In the presence of small sources of fire (smoldering cigarette butts, matches, a short circuit in the electrical, etc.) fire wood and fire spread in protected structures is not happening. The deep impregnation of wood, carried out in the impregnating cylinder under pressure and hot-cold baths, provides good protection from fire. In this case, impregnated with fire retardant wood, regardless of the source of the fire is not ignited, but only a char and not able to spread the fire. The mechanism of protection from fire retardants has not been studied in detail. It is known that at high temperatures, depending on the type of flame retardant may occur following physical changes: a flame retardant melts at a temperature below the point fire wood to form a glaze that covers it and the connection with the oxygen of the air, fire-resistant substance decomposes under the influence of high temperatures, highlighting the large amount of inert gases, these gases reduce the relative content of volatile pyrolysis products from the wood surface, because of this hot mixture is not ignited, in the process of melting and evaporation retardant consume a large amount of heat in result, the temperature of the surface layers of the timber does not reach the limit values, at which its ignition, flame retardant material, located on the wood surface, which hinders access of oxygen to the source .Produkty decomposition of wood, in violation of a protective film, the pressure released to the environment and burn at some distance from her and because of this fire Wood is not happening. Based on these provisions, the best flame retardants are substances that are under the influence of high temperatures decompose with the release of inert gases and form a protective film on the surface timber. For example, ammonium phosphate when heated allocates ammonia, which does not support burning wood, and phosphoric acids, covering the wood (or coal) with a protective film.

Fire resistance of building structures characterized by a time during which a fire they are capable of withstanding the required loads without being destroyed. Unprotected wooden structures have a relatively high fire resistance, 20-70 minutes. But big disadvantage of wooden structures in the fact that they are able to further the spread of fire and can serve themselves a source of fire. As a result, the protective treatment of wood fire-retardant wood designs can be improved by 3-4 times. In this case, flame retardants impregnated wood, exposed to fire, slowly smolders without letting the flame, after removing the source of fire wood is quickly extinguished.