What is cross-laminated timber (CLT)?

In a series of 4 blogs we will be discussing cross-laminated timber – what is it, where it comes from, who manufactures it, what its benefits are (as a building material and as a MMC) and give you our opinion on the top 5 CLT buildings from around the world.

In CLT Blog Series 1, we are looking at… What is cross-laminated timber?

Cross-laminated timber has been described in a lot of different ways including the ‘new concrete’ by Alex de Rijke, Director of dRMM architects and “jumbo plywood” by Henry Fountain in his New York Times article ‘Wood That Reaches New Heights’.  It is also referred to by number of different names – CLT, XLam, solid timber, engineered timber and massive wood.  But what is cross-laminated timber?

Cross-laminated timber (CLT) is… a multi-layered solid timber panel product manufactured to be used for wall, floor and roof elements of a building’s superstructure.

CLT is made from kiln-dried finger jointed spruce/fir planks which are laid out into sheets. The sheets are then stacked at right angles and glued together in perpendicular layers using high pressure bonding.

The majority of CLT panels currently manufactured use polyurethane adhesive to glue the sheets together.  The polyurethane adhesive is a solvent and formaldehyde-free adhesive (‘o’ emission class) which has no risk of toxic emissions at any stage in the product’s life cycle.

CLT panels can be manufactured in 3, 5, 7 or more board layers with typical widths of 0.5m, 1.2m, and 3m and lengths of up to 18m long.  Although you need to check transport regulations and requirements – in the UK it is recommended that the panels are no longer than 13.5m long for transportation purposes.  CLT panels can be supplied with three different finishes to the top layer of the panel – non-visual, industrial grade visual, and domestic grade visual.

N.B. The standard panel sizes vary dependent on which manufacturer you buy your CLT from.

The number of board layers and panels widths will depend on the purpose and structural requirements of the element (wall, floor or roof) of the building they are designed for.   Each CLT panel manufactured is cut and processed to the exact dimensions of the building using state of the art CNC technologies.   For example the board layers and panel widths for a ‘standard’ school –   with typical floor to ceiling heights in most modern schools, wall panels will, more often than not, be arranged in 2.4m wide (x required height) sections.  The thickness, and lamella build-up, of the panel will be dictated by whether or not the timber is to be left exposed as a surface finish – for example, a wall panel that will ultimately be overclad could range from 90-120mm (3 layer build up); whilst in an exposed state the thickness may have to increase (additional charring zone), and the panel may need to be a 5-layer build-up (better fire performance).

CLT panels are manufactured at full size as “master panels” and then cut to the size and shape of the individual elements required for the building – this is called panel optimisation.

Panel optimisation can be influenced by whether there are exposed surfaces or not – if the CLT panel is going to be left exposed the window/door cut outs will be cut out from the master panel but if the timber will be clad with plasterboard, then the CLT panel can be created using smaller pieces/off-cuts of CLT panels which are mechanically fixed together.

Utilising clever panel optimisation can reduce CLT panel wastage in a building, but it is important to speak to your solid timber architect, structural engineer or formatter to discuss this in more detail.

CLT Glossary 
Kiln dried – timber dried in industrial kilns to requisite moisture content levels (12% +/-2, in the case of CLT)
Finger jointed – process whereby two (shorter) pieces of timber are joined together via a set of glued complementary cuts (or “fingers”) to give a continuous longer piece.
High pressure bonding – the process whereby a CLT panel is glued during production, and is then placed in a high-pressure hydraulic press for a certified period of time (can vary with different manufacturers / press)
“0” emission class – zero formaldehyde emissions (based on a European classification system)
Superstructure – the structural frame of a building above foundation level
Non-visual grade finish – panels which will ultimately be overclad (usually with plasterboard), and not visible within the finished building
Industrial grade finish – panels which will be left exposed, but which are subject to “less scrutiny”; typically used in industrial applications, or sports halls etc
Domestic/Residential grade finish – panels which will be left exposed, and which will, typically, receive a sanded finish to the timber (which is also more carefully visually graded)
Master panels – the full size panels (or “blanks”) from which the subsequent individual elements are cut & processed

If you have any questions relating to CLT products that you would like answered, or terms you would like explained – just ask.  We will do our best to answer them all, and if there is anything we don’t know, we’ll know a man (or woman!) who will!


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