What is CLT?

It has been asked a few times recently, “So what is this CLT stuff people keep talking about?”

It’s actually pretty simple, but it has some massive implications. Cross Laminated Timber or CLT is essentially construction grade lumber (2×4, 2×6, 2×8 etc.) that’s glued together to make a panel. The key part is that the middle layer is glued in a 90-degree cross layer. This gives the 3 layer panel strength in two directions, making it very strong. This also allows for the creation of large panels made up of relatively small pieces of wood. What starts out as 2×4’s can be cross-laminated to make panels 10 feet wide and in some cases 70 feet long. The panels can also be made into more layers to create even more strength. For instance, a 5 layer panel that would be approximately 7 1/2 inches thick would be stronger than the 4 1/2 inches of 3 layers.

Depiction of a 5 layer CLT panel
Depiction of a 5 layer CLT panel

Many people are concerned about the gluing process and the strength of the bond. We need to keep in mind that we have been using wood glues to hold glue lam beams, finger-jointed lumber, plywood and furniture for decades. This process is simply doing things we’ve done for years and applying it in a different way.

These panels can now be used to supplement steel and concrete in urban environments, thus significantly reducing our carbon footprint. Wood stores carbon during photosynthesis that can ultimately be stored in the wood when we build. This environmental benefit is compounded when we use lumber from forest restoration projects in the fire-prone forests of the west. Thinning these forests prevents catastrophic wildfire. This keeps the forest green, which produces oxygen. The by-products of these projects (small logs > lumber > CLT) go into our buildings for the most effective materials for the environment.

These panels can be engineered to go into a specific part of the building, including creating the openings for windows, doors and anything else necessary for the building. This allows for very efficient construction that is also much quieter than traditional construction. One example is the Forté Building in Melbourne, Australia. According to Lend Lease, they were able to construct the main part of the 10 story structure in 38 days, as opposed to 16 weeks if it were a traditional steel and concrete building. The implications are huge.

The Forte Building in Melborne by Lend Lease Development
The Forte Building in Melbourne by Lend Lease Development

I believe that we are going to see more and more of this. As Andrew Waugh of Waugh Thistleton Architecture in London was recently quoted, “We’re beginning the Timber Age.” He would know. In the UK, they have already constructed dozens of tall timber structures. Their latest design will push things further, showing people around the world that we are capable of using timber in urban settings in ways we’ve never seen before.

Design for a new mass timber building in London by Waugh Thistleton
Design for a new mass timber building in London by Waugh Thistleton

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