A Little about CLT (Cross-Laminated Timber)

The last time we put up a blog post about a building material, we talked about one of the most common and foundational building materials: concrete. And now, we present to you some information on yet another central material: timber. But not just any old timber. No, rather than talking about timber broadly, or discussing wood planks from a particular tree, our interests for this post lie in a new (or, rather, new to North America) kind of timber material: CLT. 

CLT (Cross-Laminated Timber) is a pre-fabricated timber made by forming 3 to 7 layers of ⅝ to 2 inch thick timber panels, joined together by finger joints and/or (most commonly) some form of adhesive. As the name implies, the layers alternate direction, forming 90° angles. This method of alternating grains improves the stability of the material. CLT is load bearing and known for its resistance to high racking and compressive forces. When used in outer layers, it’s placed vertically so that its fibers run parallel to the gravity loads; this is the most efficient way to utilize CLT’s load bearing and structural capacity. When used in floors and roofs, the outer layer lumber is oriented so the fibers run parallel to the direction of the span. 

While there are some disadvantages to using the product (for instance, since it is not widely produced in North America, transportation costs can drive up the price), its advantages abound. It allows for greater design flexibility. Despite its light weight, it has a strength and stiffness that gives it reinforcement capabilities that are comparable to concrete. The air tightness allows for some amount of thermal insulation and the layers of wood serve as a thermal mass, which reduces energy use. Surprisingly, even though it’s made from wood, it has some fire resistance. In a fire, CLT can maintain its load bearing capacity for up to 90 minutes. Compare that to unprotected steel, which loses its capacity after only 15 minutes. But its most notable advantage is its sustainable nature.

While there have been materials and techniques similar to CLT, actual CLT was developed in 1994 in Austria by Gerhard Schickhofer as a part of his PhD research. Excluding marketing and shipping, the contemporary CLT process comes in 7 steps.

The first is lumber selection. Lumber is selected based on a number of factors: moisture content, visual grading, and (when used for load bearing) structural testing. The moisture content should be around 12% during manufacturing in order to avoid shrinkage, which could lead to internal stress that affects stability of the material. Further, adjacent pieces of lumber need to have similar moisture contents (a difference not greater than 5%). Works do a visual grading check to root out any deformed pieces of wood. After the lumber is selected, the pieces of lumber are then grouped so that the CLT boards are made up of consistent pieces of lumber. The pieces of lumber are then cut to the proper and desired specifications before the adhesive is applied. But, before the CLT boards are actually assembled, they undergo a panel lay-up so that the workers can find the most efficient way of constructing the panels; in order for the CLT board to be considered effective and structurally sound, at least 80% of the surface area between the lumber layers must bound together, and this step guarantees that. Finally, the boards are assembled, pressed together either by vacuum press or hydraulic press. The seventh step simply consists of the quality control measures, such as sanding.

CLT, by itself, is already sustainable and renewable. However, recent advancements from the Netherlands push this sustainability even further. C-CLT, that is Circular CLT, is made from non-virgin lumber: material gathered from demotion wood and discarded pallets. C-CLT reduces C02 emissions and is ever up-cyclable. But beyond just being more sustainable, C-CLT is also stronger (10% to 15% stronger) than traditional CLT. The (recently constructed) Urban Woods, constructed from C-CLT, is the Netherland’s tallest residential building without a concrete core. While even traditional CLT is rare here in North America, and C-CLT has only made its debut out of its European origin in 2024, it’s still (quite apparently) a development worth keeping your eyes on.