23 January, 2020

The future of engineered timber after Grenfell socialThe UK Government’s response to the tragedy at Grenfell Tower was to introduce a ban on the use of combustible materials in the external walls of buildings taller than 18 metres. Amongst other commonly used materials affected by this restriction was cross-laminated timber (CLT). As the name implies, CLT is an engineered product formed from factory fabricated bonded layers of softwood, each layer placed at right angles to its neighbour. It’s a versatile material that can be made into many shapes and sizes designed in each case to meet specific functional requirements. The resulting components are both strong and light, and significantly from the perspective of sustainability, come from a renewable resource. Against the background of much greater concern about the impact of our buildings on the climate and the changes to Approved Document B of the Building Regulations which deals with fire safety, the industry has been working hard to find innovative ways to allow CLT to continue to be used in new buildings without compromising the new - and now more prescriptively defined - standards of fire integrity.

Emerging to wider notice in the 1990s, the use of CLT grew for a number of reasons; design flexibility, good thermal performance and high sustainability credentials are all key. It is also a lighter, less dense material compared with steel or concrete which means it can deliver potential cost savings in foundation construction. Used initially for smaller projects like schools or healthcare buildings, it has been taken up to a greater extent in the multifamily and student accommodation sectors. Legal & General amongst others invested in setting up factories to prefabricate entire apartment modules from CLT which could be used to construct, for example, volume build to rent schemes. Beyond its versatility as a building material CLT also offers potential programme savings.

There can never be a choice between life safety and sustainability in our buildings, but equally they should not be seen as mutually exclusive. Materials like CLT offer us many advantages especially as a substitute for more traditional building elements with far less attractive carbon footprints like steel and concrete. Right now, we can anticipate a radical review of the standards and processes we use to establish that our buildings are designed and built to be robust and safe.

Timber trade bodies and the Royal Institute of British Architects (RIBA) both argue that the Government’s action is expedient and lacks nuance. They argue that the use of timber as a cladding material should be distinguished from its use as a structural system, where a route to demonstrate regulatory compliance has always existed. The new rules also overlook, they say, timber’s inherent fire resistance. Right now, however, it’s probably not reasonable to expect the political climate, following so significant a loss of life, to be open to so technical an argument which seems, on the face of it, to push back the margin of safety so firmly set by the new legislation. As is evident, the ban is already denying designers access to materials that were previously available, so there is something of an ingenuity gap waiting to be filled with new materials, new applications and new design approaches. Finding such an approach has been a key factor for us.

CBRE is currently project managing a residential-led urban development that includes new towers originally intended to be built with CLT. Following the amendments to Approved Document B which came into force in December 2018, a predominantly CLT clad structure was no longer an option for our client. As a result, the design team has been engaged in a series of studies to develop alternative designs for the tall buildings with a focus on avoiding compromise to the sustainability objectives in the project brief.

The design team has been working in collaboration with a leading structural frame contractor and a façade specialist to develop a hybrid design for the buildings. This new design has also been assessed by the project’s Approved Inspector to confirm it will meet the amended legislation. The proposed new design will continue to use CLT for the interior partitions, but it will work in conjunction with a steel structure supporting the exterior façade now incorporating non-combustible mineral wool insulation. Sprinklers would also be installed throughout the towers, again something we believe will be incorporated into Document B and identified in the most recent consultation about amendments in 2019.

The team has identified a further challenge in the potential risk of condensation which needs to be resolved before this can be taken to the next stage of design development. Nonetheless, this exercise shows how the industry is beginning to respond to the changes in legislation but is not being driven to relinquish the selection of sustainable materials and components in doing so. CLT is of real value from the perspective of sustainably and can also be a powerful weapon in the fight against climate change. We will all need to find new design approaches that maximise our ability to use sustainable materials like this without putting at risk any aspect of building or life safety.