The construction industry should not stop using timber but instead needs to continue to develop better ways to increase its fire safety, argues Barbara Lane
Since publication of the paper I co-authored with Susan Deeny on fire safety design in modern timber buildings, CLT suppliers and designers from the UK and Europe have engaged with us, as well as the Structural Timber Association. We see a desire within this industry to meet the fire safety challenges of timber – challenges that are not unique to this material, however. We want to be clear that we support a framework which produces robust methods for engineered timber in low-, medium- and high-rise buildings.
Our paper has been used by industry body the Concrete Centre to imply that its content suggests the need for limitations in the use of timber. This is wrong. We need all construction industry stakeholders to be more usefully focused on supporting open and honest investigations into advancing fire safety design and construction, in all material types – something that recent events show has been far too long in the waiting.
The challenge now is to know and understand the opportunities and fire risks for each material and commit to an engineered solution
The tragedy at Grenfell Tower and other recent fires have highlighted the need to re-evaluate the performance of all construction materials and systems, and the methods used to assess their fire safety. The challenge now is to know and understand the opportunities and fire risks for each material and commit to an engineered solution – one based on scientific evidence and calculation and supported by realistic fire testing, so that the risks can be mitigated and the opportunities exploited.
December’s amendments to the Building Regulations have strengthened the fire performance requirements for materials that become part of an external wall for residential buildings over 18m tall. This does not, however, amount to a “combustibles ban”, as it has often been described.
In conjunction with the creation of new tests and analysis methods, a substantial improvement in the fire performance of the resulting external wall systems with many construction materials should then be achieved. We hope for this improvement to become prevalent in the construction industry.
Timber has a key role to play in reducing our CO2 emissions and brings additional opportunities for prefabrication and the repurposing of existing buildings; these have driven a recent resurgence in its use. Timber should not be removed from the suite of materials available to the construction industry; rather, we need to develop processes to increase its fire safety.
From our own test data there is clear evidence that mass timber influences the behaviour of fire within a compartment (or room) such that:
- The mass timber may continue to burn after the fire involving the room contents has decayed – therefore, the strength and stability of the structure and the compartmentation performance, in this additional fire scenario must be designed for, noting the external wall also forms part of a compartment.
- Under low-ventilation conditions (small openings in relation to room volume) increased external flaming may occur – therefore, the performance of the external wall in this additional fire scenario must also be designed for.
At the time of writing there are up to 10 room-scale fire test programmes completed globally that support these findings. Incorporating these effects into the structural stability system, the compartmentation performance, and the detailing of the external wall, is the responsibility of any construction team utilising mass timber.
The traditional method for fire safety design of timber was to rely on the development of a thick char layer. The designer allows for the loss of structural section due to charring but also relies on the char to insulate the residual timber and doing so act as structural fire protection.
Traditional methods do not consider the additional contribution of fuel to the fire from the timber elements as they char. For mass timber that is allowed to char by design, this additional fuel created from the structure itself can be substantial. This additional fuel must be considered properly when carrying out protection solutions, for example, based on charring rates.
CLT panels have exhibited fall-off of this char layer (referred to as delamination), which is believed to be influenced by the form of adhesive used to bond the layers together. The consequence of this is an increased rate of charring and therefore a greater contribution of additional fuel to the fire. The effect of delamination on the rate of charring needs to be addressed by developing heat-resistant adhesives and/or considering the increased rate of charring within the structural calculations. Developments are in progress for both approaches and designers require clarity on the performance of panels in this regard, when creating fire risk mitigation measures.
Determining the fire performance of products and systems, their detailing in real buildings and how those details perform in fire is complex. Again, there is now acknowledgement of an industry-wide problem here, as communicated in the Hackitt review and the publicised data from the Grenfell Tower inquiry. This is not a problem unique to timber; it is a wholescale problem.
Tested and certified products for protection of the structure and the passive protection products required to seal compartments (such as fire-stopping, fire doors, cavity barriers) are critical to the delivery of fire-safe buildings. For engineered timber, we need a bespoke suite of such products, tailored to the real fire performance of mass timber.
This presents a significant opportunity for the stakeholders in the use of CLT in the use of mass timber, in a context now, where wholescale change in product fire performance has emerged as an urgent focus.