CLT — how cross-laminated timber works
A guide for developers, architects and contractors considering CLT as a load-bearing frame.
What is CLT?
CLT (Cross-Laminated Timber, in Swedish "KL-trä") is a mass timber material built from several layers of boards glued crosswise. The result is a stiff, dimensionally stable panel that can carry load as wall, floor and roof.
The material was invented in Austria in the 1990s and is today the dominant mass timber material for multi-storey buildings in Europe.
How it's manufactured
Boards are strength-graded, dried to low moisture content and glued together with a structural adhesive. Layers run at 90° to each other, typically in three, five or seven layers. The cross-layup gives the panel stiffness in two directions — unlike glulam, which mainly carries load along the fibre direction.
Properties
- Low weight — about one-fifth of comparable concrete construction.
- High stiffness — cross-layup gives dimensional stability in two directions.
- Fast erection — elements arrive ready, lifted into place and fixed with screws.
- Renewable raw material — wood stores carbon for the building's lifetime.
- Quiet installation — no piling, no vibration.
Fire
CLT chars at a predictable rate — about 0.7 mm per minute for spruce-based panels. Through over-sizing, fire-protective cladding (gypsum) or a combination, the required fire classes are achieved. For multi-family up to eight storeys there are well-established solutions today.
Acoustics
Mass timber floors are relatively light, which makes impact sound a challenge. The solution is an acoustic floating floor with heavy layers on elastic supports — and in multi-family we often recommend Ytong rather than CLT for apartment-separating walls to reach sound class B with margin.
Sustainability
CLT has low CO₂ emissions compared to concrete and steel and stores carbon throughout the building's lifetime. For projects with climate requirements this typically gives a substantial reduction of construction-phase climate impact.
When does CLT suit?
CLT suits multi-family buildings of two to eight storeys, offices, schools and civic buildings especially well — projects where fast erection, low weight and climate matter more than the lowest possible material cost.