The market seems to agree. Less than five years after its arrival on US shores, there are now CLT projects underway in almost every mainland US state. More importantly, unlike the UK – which currently imports all of its CLT – the US is investing in domestic CLT manufacturing, with factories in Montana and Oregon, and more planned in Maine, Utah, Illinois, Texas, Washington State, Alabama and Arkansas. Amazon’s new “tech-hub” building in Minneapolis is made from nail laminated timber (like CLT, but using nails rather than glue). The 2018 Timber Innovation Act also included provisions for research and development into mass timber.
Structures using wooden materials also tend to be quicker and easier to build, therefore reducing labour costs, transport fuel and on-site energy use. Alison Wring, director of Aecom, an infrastructure company, cites a CLT residential block of around 200 apartments that “took just 16 weeks [to build]… whereas if it had been done traditionally with a concrete frame it would have taken at least 26 weeks.” Similarly, says Waugh, a recent 16,000-square-metre CLT building he worked on, “would have needed around 1,000 cement truck deliveries for the frame alone. To deliver all the CLT, we needed just 92 deliveries.”
Other countries are turning to timber, too. Monika Lebeničnik, a sales engineer for Ledinek Engineering, an Austrian-Slovenian firm that makes the presses for CLT factories, sent me her order sheet going back to 2013. It begins with a trickle of orders from Austria and Scandinavia. But from 2017 onwards, there is sudden take-up from Japan, France, Australia, Latvia and Canada. “Annual capacity of such lines range from 25,000 to 50,000 cubic meters [of CLT],” explains Lebeničnik. Data suggests that 1,000 cubic metres of CLT equates to around 500 harvested trees; factories processing 50,000 cubic metres are therefore trapping the sequestered carbon of 25,000 trees per year.
There are even advantages that make the material particularly attractive to countries like Japan, since it has been found to perform well in earthquake tests. A joint Italian-Japanese research team built a seven-storey CLT building and tested it on a “shake table” (a cool but eerie video of this exists on Youtube). They found that it could withstand shaking at the level of the 1995 earthquake in Kobe, Japan, which destroyed more than 50,000 buildings. With serendipitous timing, says Waugh, “the Americans planted lots of trees in Japan as part of the Marshall Plan – that was over 60 years ago, and they are reaching maturity now”.
Counterintuitively, CLT also performs well in fires. It is designed to withstand heats of up to 270C before it begins to char – the charring on the outside then acts as a protective layer for the structural density of the wood behind it. By contrast, at similar temperatures concrete can spall and crack, and steel loses its strength.
Not everyone believes that the future is CLT, however. When I ask Chris Cheeseman, professor of materials resources engineering at Imperial College London, whether wood could usurp concrete as our primary building material, his response is blunt. “No. That isn’t going to happen. It might happen locally with some small schemes. But you’ve got to appreciate the massive use of concrete, and the massive importance of concrete to infrastructure and society. It is an exceptionally good material because of its functionality and its robustness.”