Improving the fire resistance of heat treated wood

Heat-treated timber is widely used in luxury structures, such as shopping malls, villas, Thermally treated wood lumbers are widely used in luxury constructions such as shopping malls, villas, and hotel buildings. For this purpose, thermally modified woods such as ayous, pine, ash, spruce, and iroko are commonly preferred. These materials offer significant advantages, including dimensional stability, resistance to fungi and insects, and low equilibrium moisture content. However, wood is still a combustible material. In Turkey, the importance of using flame retardants for such materials in the wood industry has increased significantly, particularly after the fires that occurred in a residential complex in İzmir in 2023 and in a hotel in Kartalkaya in 2025. Thermally treated wood products are widely used in the Turkish construction industry, especially in exterior applications where they are combined with surface coating materials. Nevertheless, these surface coatings, which improve durability under outdoor conditions, do not provide sufficient protection against fire. Today, one of the most critical issues in the Turkish wood construction industry is enhancing the fire resistance of thermally treated wood. This study investigates a potential solution to address this issue. This study aims to treat heat-treated pine, which is widely used in outdoor conditions, with a phosphate-based fire retardant and then to make it resistant to the degrading factors of the outdoor environment and to burning by using surface coatings. For this purpose, pine wood samples subjected to heat treatment in an industrial facility were impregnated at concentrations of 5%, 10%, 17%, and 20%, and then coated with a surface coating developed for outdoor use. Half of the treated samples were subjected to a leaching test based on the AWPA E11 standard. The resistance to burning of the samples was examined by determining the level of flammability when directly exposed to flame according to the TS EN ISO 11925-2 standard. The test samples were compared with samples treated with a commercial fire retardant applied industrially. As a result, all impregnated samples are protected against burning compared to control samples. The surface coating alone does not have much effect. The 17% stock solution was found to be the best fire retardant among the other concentration levels. Fire retardancy improved with the concentration increases. There were found to be no differences between the impregnated samples and references. It is recommended to use both impregnation and coating together for heat-treated wood in some areas with water contact or high humidity.