Cross-laminated timber (CLT): what it is and how it is made

What CLT is

Cross-laminated timber (CLT) is an engineered mass timber product. Several layers of kiln-dried lumber are stacked, bonded with structural adhesive and pressed into a solid, straight, rectangular panel.

How the panel is built

• Layers are stacked at 90 degrees to each other. This gives the panel stiffness in both directions, so floors and roofs can span in two directions.
• The number of layers is typically odd. Some manufacturers make panels with five or more layers that include two adjacent parallel layers.
• The boards can be visually graded or machine stress-rated. Lower-grade boards are often used in the inner layers and higher-grade boards on the outside, which also gives a use to wood with limited strength.
• Openings for doors, windows and services are cut by CNC machines before the panels leave the factory.

Size

Typical panels are about 2.4 m × 12.2 m. Depending on transport limits, panels up to roughly 3.7 m × 18.3 m are possible.

Where it is used

CLT is used for floor and roof assemblies, shear walls, load-bearing walls, and beams or headers. It can be left visible or covered. Because panels are prefabricated, on-site construction is usually shorter than with many wood-frame, steel or concrete systems.

What this means for a sawmill

CLT plants buy kiln-dried, graded boards with tight thickness tolerances and a controlled moisture content, because moisture changes after gluing cause stress in the panel. A sawmill that can dry reliably and grade by strength has a clear product to offer to CLT producers.

Standards

Design and product rules depend on the country. Examples named by Natural Resources Canada are CSA O86 (engineering design in wood) and ANSI/APA PRG 320 (performance-rated CLT). In Europe, CLT products are covered by European technical approvals and standards; check the current rule that applies to your market.