Cross Laminated Timber, or CLT, is when solid sawn lumber is glued together. Not to be confused with glued laminated timber, or glulam. There are also similarities to plywood, but CLT is thicker.
CLT is a fairly new innovation. Development started in the 1990s in Austria and Germany but wasn’t taken too seriously until the mid 90s. Its development solved the problem of sourcing old growth timber and grew in popularity across Europe in the early 2000s.
"Development of CLT started in 1990s in Austria and Germany but wasn’t taken too seriously until the mid 90s."
Cross Laminated Timber is made by gluing the sawn lumber in different directions providing more structural rigidity.
There are advantages and disadvantages that you should take in to account when considering CLT for your project.
Whilst there is a great deal of flexibility of use for CLT, the thickness of the panels can be increased depending on its application, there are higher production costs involved. That said it’s a super eco friendly material. No need to burn fossil fuels and the wood that the lumber is harvested from can be replaced through effective replanting strategy.
It’s still a relatively new material, so more research needs to be done in to its properties and limitations. For instance it has potential for high thermal insulation but shows a weakness with regard to sound insulation. It requires at least two CLT panels to reach the minimum requirement for sound proofing.
"More research needs to be done in to its properties and limitations."
How do we make this wonderful construction innovation?
Step one involves selecting the lumber. Checking the moisture content and the structural integrity of the pieces.
They are then categorised as either being construction grade or appearance grade. Once they have been graded they are grouped together in appearance or construction. Construction grade lumber is for the inner layers of the CLT panel and the appearance grade goes on the outer layers - makes sense!
"Construction grade lumber is for the inner layers of the CLT panel and the appearance grade goes on the outer layers."
The next stage is planing. The purpose of this is to improve the surface of the timber so that it is more even. The standard planing method is to shave 2.5mm off the top and bottom and 3.8mm off of the sides. In some cases the surface is treated but not always. The the pieces of lumber are cut to the specified needs of the buyer.
Glue is then applied. This is usually done with a machine. However it’s done it needs to be done airtight so prevent gaps and air holes forming in the glue and should be applied at a constant rate - similar to concrete. The pieces of lumber are then aligned and joined. The lumber must be joined in such a way that 80% of the surface area between the layers is bound together.
"Gluing needs to be done airtight so prevent gaps and air holes forming in the glue and should be applied at a constant rate - similar to concrete."
The final stage of the manufacturing process is to press the layers together. This can be done in a vacuum or using hydraulics. The benefit of hydraulic pressing is that more pressure can be added than with vacuum pressing. However, vacuum pressing allows for more than one panel to be pressed at a given time.
The panels are then quality tested and sent off to be sold.
It’s a surprisingly complex and quite delicate process.
"It’s a surprisingly complex and quite delicate process."
We really love this hero material. It’s still a young construction material but it’s versatility and sustainable attributes make it extra special. We’re keeping our eye out for more underrated materials like this in the future.