Is Cross-Laminated Timber the New Concrete?

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CC BY 2.0. CLT in Sudbury, Ontario/ Lloyd Alter

Jorge Calderón thinks that it can take the place of concrete in buildings, inside and out.

For some time we have been asking What's the best way to build with wood? When Cross-Laminated Timber (CLT) first came out, everyone thought it was the greatest thing since sliced wood. Then Nail-Laminated Timber (NLT) came back from the dead and Dowel-Laminated Timber (DLT) came onto the scene, and good old-fashioned stick-framing started looking good again because it is so efficient in its use of wood.

detail CLT

Detail of Murray Grove Tower from V&A; show/ Lloyd Alter/CC BY 2.0In ArchDaily, José Tomás Franco interviews industrial designer Jorge Calderón,who is also manager of CRULAMM, a Chilean CLT manufacturer, and who explains the particular virtues of CLT. He sees it as the concrete of the future. He explains that it's better for the environment:

We usually design and build with concrete, but concrete's environmental footprint is enormous compared to that of wood. One ton of CO2 is emitted into the atmosphere for every cubic meter of concrete created. In contrast, CLT contains "sequestered carbon," or carbon naturally stored in wood during tree growth. Thus, despite all the energy used in the extraction and manufacturing processes, emissions from wood construction will never match the amount of carbon that is kept "sequestered" in the CLT.

Calderon notes that it is much lighter than concrete, yet has the same structural strength as concrete, "but it's a material with a high degree of flexibility that has to undergo great deformations to break and collapse – unlike concrete. Moreover, 1 m3 of concrete weighs approximately 2.7 tons, while 1 m3 of CLT weighs 400 kg and has the same resistance. The same goes for steel."

Detail of CLT on wall

Detail of CLT on wall, Susan Jones' house, Seattle/CC BY 2.0

Because NLT and DLT have the wood all lined up with the grain running in the same direction, they can expand and contract. CLT is different:

Due to the cross orientation of each of its longitudinal and transverse layers, the degrees of contraction and dilation of the timber at the level of the boards are reduced to a negligible amount, while the static load and shape stability are considerably improved.
CLT in Susan Jones' roof, Seattle

CLT in Susan Jones' roof, Seattle/ Lloyd Alter/CC BY 2.0

CLT is easier to transport than concrete, and assembles really quickly because it is cut so precisely. I remember this roof in Susan Jones' house, where the panels were designed in Seattle, digital instructions were sent to the factory in Penticton BC where they were cut, then shipped back to Seattle where they fit together perfectly. "CLT behaves with the precision of a piece of furniture, working with margins of error of 2 millimeters."

Calderón is fine with CLT being used outside, which I have not seen a lot of yet. I am not sure if it is even legal in many places, but he suggests it's not a problem and can protect it for 25 years if reapplied every 5 years.

Vegetable oils are recommended for indoor use, while mineral paints work best outdoors, mainly on walls. These products, which are odorless and high performance, can be applied by anyone, following basic instructions and taking necessary precautions.

I have my doubts about that, as I think anyone who has a wood boat or wood-sided building might. CLT is made from softwoods like pine and spruce, not what you usually put on the outside of buildings. For longevity it is nice to separate the structure from the cladding, so that you can fix or replace it without having to rebuild the entire structure. Wood is still a very high maintenance material, which is why most CLT buildings are covered up with something else; perhaps things are different in Chile.

CLT being pressed in Milan, Italy

CLT being pressed in Milan, Italy/ Lloyd Alter/CC BY 2.0

CLT is still in relatively short supply and is more expensive than other wood technologies, requiring big fancy presses, while anyone can make their own NLT in a shop or on site, like they did through the 19th and early 20th century in almost every wooden industrial building. But Calderón makes a persuasive case that CLT is indeed special stuff: light, fast, and precise. Read it all in ArchDaily.