Fire resistance and fire classes of wood

Contrary to common belief, structural timber is not a highly flammable material. Moreover, it has an advantage over many other construction materials: it insulates heat, preventing its spread to other rooms. The precise characteristics are defined by the fire resistance class. The type of impregnation used also plays a major role.

Wood, due to its natural properties, but also the high standards related to strength and safety, is a widely used building material not only in all-year cabins, holiday cabins, but also in tall and high-rise buildings. Their main structure (aside from the foundations) is made of wooden components. An example is Stadthaus Murray Grove, a 9-storey apartment building in London completed in 2009, and the 14-storey apartment building erected in 2014 in Bergen, built with glued timber and cross-laminated timber (CLT) panels. Therefore, the high combustibility of wood used in construction should be treated as a myth. A precise attribute for determining it is fire resistance.

Fire resistance of wood – what is it really?

Fire resistance classes of structures, here wooden ones, indicate the ability to perform defined functions during a fire. The fire load-bearing parameter is denoted by the letter “R” and numbers, e.g. 15 (the number of minutes after which load-bearing elements cease to fulfill their function). Important parameters also include fire tightness “E” (preventing flames and gases) and fire insulation “I” (preventing heat). Based on these parameters the fire resistance class of a building is determined. Fire resistance is most often established by the architect and the firefighter in accordance with legal regulations.

Advantages of wooden construction in a fire

Wood in a fire behaves in a predictable manner, which is certainly an advantage compared to, for example, steel. Glued wood is more fire-resistant because it has a more regular shape, fewer cracks and a smoother surface. In practice, wood burns when it is too thin, in conditions of good air supply. Thick logs do not burn well, but char on the surface. The fire loses momentum when the coating is unfavorable for its spread (it limits oxygen flow into the interior). During a fire, wooden beams maintain their load-bearing capacity longer and give more time for evacuation (up to an hour) compared to concrete structures. Moreover, they do not spread high temperatures to adjoining rooms. The combustibility of wood can be significantly reduced thanks to the use of appropriate finishes. They protect the timber not only from adverse factors (moisture, fungi), but also make the material less flammable. For example, solid timber beams with a width not less than 14 cm have an average fire resistance of R30. Polish manufacturers already offer building materials that are far less flammable, e.g. wooden walls with an resistance EI60, floor and roof structures REI30 and floors REI60.

A brick building – not as safe as it seems

Bricks, commonly used in construction, are non-combustible. It may seem that a brick house is completely safe and fire-resistant. Unfortunately, that is not the case; this is another common misconception. Such buildings have weak points, namely floors supported by embedded steel reinforcement. Heated steel bars may pull out, pose a direct danger, and can also crush whole structures. During a fire, after just 15 minutes, concrete and reinforced concrete walls can reach temperatures of 600ºC, causing melting of installations and short circuits.