The thermal treatment of wood is an ancient and natural method for improving the durability of wood. SLP Thermowood is made with a modern high-quality thermal treatment process. Thermal treatment processes the natural features of the wood using a high temperature (185 – 215°C) and water steam. No chemicals are used in the production process. Thermo-treated products are thus natural and environmentally friendly.

The production process can be divided into three stages. In the first stage, the temperature of the wood is raised rapidly to 100°C and then more slowly to 130°C. The moisture content of the wood is reduced to nearly zero. At the same time, the steam prevents the wood from splitting. The actual thermal treatment occurs during the second stage of the process. Depending on the degree of treatment, the temperature rises to more than 185°C and is kept there for 2-4 hours. Water steam protects the wood from igniting and influences chemical changes. In the last stage, the temperature is reduced using water spray. The moisture content of the wood stabilizes at over 4 percent.

The raw material for the products is primarily Finnish, PEFC-certified pine and spruce (Pinus Sylvestris and Picea Abies). Slowly grown, dense northern wood is an excellent material for thermal treatment. The wood has a PEFC-certification, which assures that the origin of the products is known and that it has been monitored by an outside, independent auditor.

There are two product classes of thermo-treated wood, Thermo-S (stability) and Thermo-D (durability). The essential features of Thermo-S products are appearance and stability. Classified according to the standard EN 350-1 (scale: 1 very durable – 5 not durable), Thermo-S is significantly durable and belongs to decay resistance class 3. In Thermo-D products, the essential feature is biological durability. According to the standard EN 350-1, it is classified as durable and belongs to decay resistance class 1-2. A decay resistance class 1 product can withstand contact with the ground for approximately 25 years (depending somewhat on the moisture content of the soil).