Architect Anthony Thistleton gives the lowdown on the carbon realities of timber, and explains why misconceptions around CLT on fire safety grounds need to be corrected
As the world finally wakes up to the task ahead in mitigating climate breakdown, there is a lot of discussion about trees and urging reforestation to help reduce atmospheric carbon and limit global warming. Many commentators refer to trees using terms like “miracle machines that build themselves and convert CO2 to oxygen.”
Of course this is true, we need to plant trees on an unprecedented scale and pace if we are to have any chance of meeting the IPCC’s targets for 2030. However, while forests are great carbon stores, once the trees are mature, their carbon emissions balance the absorption leading to a net zero contribution to GHG reduction. For a fast growing spruce or pine, this takes about 50-80 years, for an oak, this takes around 200-250 years.
If we are serious about using trees to combat climate change, we need to therefore remove the trees once they are mature, store the timber and plant new trees. One of the best long-term stores for timber is in buildings. We can construct buildings that last over 100 years using timber and using modern forms of engineered timber, such as glulam and cross-laminated timber (CLT). We can also replace materials like concrete and steel, which have large carbon footprints – together these two materials are responsible for 15 per cent of global CO2 emissions.
So the arrival of these methods of timber construction is timely. A number of innovative architects, engineers, developers and contractors in this country have pioneered the use of CLT as an alternative to concrete and steel in more, and larger buildings. As a result of this, the UK leads the world in the range and scale of implementations of this technology with around 600 completed CLT structures.
However, in the aftermath of the tragedy at Grenfell Tower in 2017, the UK Government introduced legislation that threatens to damage the perception of timber as a construction material and could have a significant impact on the industry.
As a result of the ban on combustible materials in the external walls of residential buildings taller than 18 metres, there is an erroneous perception that CLT may be a fire risk. This is completely untrue.
It is important to understand how engineered timber performs in combustion, as it actually has a number of benefits that offer an improved performance in the event of a fire in comparison to other building technologies.
In the first instance, in a CLT building there is no increase in the likelihood of a fire starting. The majority of fires ignite through electrical faults and accidents, such as cigarettes left on furniture. If a fire starts in a CLT building, the burning causes the formation of a char layer on large timber elements which actually protects the timber beneath and inhibits further deterioration.
Once a fire starts, the key issue is that it should be contained. Most catastrophic fires occur because a fire has spread dramatically from the source of ignition. In a CLT building, with walls and floors made from solid timber, the fire is unlikely to break out of the compartment in which it starts, even if the fire fighters take a long time to attend. When we design in CLT we often protect the CLT with plasterboard, and, where we rely on the charring for fire protection, we increase the size of structural members to account for the loss of the charred layer.
In many framed forms of construction, a 30-minute firewall will be exactly that, and after half an hour the fire will breach that wall, spreading to adjacent spaces. In a CLT wall, it will continue to act as a barrier to the fire well beyond the fire rating.
The fact is that all materials can be adversely affected by fire – steel melts and concrete spalls (explodes). The key is that designers understand the material’s performance and design accordingly. We are convinced that when designed properly, CLT structures are at least as safe as other forms of construction.
The challenge we now face is to provide the testing data and other material required to ensure that buyers, funders, underwriters and politicians understand this as well as the professionals who use CLT so that we can rely on regulations that are based on evidence.
Across the world, countries are amending their building regulations to support taller engineered timber structures, supported by extensive testing. In the next decade, the UK’s lead in this ground breaking technology is likely to be eclipsed as the rest of the world adopts the techniques that were initiated here.
More than this, however is our responsibility to the environment – by creating a long term store for CO2 absorbed during a tree’s growth and substituting high polluting materials like concrete and steel, CLT can be a key agent in the drive to reduce global atmospheric carbon, but there is another factor. If we create a large, sustainable market for timber, the widespread adoption of CLT can drive the reforestation needed – over the last two centuries, every country that has increased its timber manufacturing capacity has increased its forest cover by a commensurate amount. By moving to CLT we can better ensure large scale tree planting than by Government urging, and grants.
Anthony Thistleton is director and co-founder of Waugh Thistleton Architects