FB 50.
• Batt.
• BS 476.
• Cavity barrier.
• Compartment floor.
• Compartment wall.
• Coppicing, pollarding and shredding.
• Fire resistance.
• Fire separation.
• Fire.
• Fire-separating element.
• Fire-stopping.
• Pyrolysis
• Physical Properties of Wood.
• Sacrificial timber.
• Sapwood.
• Testing timber.
• The Art of Pyrography.
• The history of timber construction in the UK.
• Timber species.
• Timber preservation.
• Timber vs wood.

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