CLT: One Minute Briefing

31 Jan 2019

Published in: Building, January 2019

What is CLT? Why use it? How is CLT different to standard timber frames?

What is CLT?

Cross-laminated timber is a form of engineered wood made from gluing layers of solid-sawn lumber together, with each layer of board oriented perpendicular to its adjacent layers – similar to plywood but with much thicker layers. The particular arrangement gives much higher structural rigidity.

Why use CLT?

Compared to concrete construction, CLT buildings are much lighter, and require far fewer deliveries to site. CLT panels can be pre-fabricated off-site, vastly reducing time spent on site, speeding up construction and minimising site-safety risks. The technology’s proponents say it also eases the work of follow-on trades to, who can fix into panels using hand tools rather than the heavy power tools required for concrete or steel.

How is CLT different to standard timber frame?

The greater structural strength of CLT allows it to be used to construct much taller buildings than standard timber frame allows. Waugh Thistleton’s 10-storey Dalston Lane building, constructed by B&K Structures, is a leading example, while Lendlease’s dRMM-designed Stirling prize-nominated 10-storey Trafalgar Place is another. Hybrid CLT-glulam buildings of up to 18 storeys have been produced elsewhere in the world.

According to manufacturer Stora Enso, CLT is rated anything between Class B and Class D under the Eurocode system, depending upon whether the product is combined with flame retardants. According to the Structural Timber Association, CLT has inherent fire resistance given that it is most commonly used in wall panels with a large section size, which limits the surface area exposed to flame. This is very different from traditional timber-frame construction, which comprises discrete studs.

Is CLT the future?

Proponents claim huge reductions in greenhouse gas emissions compared with concrete construction, while maintaining it is perfectly suited to digital design and modern methods of construction. dRMM’s Jonas Lencer says: “The most exciting thing is around the design for manufacture and digital design agenda. CLT can all be machine cut by computers to CAD.

After hundreds of years of pushing apart design and manufacturing in the construction industry, CLT allows these to come together.” However, critics such as the Concrete Centre say that combustible materials such as CLT should not be used as structural materials in high-rise buildings. The Concrete Centre’s Tony Jones says: “In the current consultation on building regulations, I’ll certainly be making that point.”

By Joey Gardiner

Contributor - Tony Jones, principal structural engineer at The Concrete Centre