Treated wood only for special uses
Wood's natural susceptibility to weather and decaying caused by organisms should be taken into account when building outside. The resistance of treated wood has been enhanced with chemicals. It can withstand changing weather conditions and does not decay easily. Chemicals dissolve from treated wood into the environment. Therefore, treated wood should only be used for projects that require especially long durability, and the waste resulting from such projects must be appropriately disposed of.
Impregnated timber usually falls into preservation class A or AB as defined by the Nordic Wood Preservation Council.
- Class A timber contains more preservatives than class AB timber and it is intended for structures that have permanent ground contact, such as buried fence posts and patio foundations.
- Class AB timber can be used for other structures that are exposed to wet conditions and must have longer durability for reasons such as safety. These structures include boarding of terraces, external stairs and banisters, for example.
- Class C timber primarily includes timber used for the wood product industry. This timber is used for products that are exposed to the weather but do not need to be as durable (e.g. window frames).
- Untreated wood can be used for other purposes, such as boarding fences and houses. Easily replaceable structures, such as pier boarding, can also be build using untreated wood.
Tips for builders
Thousands of cubic metres of impregnated wood is used in Finland annually. Some of this use is unnecessary and increases the environmental load. For this reason, untreated wood should be used whenever possible. The useful life of untreated wood is often long enough.
Chemically treated wood must not be burned. Instead, it should be brought to a waste treatment site for separate collection. By using untreated wood, you can also reduce the amount of separately collected waste.
If you build using treated wood
- Use wood that is appropriate for the purpose, quality graded and marked. Class A treated timber should only be used for structures that are in permanent contact with ground or water. Class AB treated timber provides adequate protection for above-ground structures.
- In load-bearing structures, impregnated timber should also be strength-graded and stamped, the strength class used is determined by the structural calculations.
- Always find out what substances the timber has been treated with and what conditions exist for the handling, use and waste management of treated wood. Contact the seller of the treated wood for more information.
- Buy dimension timber that is as ready for use as possible. Treated wood should be case hardened. This helps you avoid exposure to harmful substances and reduce the amount of working residual waste.
- Use safety gloves and respirator when working on treated wood.
- Do not use treated wood for structures or products where bare skin often comes into contact with the treated wood. Treated wood must also not be in contact with food products or drinking water. It may not be used in the shelf structures of underground cellars or in well covers.
Throughout the years, Finnish people have skilfully used wood that has not been chemically treated in ways that have allowed it to last decades and even centuries. Decaying of wood has been taken into account when building. For example, heartwood, slowly grown close-grained wood and larch are more durable than ordinary wood when used for external structures. In recent years, heat-treated timber that has better moisture-endurance and certified hardwood have entered the market. Untreated wood that is no longer used can be burned or decomposed. Compound materials, such as composite wood, are also currently available for building patios. Pieces left over from composite timber can be disposed of as mixed waste.