Are cement-bound wood boards really non-combustible?

Compared to pure wood products, cement-bound­ wood boards have improved fire properties.

Cement-bound wood boards are widely used

Cement-bound wood boards have been manu­factured and used for construction purposes­ since early in the 20th century, and widely used throughout Europe since the 1930s. The boards are composite­ materials and come in several forms.

Common to these are wood fibres bound with cement to create good insulation properties, durability, and improved fire-resistant properties of the wood-based material.

Improved fire properties compared to pure wood panels

Fire ratings are often significantly better than pure wood panels, with approvals B-s1, d0, meaning difficult to ignite and a slight contribution to fire growth. Some panels can even be classified as A2-s1, d0, which signifies no contribution to fire growth, non-combustible, with combustible material to a minor extent.

However, the tests used for such a classification are the approved “Reaction to fire test”, in which the material is exposed to a relatively small fire source for 600 seconds, after which the energy source is removed. In a real fire, the energy source cannot be removed, and would instead grow rapidly, and in our experience, even cement-bound wood boards contribute to the fire.

The boards are often only part of an insulating construction, and roof constructions, in particular, can consist of a layer of cellular plastics above or in between wood panels. Any penetration or damage to cement-bound boards will expose the cellular plastics, which can create fierce fire conditions.

Cellular plastics such as EPS and XPS normally melt when presented with temperatures above 150°C, creating pool fires that penetrate the wood panels. We have also seen real-life fires in which the structural wood bars used for mechanical strength inside the boards are damaged by heat exposure from a fire, with the risk of collapsing elements.

Compared to pure wood products, cement-bound wood boards certainly have improved fire properties. How­ever, the boards are nonetheless flammable, and will burn in a real-life fire and flashover. Other factors, such as aging and drying of the components, but also accumulated oil vapours on the panels, are likely to have a negative effect on the combustibility of the panels.

Take actions to prevent fire

In line with all other construction materials with a grading of B or D (combustible), we recommend taking actions to prevent fire from becoming established in the construction element. Strict guidelines and limitations for hot work, awareness of electrical installations, and the location of waste bins should be addressed and implemented when such boards are used.

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Anders Rørvik Ellingbø

Head of Risk Management Services, Norway

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