This paper aims to discover how organic wood preservatives containing different fungicides and insecticides used to protect wood against fungal and insect pests affect the fire performance and combustion properties of wood materials commonly used in the domestic wood and construction industry. The aim of this article was to investigate the charring depth. From the point of view of the loss of load-bearing capacity of the wood, only the charring depth matters, since the reduction of the load-bearing capacity will be determined by the reduction of the cross section of the wood. We examined the effects of wood preservatives applied to undamaged samples of Scots pine and Norway spruce conditioned under normal climate conditions. The test samples underwent a second treatment application after which we examined the effect of different preservative combinations. Seven treatment mixtures were selected from the organic solvent and water-based wood preservatives available on the domestic market. The soaking experiments demonstrated that the preservative impregnation of Norway spruce differs significantly from that of Scots pine, which affected the comparability of the test results. We have experimentally investigated the moisture content, the loss of mass and we have calculated the char depth. Adhering to the MSZ 9607–1:1983 standard, the Linder method was performed to analyze the wood and wooden-based construction products used in construction. The effect of the different treatment materials was more significantly detectable when analyzing the size of the surface burn patterns. The Scots pine specimens suffered much greater surface burn than the Norway spruce specimens did, regardless of the type of preservative used. The higher body density, resin content (1–10%) and higher initial moisture content of Scots pine caused it to bend and, therefore, to burn better.