CLT – cross-laminated timber – is made of layers of softwood boards glued crosswise, at right angles to each other. This creates large, stiff panels from which walls, floors and roofs are built. But why would you choose CLT instead of conventional structures? We have collected the most important arguments – and also what to watch out for.

Light – so you can build where other materials are too heavy

The density of softwood is 450–600 kg per cubic metre, that of normal concrete 2000–2800 kg. Timber is therefore roughly four to five times lighter. This means a smaller load on the foundations and opens up new possibilities:

  • roof extensions and loft conversions on existing buildings, where the old walls and foundations can only take a small additional load;
  • extensions and conversions on tight urban sites, where light, easily transported elements make a big difference.

Fast, dry and quiet construction

CLT elements are made in the factory and assembled on site. There is no drying time and no formwork: assembly is short, the construction method is dry, and the building can be occupied quickly. In city centres this also means less noise, dust and traffic disruption for the neighbours.

Precise, because it is made in the factory

The elements are made on computer-controlled machines from the 3D model – including openings and penetrations for building services. What is in the design is what arrives on site, and there is no need to improvise.

It stores carbon

As it grows, a tree binds carbon dioxide, and this carbon is stored in the building for the long term. The amount can be calculated with a standard method (EN 16449). How this is reflected in the new, mandatory carbon footprint calculation for buildings in the EU is the subject of our article on the EPBD.

Predictable in fire

Timber burns – but the charred layer that forms on its surface protects the inner cross-section, so the load-bearing capacity of the structure can be calculated even in a fire. The new Eurocode 5 provides a standard method for CLT too, for up to 120 minutes of fire resistance – read more here.

What to watch out for

CLT is a great material, but good results require careful design:

  • Moisture during construction. Installed elements must be protected from moisture during assembly – we plan this together with production and the assembly sequence.
  • Acoustics. Floors achieve the required sound insulation with a layered build-up – screed and impact sound insulation; this is designed by our partner office.
  • Fire safety. We verify fire resistance by calculation, and prepare the fire safety design with a partner office experienced in CLT.
  • Stairwell and lift shaft. A reinforced concrete core can slow construction down and is built to a different accuracy – it is worth deciding at concept stage what it will be made of.
  • Visible surfaces. CLT is a structural element, not a furniture panel: the joints between panels are visible, and drying cracks may appear. If you want a furniture-quality surface, there is a solution for that too – see our FAQ.
  • The right structure in the right place. Not every part of a building is ideal in CLT. Where it works best, and where another structure is the better choice, is something we compare at concept stage.

How we help

From the first sketch we show you what CLT can do in your building: spans, floor thicknesses, cost. We calculate the structure, prepare the fabrication drawings for every element and are there during assembly too – independently of manufacturers.