Wood is a versatile material for sustainable, climate-positive construction. It is, therefore, no wonder that the popularity of wood in construction is growing. However, it is good to know the properties of wooden building materials, such as fire resistance. Fire-resistant building materials and structural fire protection can play a vital role in the event of a fire.
The good fire resistance of plywood is demonstrated by the fact that plywood can be used in fire-resistant structures, as it embers at a steady rate during combustion (about 0.6 mm per minute). Plywood will only spontaneously combust at temperatures above 400°C and, when exposed to flames, at about 270°C.
Plywood also has a very wide operating temperature range, as it can be used at temperatures of 100 °C continuously and 120 °C for short periods. Plywood is more resistant to cold than to heat and can be used at temperatures as low as -200 °C.
Changes in temperature do not cause as significant dimensional changes in plywood as in metal and plastic. In fact, the dimensional changes are usually so small that they can be ignored altogether.
Plywood has good fire resistance as a building material
The fire class of building products indicates how the building material will participate in a fire or behave in a fire. There are seven fire classes: A1, A2, B, C, D, E and F.
Class A1 and A2 are non-compustible materials. A1 materials (e.g. glass, stone, concrete) have no contribution to fire and A2 material (e.g. gypsum board) has an extremely limited contribution to fire development.
Classes from B to F are for compustible materials. B, C and D have very limited to medium contribution to fire while E and F classes contribution to fire is from high to easily flammable.
In addition to these main classes, there are additional classes to indicate the smoke formation characteristics of the product (s1, s2 and s3) and the formation of flaming droplets and particles (d0, d1 and d2). The additional prefixes s1 and s2 indicate very low or low smoke production, whereas d0 and d1 indicate that there are no flaming droplets, or, that the flaming droplets will extinguish quickly.
Untreated wood material has a fire classification of D-s2. Untreated plywood such as WISA-Spruce and WISA-Birch plywood has a fire rating of either D-s2, d0, D-s2, d1 or D-s2, d2, depending on the type of installation.
Although plywood cannot be made completely flameproof, the fire protection treatment of plywood can go a long way: UPM's WISA-SpruceFR plywood meets the requirements of the highest possible fire rating for wood materials, B-s1, d0. Fire class B-s1, d0 means the product as a very limited contribution to fire, produces a very limited amount of smoke, and does not produce flaming droplets.
The fire resistance of plywood can be further improved
The fire resistance of plywood can be improved by using fire retardant chemicals or by coating plywood with fire resistant layers. However, pressure impregnation reduces the strength properties of plywood and is likely to cause distortion such as twisting.
In the case of fire-retardant WISA-SpruceFR plywood, the fire protection treatment is applied at the manufacturing stage by impregnating the surface veneers. This improves the fire resistance of the plywood but does not change its technical properties. Fire retardant treatment at the manufacturing stage saves time, material, and costs in several phases of the construction project – the product has the required fire classification built into it and at the same time it works as a structural material.
The fire retardant used in the treatment of WISA-SpruceFR is non-toxic and does not contain halogenated compounds, heavy metals, or boron. WISA-SpruceFR plywood retains its high fire retardancy even when sawn or drilled with a small number of holes.