What is CLT construction?

Cross laminated timber is a strong yet lightweight structural method that can e used r lls r s d rs

Right: CLT’s inherent strength means it can be used on timber buildings to achieve heights previously thought impossible

Below: CLT panels being craned into place, with indo s tted in the factory. This engineered material is mostly imported from Europe from brands like Stora Enso

Cross laminated timber, known as CLT, is formed from cross-layered engineered wood. It’s particularly strong and its versatility means it can be used in many ways in construction, from walls to stairs. This method was pioneered in Europe during the 1970s and is still mostly imported to the UK, mainly from Austria, Germany and Scandinavia.

How does it work?

The strength and stability of crosslaminated timber comes down to how it’s formed, which is in a similar way to plywood. Multiple layers of cross-bonded timber, typically 20-40mm thick, are glued together under pressure for stability. The number of layers is always odd, with a minimum of three, and each one may be of varying thicknesses, arranged symmetrically around the middle layer. The panel’s strength is achieved by having the grain of each layer at right angles to the next. The overall thickness, as well as the load-bearing performance of the panel, is determined by the build-up of the individual laminates. The result is big timber sheets that can be used to create large-format load-bearing panels for walls, oors and roofing elements spanning up to 24m in length. This can allow for coveted design features such as vast open-plan layouts, cantilevers, vaulted ceilings and liveable loft spaces without needing to rely on reinforcing steels. It also enables ultraquick build speeds on site.

CLT structures are typically built using a platform frame, where walls are temporarily braced with raking props before oor panels are lowered onto them and fi ed. The completed oor structure provides the platform for the erection of the wall panels to the following storey.

As with other engineered timber systems, however, you’ll need to nail down your detailed design before the house is fabricated as it will be di cult and expensive to make changes later on. The panels’ size means they’ll normally need to be craned in, which could be di cult to arrange on tight sites.

What are the benefits?

In comparison to conventional softwood wall framing and joisted oors, CLT has a strong load-bearing capacity when used as a wall or slab. It’s got high in-plane strength (resistance to wind and seismic forces). As a solid wall panel (rather than a framed construction) cross-laminated timber panels distribute weight evenly, reducing the requirement for localised pad foundations.

This material is also inherently fire-resistant the dense structure will char like oak if subjected to a blaze, thus protecting the internal wood core) and provides impressive acoustic performance compared to many timber techniques. As it’s manufactured from timber, it can represent a lower impact, more sustainable option than concrete or steel although the glues used can offset some of the environmental gains.

CLT panels can come pre-insulated, with U-values that can comfortably rival those achieved by other modern forms of construction. What’s more, cross-laminated timber is highly airtight out of the box (especially so if supplied in thicker panels). A breathable roof underlay and rigid insulation are usually installed above the CLT panels for a warm roof construction, which can be prefabricated as an insulated cassette. This makes them well suited to room-in-the-roof designs where there is a simple arrangement of continuous ridges and gable ends.

One of the most impressive aspects of CLT panel construction is its potential to overcome height limitations of traditional timber structures. In fact, the tallest timber buildings in the world use this method of construction, including the Mjostarnet tower in Norway, which is 85m high.

CLT currently represents a higher-cost option for full one-off homes. So it’s more commonly used in hybrid constructions – where its impressive span capabilities can complement another building system to achieve the desired outcome.

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