Production process and raw material

Production process

The thermal treatment of wood is an ancient and natural method for improving the durability of wood. SLP has developed thermal treatment into a modern, high-quality and strictly-monitored 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 exceptionally natural and environmentally friendly.

At SLP, the production process is strictly monitored and can be controlled to produce a product of the desired type by adjusting the temperature, the amount of steam and the treatment time. Through experience, we have been able to refine the process to make it suitable for different species of wood and materials of different sizes.

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).

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.


We want to keep up with developments in technology and we seek to constantly develop the efficiency of our process as well as its other aspects, such as environmental friendliness. In recent years, we have been working on utilizing the sawdust created in planing the wood as the energy source of the thermal treatment process.

Raw material

The raw material for our 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. We want to use only high-quality, healthily-knotted lumber and seek to build long-term supplier relationships with local sawmills in order to assure the supply of homogenous materials. Both raw materials and the production process are constantly monitored, because we want to meet our customers’ expectations for high-quality products.

Our products have a PEFC-certification which assures that the origin of our products is known and that it has been monitored by an outside, independent auditor. PEFC-certification is reliable proof of how Finnish forests are cared for and utilized. The certification system includes all the essential parts of forest certification: requirements pertaining to forest care and to the chain of forest management and utilization, as well as the requirements for outside auditors. Our production, our working habits and quality assurance practices are all constantly monitored. This requires us to evaluate our activities year-round.