Building related embodied greenhouse gas (GHG) emissions in the UK are estimated at 6% of total emissions. The wider policy drivers are clear; GHG emissions need to be cut to net zero by 2050. This means the building industry is increasingly looking towards new methods to improve its environmental impact, one of which is to make greater use of timber.
Timber is one of the oldest construction materials and has a huge advantage over many more modern alternatives: it plays a role on both sides of the net zero balance. Trees absorb CO₂ as they grow, and is far less carbon intensive to manufacture, transport and erect than steel or concrete. One increasingly popular timber construction material is cross-laminate timber (CLT), a wood panel product developed in Europe in the 1970s, which has a projected global market of $1.6 billion by 2024.
The basic manufacturing process is relatively simple. Planed and kiln-dried timber is stacked into layers (lamellas), with each layer being placed at a 90° angle to the one beneath. These layers are glued using a non-toxic adhesive (which also aids end-of-life reuse or recycling), and hydraulically pressed together to produce high-strength structural panels. Almost any shape of panel can be produced and any waste is readily used for biomass fuel, often on-site.
The carbon benefits of the material are also clear – a report commissioned for the Committee on Climate Change by the BioComposites Centre estimates that, compared with a concrete frame apartment block, one built from CLT would (per flat) save up to 18 tonnes CO₂ equivalent in construction and sequester up 17.3 tonnes of CO₂ equivalent.
CLT continues to benefit from wood manufacturing innovation, as well as software advances enabling complex algorithms which are revolutionising building design. As a consequence, the applications of CLT now include curved load-bearing wall structures, stair flights and lift shafts, whilst the feasible height range for buildings continues to be extended. The Mjøstårnet Building in Brumunddal, Norway, is 18 stories high (85 metres) and built entirely out of CLT, including the elevators. The development consists of office space, apartments, a hotel, restaurant and an adjoining public swimming pool.
The UK’s first large bed vacuum press for the production of CLT on a commercial scale only came online in 2017. Despite the UK being a late adopted of CLT, the material has the potential to make a sizeable contribution to greening the economy in pursuit of net zero goals. While UK production of CLT may be in its infancy, the UK already has one of the most diverse ranges of CLT buildings in the world. Uptake has been aided by the UK’s common law legal system, where architects and engineers must demonstrate that new buildings conform to building regulations rather than having to follow prescriptive design parameters, providing flexibility to interpret requirements in the light of changes in technology, society and legislation.
Construction using CLT has been especially concentrated in London, as CLT has particular advantages in dense urban environments. Prefabrication from CLT results in fewer site deliveries, lower noise levels and greater speed in construction, reliable on-site programming and ease of dismantling at the end of service life. Furthermore, CLT buildings are relatively light, which can be a considerable advantage in urban sites where piling depths can be constrained, as they are in London by factors such as the Underground network.
In 2017 there were 24 CLT buildings either completed or in construction in the London Borough of Hackney alone, representing approximately 5% of all UK CLT buildings. These include Dalston Works, the world’s largest CLT complex when completed in 2017, incorporating 121 homes and 38,000ft² of commercial space. The complex weighs a fifth as much as a comparable concrete structure, enabling the construction of an additional 35 homes; it required 111 deliveries of CLT, when 700 deliveries of steel would have been needed; and achieved a net embodied carbon benefit of 196 tonnes (though it isn’t clear whether the four specialist carpenters’ flights between Germany and London each week during construction were included!).
Hackney’s unusually high concentration of CLT buildings is attributable to the borough’s progressive stance on planning policy: it became the first English local authority to actively promote timber construction in 2012. While the council stopped short of a ‘timber first’ policy (whereby planning applications would have to demonstrate that a timber solution had been investigated as an option for each scheme proposed) due to the potential for legal challenge, taking embodied emissions into account during the planning process is a huge step forward, that has promoted CLT construction within the borough.
Fighting the wood fight
CLT would appear to have advantages as a sustainable building material, but there are barriers to its wider uptake in the UK. One issue is awareness, perhaps connected to the fact that many CLT buildings are not recognisable as such. For example, whilst the Dalston Works’ structure is pure CLT, the external cladding is brick, making it difficult for the casual observer to see that it’s anything out of the ordinary.
The London Borough of Hackney’s planning policy amendments have been shown to be effective, but it remains an exception amongst UK authorities. True ‘timber first’ policies have been introduced at municipal or regional levels of government in Canada, New Zealand, Germany, Finland, the USA and Australia. In Amsterdam, from 2022 onwards, all new buildings will have to meet circular criteria which will greatly increase the use of recycled and biobased materials throughout the city.
It is notable that countries with a timber first policy have an active local forestry industry, while the UK has a relatively small forestry base and is the world’s third largest net importer of timber behind China and Japan. The power of the UK’s concrete and steel sectors may be difficult to overcome if they can defend their position as ‘domestic producers’ against imported CLT products, mainly from Europe.
Furthermore, wood construction triggers concerns around safety in a country still scarred by the Grenfell Tower tragedy in London, where a fire exacerbated by inappropriate cladding material in June 2017 led to 72 deaths. This, and the subsequent inquiry, has brought renewed scrutiny on cladding materials and their potential combustibility. In response, in 2018 the government banned the use of any material in the external walls of residential buildings over 18 metres high that does not have one of the top combustibility ratings. This effectively bans CLT’s use in this application. The government’s impact assessment acknowledged that this was “likely to slow down the use of engineered timber… in the medium to long term”.
The immediate response from the UK CLT industry was one of understandable concern. Importantly though, the restrictions don’t rule out using CLT entirely – just in the external walls. Hybrid designs are now being looked at where a structural CLT core and floors sit alongside external walls made from another material, typically light-gauge steel.
Advocates of CLT point towards testing that shows the material’s fire performance to be better than one might expect. During a fire a charred layer forms on the surface of the CLT, which insulates the remaining CLT section, thus reducing the entry of oxygen and heat from outside. This enables the section to retain its load-bearing capacity, and significantly delays the further surface spread of flame.
However, CLT remains classed as a combustible material, and some within the construction industry continue to express concerns about its use. Tony Jones, principal structural engineer at the Concrete Centre commented:
“The issue I have is that the ban covers the cladding aspects of a structure. I think it should be extended to the whole internal structure. It’s bizarre you can build a 15 storey CLT building, just as long as its cladding is not CLT”.
It seems reasonable to say that the structural safety, or otherwise, of CLT under fire conditions is a complex issue. Studies looking specifically into this raise questions regarding the applicability of standardised fire resistance furnace tests in qualifying CLT for use in structural applications, such as walls, in tall timber buildings. Instead, more holistic approaches to testing are advocated to establish the fire performance of both the wood and adhesive elements of CLT.
CLT would appear to have great potential for a UK construction industry that seriously needs to reduce its carbon footprint, but there are legitimate safety concerns that need to be addressed. To explore safety concerns, the government needs to implement or encourage a thorough evidence gathering process to inform policy governing the use of CLT. Whilst this will undoubtedly lead to some limitations for the UK CLT industry in the short-term, the potential carbon benefits of this material lead one to hope that the sector can make any necessary adaptations to the material, and to building design and construction, to allow CLT to be used safely and widely.
Featured image: Oregon Department of Forestry (CC BY 2.0), via Flickr