Construction Specification

Sustainable building elements: cross laminated timber

Stora Enso’s Rory Doak explains the benefits of cross laminated timber and how the industry is responding to the government’s controversial extension to the ban on combustible cladding.

Stora Enso’s Rory Doak explains the benefits of cross laminated timber and how the industry is responding to the government’s controversial extension to the ban on combustible cladding.

With high profile projects such as Dalston Works in London and Earth Tower in Vancouver hitting the headlines, cross laminated timber (CLT) is seen by many as a fast route to decarbonising the construction industry, which is critical to achieving national net zero goals.

Last month, for example, European Commission President Ursula von der Leyen said in her state of the union speech that a major focus will the use of organic building materials such as wood to turn the construction sector from a carbon source into a carbon sink.

Also known as massed timber, CLT are prefabricated structural panels typically made up of three or more layers of timber cross-bonded for strength. Used for walls, floors and roofs, CLT is a key component of the offsite construction drive to improve construction’s longstanding productivity issue. Indeed, in 2017, the UK government announced a presumption in favour of offsite for all capital projects from 2019.

Image courtesy of Stora Enso

However, CLT is now threatened by the government’s proposed extension to the ban on combustible cladding, which would include material used in the structure of external walls of buildings of ‘at least 11m’. This could mean that the maximum height is effectively reduced from six storeys to four and could halt the use of structural timber in its tracks. The Structural Timber Association is therefore urging the government to rethink, as are other groups who say such a ban would prevent the government from meeting its own net zero commitments.

There are opportunities to save money on materials and labour costs, typically resulting in a 25-30% programme saving versus traditional materials

Rory Doak, Stora Enso

To explain the issues, EcobuildingNow caught up with Rory Doak, Stora Enso’s Business Development Manager UK & Ireland. The company with headquarters in Helsinki and Stockholm, last month invested in new facilities in the Czech Republic to meet increased demand for CLT, and counts Lendlease, B&K Structures, Hybrid Structures, and Swan Housing among its clients.

Why is CLT seen as critical to the decarbonisation of construction?

Given that trees remove carbon from the atmosphere as they grow, locking away the carbon for the lifetime of the material, engineered timber such as CLT is potentially carbon negative. With 11% of global emissions associated with construction materials, wood has an important role to play in offering low-carbon construction of buildings which store carbon throughout their lifespan.

However, building with wood is not only about storing carbon in the structure. The speed of construction itself provided by lightweight pre-manufactured CLT reduces carbon emissions since there are fewer site deliveries, while massed timber is highly energy efficient and airtight, and so can help buildings to meet PassivHaus standards. Even when wood products reach the end of their life and can no longer be reused or recycled, they can still be used to generate bioenergy.

Can CLT really help the UK build more quickly and efficiently?

The process of building with CLT is much simpler than with other materials such as concrete, which means it can be cheaper. For example, there are opportunities to save money on materials and labour costs, typically resulting in a 25-30% programme saving versus traditional materials.

Take for example the Green House in Bethnal Green, which used CLT extensively in the refurbishment of a 1960s concrete office block. The superstructure was completed over an eight-week construction period, using a team of five, who installed over 800 CLT panels.

Furthermore, prefabrication is well suited to 3D digital modelling and can be delivered to within a millimetre accuracy, which combined with digital cutting, processing and logistics, provides a high degree of transparency to the building process.

What about cost?

If you compare the build cost of an office, for example, the difference is between 3-6% against steel and concrete. However, the speed of building with CLT should be taken into account as should other factors such as less groundworks and shorter loan periods.

A recent study by the UK Green Building Council has also shown that while the cost reduction of building greener are relatively marginal they are likely to be offset overtime by enhanced value. This is why Stora Enso provides its customers with Lifecycle Analysis, which takes into account everything from the sustainable sourcing of the raw material to extending the lifespan of the used wood.

How are CLT manufacturers responding to the proposed extension to the ban on combustible cladding?

Safety is clearly of utmost importance and every CLT project has a bespoke fire safety strategy relating to a scheme’s specific details. The industry is therefore carrying out a testing programme to prove CLT’s safety performance for buildings over both 18m and 11m. This research will provide a testing framework for all CLT manufacturers that includes enhanced understanding of fire characteristics.

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