Fire resistance is the ability of a material to confine a fire while providing its structural function. This determined by timing the start of the fire up to the point where the material can no longer function as required.  Timber is known to be vulnerable to fire whether in a wet or dry state. In a Cross Laminated Timber (CLT) structure, fire resistance is provided through ‘charring’.

Charring is a chemical process of incomplete combustion of certain solids when subjected to high heat. In wood, charring rate is affected by density, wood moisture content, heat intensity and the oxygen concentration of the surrounding air. How?

Density is the mass of an object compared to its volume. In simple terms, it is the compactness of a substance. An object of higher density, due to its compactness has a high fire resistance. When the timber panel is exposed to a fire of about 4000C, the surface timber-layer ignites and burns at a steady rate forming 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 the unaffected section which continues to function for the period of the fire resistance.

Therefore, to achieve the designed fire resistance period, there must be sufficient solid timber behind the char layer to sustain the loads applied. This is achieved due to the various panel layers embedded in Cross Laminated Timber. CLT is equivalent to a standard steel-studded wall because the panels contain multiple laminated layers, the remaining wood provides additional strength at the point of rupture.

Moisture content is the ratio of the weight of water to the weight of the solids in a given mass of an object. Timber, always has a natural moisture content which must be drained for effectiveness in furniture use. The higher the moisture content, the higher the fire resistance. However, in timber, a high moisture content is associated with dampness and moulding which weakens its structural properties. Due to this, the moisture content of CLT is kept as low as possible while other fire-resistance solutions are of more consideration.

NoteFire resistance testing of CLT panels is according to the ISO 834 (the standard used for door sets which most of us are familiar with).

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