CLT is a term that is used increasingly frequently in architecture, and particularly where sustainability and timber are being discussed.
It stands for cross-laminated timber, which is a relatively new way of making large panels from small planks of wood that can be used for the walls and floors of entire buildings. CLT is a ‘sandwich’ construction, in which the planks are placed at right angles in each successive layer, so that if you could look through the CLT from above you would see a kind of grid of wood fibres.
This is important because timber is strong along the directions of the fibres, but weak in the cross direction. Laying up the planks in this way creates a large panels that have equal strength in both directions. This means that timber can now be used as a replacement for reinforced concrete walls and slabs.
Not only are the CLT panels much lighter than precast concrete slabs, but they can also be factory machined to incredibly tight tolerances. Being lightweight makes them easy to transport and being machined to tight tolerances makes them ideal for rapid prefabrication – a CLT building can be assembled as fast as an Ikea shelf unit. This enables much faster, safer construction and it’s quieter too because the panels can be joined with long screws installed with a simple rechargeable screwdriver. Most people looking at CLT construction for the first time, particularly in housing, are amazed by how fast it is. Typically a truck will arrive and a small number of skilled workers will erect them and join them together, creating a weatherproof enclosure, in just a few days. There are no wet trades and no mess or waste.
The manufacturers of CLT panels, most of whom are based in northern Europe, use softwood – most commonly spruce. Until recently, there was no alternative to softwood CLT, but 3 years ago AHEC (the American Hardwood Export Council) started a process of experimenting with using hardwood, and specifically tulipwood, which culminated first in the design and construction of Endless Stair, a structure created for the London Design Festival in 2014. Designed by architect de Rijke Marsh Morgan with Arup as the engineer. Arup carried out considerable research and testing into its behaviour, including its strength in rolling shear. This showed that in addition to its superior appearance, the tulipwood CLT is significantly stronger enabling thinner panels.
The crucial difference between the way that tulipwood CLT was used in the Endless Stair and in The Smile related to the way it was made. Because the material was effectively being prototyped on the Endless Stair, it was made by a far more small scale furniture-making approach. In contrast, the CLT for The Smile has been made in a large scale CLT production plant, alongside softwood CLT. When the material comes to be used on other commercial projects, this is the way that it will be made, so this is an important step forward.