Watch our webinar in partnership with Medite Smartply, which explores fire safety in timber buildings.

Mass timber is an immensely attractive material, aesthetically and environmentally, and also because of its lightweight properties. But barriers to its use are becoming greater, due largely to worries about its behaviour in fire. This is coming from two main directions – from changes to the Building Regulations, and from increasing concerns by the insurance industry. This webinar examined the issues and suggested some ways forward.

Speakers (from left to right) Steven Wallis, Jim Glockling, Anna Beckett, and David Murray

As Anna Beckett of engineer Webb Yates said, ‘Timber doesn’t burn as you would expect it to. The perception doesn’t agree with the way that it actually behaves.’  The fact that timber is not as flammable as some fear, and that it is usually protected from burning through by a char layer, does not mean that there are no problems.

Beckett explained how regulatory changes make it necessary to adapt designs. She showed some of the projects that the practice has worked on previously and discussed how they would have to change if built today.

Exposed glulam columns and CLT slab forming the front extension at York House, London, designed by dMFK Architects with Webb Yates Engineers (ph: Agnese Sanvito)

For example, at York House in London’s King’s Cross, designed with architect dMFK, the front and roof extensions are in CLT and glulam. The CLT was sized to allow for the thickness lost to charring. At the time – 2019 – this was considered adequate and the CLT could all be left exposed. Today, Beckett said, the fire treatment would be different.

On a project at Old Street with Mary Duggan Architects, the team is looking at a hybrid steel and concrete structure. They were advised that either the ceiling or the walls of the CLT elements needed to be covered, because of concern about a build-up of heat in the joints.

CLT panels being lifted into place during construction at 134 Old Street, London, designed by Mary Duggan Architects with Webb Yates Engineers (ph: Agnese Sanvito)

Beckett provided several considerations for the design of future timber buildings:
• Choose a fire engineer and building control who have experience and are familiar with timber buildings.
• Have conversations about fire design early in the design process
• Full-scale testing or modelling for fire is likely to required
• It is unlikely both the walls and ceiling can be exposed
• Consider the charring depth for solid timber and charring with delamination for CLT.
• Sprinklers may be required even in low-rise buildings.

The Fire Protection Association’s (FPA) recent white paper explores the insurance challenges of massive timber construction

Jim Glockling, technical director of the Fire Protection Association, and director of RISC Authority – a research organisation funded by the insurance industry, gave an insurer’s point of view. He explained the concept of estimated maximum loss, which means the largest part of a building that could be expected to be lost in the event of a fire. This is not only the damage that is caused by burning, but also the consequent water damage from putting the fire out. ‘Designers,’ he said, ‘need to build in features that allow insurers to assign an EML that is less than 100 per cent.’

The problem, he said, is that our enthusiasm for timber has led to new ways of building without fully understanding some of the consequences. In addition, he argued, our Building Regulations, unlike those of some other countries, are concerned only with protection of life and not with preservation of fabric. A particular problem, he said, comes from fires in combustible voids which can smoulder for a long time without going out.

Because of wood’s environmental advantages it is, Glockling said, ‘probably the material for the future – but it needs to be used in hybrid structures. At the moment, the desire to build is running ahead of research.’

David Murray, head of technical affairs and Ireland sales at Medite Smartply, addressed a specific and important issue – the combustibility of timber during construction when, by definition, not all protective measures are in place. He explained the rigorous independent testing regime that his company’s flame-retardant OSB goes through. Because the material is impregnated throughout, there is no loss of protection when boards are cut. Using this material can be of significant benefit to a developer. The main means of protecting adjacent buildings from construction fires is by specifying a separation distance. With flame-retardant OSB this distance reduces. So the building can be bigger and generate more revenue.

SMARTPLY MAX FR BUILD (flooring) at The Edge student accommodation, John Moores University and University of Liverpool (ph: Datum Timber Frame)

The fourth speaker, Steve Wallis of architect dRMM, presented a case study of the Charlton WorkStack, a building near Greenwich, east London, that provides light-industrial workspaces. The units are stacked and cantilevered to provide a visually striking entrance to a larger-scale development.

Designed by dRMM, Charlton Workstack is a new model for low-carbon, high-density light industrial units

Based on the idea of stacked logs, this approach provides units in a range of sizes, and the overhang also assists with solar shading. Wallis explained that the design uses the maximum lengths of CLT panels available, with long floor panels spanning the width of the building, and wall panels spanning the length. The CLT is left exposed internally on the walls and soffits, with a fire strategy that depends on having a sacrificial char layer.

Located in south-east London, the five-storey building is constructed from CLT

There have been many imaginative timber buildings from this practice and others. Let’s hope that the regulatory regime and the concerns of the insurance industry can be satisfied so that we can have many more exciting and safe ones.