One of the major advantages of Cross Laminated Timber is its inherent fire resistance. CLT can be designed to accommodate substantial fire resistance and unlike steel remains structurally stable when subjected to high temperatures. CLT panels can be produced with fire resistances of 30, 60 and 90 minutes.
It’s easy to see why any form of timber construction can pick-up a reputation for fire risk, but for solid wood and Cross Laminated Timber, performance in a fire is a long way from their timber frame cousins.
To understand how efficient CLT is in a fire, we should start with the understanding that fire resistance is the ability of a material to confine a fire or to continue to provide a structural function or both.
The measure of fire resistance is the time elapsed from the start of the fire up until the point where the material fails to function. Typically resistance is expressed in minutes eg FR 30, 45, 60 or 120.
CLT’s fire resistance is provided through ‘charring’. As the face of the timber panel is exposed to a fire that ramps up to a temperature in excess of 400 degrees C, the surface of the timber ignites and burns at a steady rate. As the timber burns it loses its strength and becomes a black layer of ‘char’. The char becomes an insulating layer preventing an excessive rise in temperature within the unburnt core of the panel. It is this unaffected core which continues to function for the period of the fire resistance.
To achieve the designed fire resistance period there must be sufficient virgin solid timber remaining behind the char layer to sustain the loads applied. Therefore each CLT panel within the building must be designed for the fire resistance period and the specific loadings applied to that panel.
Fire resistance testing of CLT panels is to ISO 834 – the same standard used for doorsets which most of us are familiar with.
Illustration showing a typical temperature / time gradient through solid timber under fire conditions
(image courtesy of Structure magazine)