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Masterclass: Cross-laminated timber and fire safety

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The AJ and James Hardie recently hosted a CPD-accredited talk on fire safety by building regs expert and AJ columnist Geoff Wilkinson, followed by a panel discussion

‘We should build a three-storey, timber-framed building, put a mattress on the balcony, a motorbike in the front room with a full tank of petrol, and set fire to it,’ says Waugh Thistleton Architects co-founder Anthony Thistleton.

Far from being a pyromaniac, Thistleton is a fierce ambassador for the environmental and fire-safety benefits of modern timber construction. He is speaking at a masterclass event at the Science Gallery in central London on Friday, 28 June, organised by the AJ and partner James Hardie, following a talk on fire safety by Geoff Wilkinson, managing director of Wilkinson Construction Consultants, AJ Specification columnist and a building regulations expert.

Anthony Thistleton

Anthony Thistleton

Anthony Thistleton

Thistleton says more could be done to shift public and political opinion towards timber-framed buildings. A demonstration project in which a mock residential tower was built with a cross-laminated timber (CLT) or glulam frame, loaded with the fire risks of modern life and then ignited, would go a long way to proving just how safe the material is, he says.

Thistleton – whose practice has published a book showcasing timber frame construction – says the government’s ban on combustible materials in external walls of new high-rise buildings has hit the sector hard. 

‘We need to look to testing,’ he says. ‘In the USA, you also can’t use combustible materials in the external walls of buildings above a certain height – unless they’re proved to be safe by testing. This lack of testing as a system prevents the use of CLT in the UK, which we know is safe if it is installed correctly.’

Fellow panellist Kelly Harrison, an associate structural engineer at Heyne Tillett Steel, adds that experiments across the Atlantic had shown just how safe timber frames are.

Kelly harrison crop

Kelly harrison crop

Kelly Harrison

She says: ‘I saw some interesting tests from the USA where they showed that all fixtures and fittings in an apartment room built in CLT were lost before the structure of the CLT started to go, even when it was completely exposed. The tests are out there; it’s just about doing more of them and putting them into the public domain.’

We need to get main contractors more familiar with timber frames

Harrison adds that main contractors need to be convinced, as well as clients and policy-makers. She says: ‘We need to get main contractors more familiar with timber frames – they often “value engineer-out” the glulam or CLT frames. Test projects publicised in the right way would help the cause.’

The third panellist, Geoff Wilkinson, says a lack of confidence persists in the fire performance of CLT. ‘The way timber achieves fire-resistance is that its outer layer chars, which prevents ongoing fire spread,’ he says.

‘You oversize a member, which chars around itself, self-extinguishes and remains in place with the strength needed to hold the building up. 

‘But, with laminated timber, it can come down to the adhesives whether it delaminates. Also, when a fire gets into a cavity structure itself, you can end up with a situation where the fire service thinks a fire is extinguished but it reignites. 

‘We need to be sure that we’re confident in fire protection before we rush into offsite manufacture of timber frames.’

Thistleton responds: ‘In most cases in apartment blocks we’re encapsulating CLT, so that it’s the primary line of defence. Even when that fails, the CLT behind it will continue to perform structurally and as a compartment. Compare the situation with a metal stud: the wall gives you 30 minutes’ protection; after that the wall has gone and the fire is on the other side.’

He adds that timber frame specifiers are often more focused on fire safety than those going down the steel or concrete frame route. He says: ‘We go well beyond what is required. That can be contrasted with traditional building methods, where a lot of people are asleep, doing things by rote, not thinking about fire safety in the way we are when designing with timber.’

Aj 151

Aj 151

Thistleton emphasises that the pressing reason for promoting use of timber frames in construction is environmental. He says: ‘Timber, and mass timber in particular, is made from fast-growing softwood that absorbs CO2 during its growth. Once a tree reaches maturity, it stops having a significant carbon absorption net benefit and you need to cut it down, store the timber somewhere and plant new ones. Putting timber into buildings is a fantastic long-term use.’

Harrison adds: ‘We have net carbon targets for 2050 and industries that have to emit carbon. We have to encapsulate it somehow and reforestation is the main way. We are always refurbishing and extending existing buildings in inner city areas and timber has a massive strength-to-weight ratio, so it is a great material to work with.’

She says that on a large glulam-framed extension of an office building in north London, Heyne Tillett Steel was putting in 120-minute fire cores as part of a concerted effort on fire safety. ‘A timber building that burns down is not saving carbon,’ she says. 

Thistleton acknowledges that timber suppliers could come together more effectively to lobby for policy change. 

But he adds: ‘All the oxygen of Parliament has been taken up by Brexit. It will also be difficult to find MPs to challenge the ban on combustibles, given the focus on “letting people sleep safe in their beds” after the Grenfell Tower fire.’  

Key points of Geoff Wilkinson’s talk

  • ‘We are all part of the industry that created the situation that killed 72 people at Grenfell Tower. It is unacceptable to continue in the mindset that allowed that to happen.’
  • Although Approved Documents help explain Building Regulations in context, there is no legal obligation to follow the processes in the documents if you can meet the regulations in other ways.
  • Building safety requires a fundamental culture change, considering fire safety in all buildings, not just those above a certain height.
  • Architects must take responsibility for fire safety and ask questions about construction products.
  • We need the equivalent of the car industry’s NCAP rating, whereby certificates might be posted on buildings, giving information about fire safety features.
  • Technology might provide further practical aid. Building control officers could ‘virtually inspect’ projects using robots and video uploads.

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