Plywood is growing in popularity as a building material due to its positive climate impact and versatile uses. When using plywood, it is important to consider its specific characteristics, such as its biological durability. This article discusses the influence of wood raw material, glueing, coating and treatment as well as conditions of use on the biological resistance of plywood.
In general, the biological resistance of plywood is similar to that of the wood used in its manufacture. UPM's WISA plywood is made from spruce or birch. Spruce plywood’s biological durability is naturally slightly better compared to birch plywood. Thanks to the fact that spruce is less hygroscopic and therefore absorbs moisture more slowly, the natural durability of spruce can be considered rather better than the standard classification, particularly above ground. Therefore, spruce can withstand various conditions in construction applications.
Choose the right plywood based its use class
Plywood is made by glueing together several layers of thin veneers. Almost all WISA products are bonded with weather-resistant glueing according to EN 314-2/class 3 Exterior. This means that the plywood panel’s glueing can withstand changes in humidity.
Precise, product-specific biological durability properties for each WISA plywood product can be found in the CE mark Declaration of Performance (DoP) or UKCA mark Declaration of Conformity (DoC). The documents are easily available on UPM Plywood's website either through the DoP search tool or the DoP reference list (or DoC list for the UK).
In the DoP or DoC document, you can check the product’s use class to find out if it is safe for outdoor use. The use classes are based on differences in environmental exposure that can make the panel susceptible to biological deterioration. UPM’s WISA plywood products are used in classes 1, 2 and 3.1.
- Use Class 1: For use inside a construction, not exposed to the weather and wetting.
- Use Class 2: For situations where the plywood is covered and not exposed to the weather, but where occasional wetting can occur.
- Use Class 3: For outdoor use where the plywood is above ground and exposed to the weather.
- Sub-class 3.1: For situations where the plywood will not remain wet for long.
- Sub-class 3.2: For uses where the plywood will remain wet for long periods.
- Use Class 4: For uses in which the plywood is in direct contact with ground and/or fresh water.
- Use Class 5: For uses in which the plywood is in direct contact with salt water.
Proper conditions prevent rotting
Despite the weather-resistant glueing, the resistance to moisture of uncoated plywood with unsealed edges is limited in outdoor conditions. Decay in wood is caused by fungal attack. If the relative humidity of the air exceeds 85%, the moisture level of the plywood will rise to more than 20%. Under these conditions, if the temperature is +3 ... +40°C and oxygen is available, rot can occur in the plywood.
The risk of fungal attack can be avoided by using correct storage and use conditions as well as correct construction methods that eliminate some of the above factors.
Wood protective agent prevents discolouration and decay
Mould or blue stain fungi can also appear on plywood. These mainly cause discolouration of plywood. Mould grows only on the surface of wood. Blue stain lives on the soluble substances in the wood cells, so it does not significantly weaken the strength of plywood.
Discolouration can be avoided by applying a wood protecting agent, usually during the manufacture of plywood. The specially constructed WISA-Roof designed for roof structures and WISA-SpruceBT are treated with wood protective agents on both sides of the panel. The treatment gives good protection against wood colouring and decaying fungi during construction and works as a primer. The panel should be painted with suitable paint on those parts which are left visible.
The wood preservative offers additional protection, but it is good to remember that plywood can always be best protected by taking care of proper storage and use conditions.