CLT – cross-laminated timber, also known as X-Lam, in German Brettsperrholz – is a large, solid timber panel made of layers of boards glued crosswise on top of each other. The walls, floors and roofs of a CLT building are made from it. On this page we summarise how it is built up, what makes it special and where it works best.
How is it built up?
- Crossed layers. The board layers run at right angles to each other. An element usually has 3–11 layers and is generally 60–400 mm thick.
- Always an odd number of layers. Thanks to the symmetrical build-up, the element does not warp.
- Made of softwood. Usually spruce, fir or pine; for a decorative or harder top layer, oak or birch can also be used.
- Also airtight. If the boards of the top layer are also glued along their edges, the element becomes airtight and smoke-tight – which also improves fire resistance.
What makes it special?
Dimensionally stable. Timber swells and shrinks as its moisture changes. The crossed layers largely prevent this, so a CLT element keeps its size.
Uniform. The glued, crossed build-up evens out the natural irregularities of timber – the behaviour of the element can be calculated reliably.
Load-bearing in two directions. A CLT floor has a main and a secondary load-bearing direction. It can therefore cantilever in two directions and can be supported at points, on columns – which is not possible with beam floors.
Bracing. In its own plane, a CLT wall resists horizontal forces – wind and earthquakes – well, and well-connected floors act as stiff diaphragms. This makes CLT suitable for bracing multi-storey buildings.
How does it get into the building?
The elements receive their final dimensions in the factory, based on the 3D model – including openings and penetrations. A window or door can be cut out without a lintel if it is far enough from the edge of the element. Walls usually arrive on site as storey-high elements, where they are quickly assembled with simple fasteners, for example inclined screws.
In principle, the size of the elements is almost unlimited; in practice, production and transport set the limits: floor elements, for example, can be up to 4 metres wide and 22 metres long. The exact formats differ between manufacturers – this is worth considering at the start of the design, because a good panel layout means less waste and fewer joints.
Where does it work best?
- Walls, floors, roofs – in houses, schools, kindergartens, offices and hotels.
- Medium spans. The thickness of a timber floor is usually determined not by load-bearing capacity but by vibration and deflection. A CLT floor is ideal for medium spans; for larger spans, a timber-concrete composite floor or a box floor may be the better choice.
- Extensions and roof extensions. With light, prefabricated elements you can build quickly and with little disruption – we write about this in our article Why build with CLT?.
Where CLT works best in a building, and where it is worth combining with another structure – for example a glulam frame – is something we compare at concept stage. Here is an overview of engineered timber products.
Carbon dioxide
As it grows, a tree binds carbon, which is stored in the installed timber; the amount can be calculated according to EN 16449. Because of their greater mass, solid CLT floors store more carbon than lightweight timber floors. How this is reflected in the new, mandatory carbon footprint calculation in the EU is the subject of our EPBD article.
Standards
- EN 16351:2021 – the European product standard for CLT: requirements, testing and certification.
- EN 1995-1-1:2025 – the new Eurocode 5, which includes the design of CLT for the first time; we write about the transition in this article.
CLT is not a new invention: the first CLT products were approved in Germany and Austria in 1998, which marked a turning point in modern timber construction.
How we help
We design CLT structures independently of manufacturers: from the first sketch we show you the spans and floor thicknesses, calculate the structure and prepare the fabrication drawings for every element.