The use of cross laminated timber (CLT) and other engineered timber products is on the rise around the world but struggling with low awareness levels and concerns about fire risk and toxicity. Now the industry is fighting back.

Last year in the wake of the Grenfell fire disaster the UK government banned combustible materials from the walls of residential high-rise buildings – and this included CLT. The industry was up in arms because the structural properties of CLT allow taller timber structures and carbon sequestration, amongst other benefits.

The British ban covers buildings above 18 metres (around six storeys) that are residential, but only their use in external walls.

CLT’s supporters such as the Timber Trade Federation advised the authorities there was no evidence that banning CLT would improve fire safety, and believe it excessive that this blanket ban covers the whole of the external wall construction, not just the cladding.

The ban was slammed by Anthony Thistleton, founder of architect Waugh Thistleton, which designs virtually all its projects in timber, who warned at the time that it could prompt a collapse of CLT use in the UK, taking the country from “a world leader to a backwater”.

Thistleton now is of the opinion that “it just means we need to do something else for external walls. We might even be able to increase the level of prefabrication”.

Lendlease backs out of CLT – then backs CLT

Australia’s Lendlease was among the companies affected and announced following the ban that it would no longer pursue projects using cross-laminated timber, or CLT.

But since then it has issued a statement in support of CLT, saying that “CLT is an innovative product we use and support around the world. It offers a range of benefits over alternative building materials – not least of which is its sustainable credentials. And we plan to continue using the innovative product while adhering to relevant building regulations.”

Lendlease used CLT extensively in the construction of Australia’s tallest and largest engineered timber office, Brisbane’s 10-storey 25 King, designed by Bates Smart, as well as at International House at Barangaroo in Sydney.

Other companies have voiced similar statements. “We are confident in the fire safety of CLT construction, but the industry really does need to support more research and testing to demonstrate this unequivocally.” say manufacturers Simpson Strong-Tie.

According to Lendlease, building with CLT saves time and labour. The company compared the time it took to build a 10 storey timber apartment building in Melbourne with the time required to build an equivalent concrete structure.

It took 38 days and six technicians to build a wooden building and 20 weeks and 30 technicians to build a concrete one.

The US is playing catch-up

The USA has been lagging behind the engineered timber trend. At the moment the tallest building in the US, the Carbon 12 building in Portland, Oregon, is just eight storeys high, a titch compared to the newly-opened 18-story Mjösa Tower in Norway.

It is now amending its building code to allow its use in buildings up to 18 storeys.

The change to the building code will be added to the 2021 International Building Code (IBC), slated for release in late 2020.

Up to nine storeys high, timber will be permitted to be unprotected. Between nine and 12 storeys the amount of exposed timber will be limited, and up to 18 storeys it must be fully protected with noncombustible materials.

Are there enough trees in the world?

According to Tallulah Chapman of the Forest Stewardship Council, “Timber has a great sustainability story to tell, offering benefits in terms of carbon, energy and performance when substituted for materials such as concrete or steel”.

But if the world switched from steel and concrete timber would there be enough forest to supply the demand?

Chapman acknowledges that “increased demand for timber places pressure on forests, many of which are already threatened by activities leading to deforestation and degradation”.

Timber is a scarce resource but while increased demand is a potential threat on the one hand, on the other it is also an opportunity, incentivising responsible management of forests

In response she believes that we require more responsible forest management. “This is where the FSC certification system enables businesses and consumers to choose timber products made with materials from well-managed forests and/or verified recycled sources.

“But while increased demand is a potential threat on the one hand, on the other it is also an opportunity, incentivising the responsible management of many more hectares of forest around the world,” she concludes.

To support the rising trend for highlight buildings made of engineered timber, TRADA (Timber Research and Development Association) has produced a clients’ guide.

This is intended to overcome the lack of familiarity or working knowledge that many specifiers may have in regard to these novel materials. TRADA hopes this will improve take-up.

Adhesive free timber buildings?

One criticism of engineered timber is that it contains a large amount of toxic adhesive which can have a bad effect on internal air quality and the environment.

  • UPDATE 13 June 2019: Industry feedback suggests  modern practice is now to use low VOC products.

In response there is now a move to go beyond the use of adhesives with a €4.81 million project whose members include the UK, Ireland, France, Germany, Luxembourg and Belgium.

The technologies being investigated include compressed wood fasteners (dowels) and their metallic fastener counterparts, in laminates of three and seven layers. The dowels are inserted in a tight grid. The resulting products are then tested for compressive strength.

The project is supporting 10 businesses in north-western Europe to adopt and produce a range of adhesive-free engineered wood products.

This will help, they believe, to contribute to sustainable management of forests in the region as well as add value to locally available low quality timber following from increased use of timber in buildings in North West Europe.

Presently the majority of engineered wood products go to landfill or incineration and help contribute to the 15 million tonnes of timber construction waste in landfill across Europe each year.

Adhesive-free products will also be 100 per cent reusable and recyclable.

UPDATE: 12 June 2019 – This article has been amended to remove an inaccuracy that the US lacks engineered timber manufacturers. In fact there are several.

David Thorpe’s books include the Passive Solar Architecture Pocket Reference, Energy Management in Buildings and Sustainable Home Refurbishment.

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  1. “One criticism of engineered timber is that it contains a large amount of toxic adhesive which can have a bad effect on internal air quality and the environment.”
    – This is very misleading statement.

    Most modern engineered timber products are being made with low-VOC, formaldehyde free-adhesives and are arguably far less damaging to the environment than concrete and steel.

    XLam is the only CLT supplier currently with Declare status for its products under the Living Building Challenge.

    1. Thanks for the clarification Nick. Good to hear. Maybe you could put us on your mailing list for important updates such as these. We will also update the article.

  2. Hempcrete (hemp fibre mixed with lime and water) is extremely flame-resistant, lightweight, and sequesters carbon. It would be interesting to see more large-scale buildings use hempcrete as a fire-proofing medium to surround timber frames. You would see double the carbon-sequestering benefits, along with improved indoor air quality, and long-lasting, all natural facades and finishes. It’s a niche market at the moment, so slightly more expensive, but as soon as it becomes a bit more mainstream – imagine the possibilities for green buildings!