What cross-laminated timber is
Cross-laminated timber — CLT, in German BSP — is a planar wood product made from softwood board layers bonded crosswise, usually spruce. The number of layers is always odd, with a minimum of three.
The crosswise build-up is the whole trick. Because the fibres of each layer run at right angles to the next, swelling and shrinkage largely cancel each other out, and the panel spans in two directions instead of one. A stick becomes a plate.
CLT is supplied as large-format panels, in industrial or visual grade. Elements are cut to size in the factory — openings for windows, doors and services are already routed out when they arrive on site.
Solid timber elements
Solid timber elements are planar wall and floor elements. They are frequently glulam elements: the laminations lie parallel to the grain and are bonded — the same principle as a glulam beam, only built into a surface rather than a beam.
The difference to CLT lies exactly there. In CLT the layers cross, the element spans in both directions and provides diaphragm action at the same time. With a parallel build-up everything runs one way: the element carries in the span direction and brings a lot of capacity there, but almost none across it. What is gained in one direction is missing in the other.
For the user that means: solid timber elements are the right answer where a floor or wall spans in a clear direction and stability is resolved elsewhere. Where a plate must span two ways or brace the building, the route leads to CLT.
Advantages
- Spans in two directions. Walls, floors and roofs become load-bearing and bracing diaphragms at once.
- Very dimensionally stable. The cross effect substantially limits swelling and shrinkage.
- High degree of prefabrication. What arrives is assembled, not built. Shell times of a few days are realistic.
- Compact build-ups without cavities. No stud bays, no voids — simpler building physics and more robust against workmanship errors.
- Light relative to capacity. That is why CLT is often the only option in existing buildings and vertical extensions.
- Usable exposed. In visual grade the soffit is the finished surface.
- Simple connection details when walls and floors come from the same system.
- Solid timber elements use material more deliberately where the span runs one way only — CLT cross layers do not contribute there and still cost money.
Drawbacks
- High price. Per square metre of component the most expensive of the common timber products.
- Design must be complete before production. Every socket sits in the shop drawing. Redesigning during the shell phase costs twice.
- Changes to finished elements are difficult. A subsequent opening in a load-bearing diaphragm is a structural intervention.
- Crane and access are mandatory. Element weights and lengths impose conditions on the site.
- Element sizes limited by transport, not by production.
- Long lead time. Weeks pass between release and delivery.
- Adhesive in the component, with correspondingly limited separability at end of life.
- Sensitive during construction. Unprotected elements tolerate prolonged wetting poorly; weather protection belongs in the concept.
- Recurring inspection duty at large spans. From 12 m span or 6 m cantilever, consequence class CC2 applies: annual walk-through, inspection every 4 to 5 years, detailed examination every 12 to 15 years, covering deformation, cracking, moisture ingress and the condition of the glue lines.
- Solid timber elements carry in the span direction only. Bracing must then come from other components, and the installation direction is not negotiable.
Applications
- Load-bearing and bracing external and internal walls
- Floor elements with large spans and no downstand beams
- Roof diaphragms
- Multi-storey residential and office buildings in timber
- Vertical extensions and densification of existing stock, where weight is decisive
- Stair flights, lift shafts, balcony slabs
- Exposed soffits and walls without additional lining
- Solid timber elements as one-way spanning floor and wall panels, for instance over corridors or in combination with a bracing core
Where to avoid CLT
- Small, simple projects. A single family house with straightforward statics is usually considerably cheaper in timber frame.
- Projects without fully developed design. CLT does not forgive improvisation on site.
- Linear structures. For beams and columns, glulam is the answer, not the panel.
- Permanently damp uses. CLT is designed for dry to occasionally damp conditions, not constant wetting.
- Sites without crane standing area or with tight access.
- Components that will be altered often. Flexibility is not the strength of the system.
- Solid timber elements where two-way spanning or bracing is required. That is a case for CLT.