(BS) EN 335:2013 and Moisture Content
Use & Service Classes for Moisture Content
The updated (BS) EN 335:2013 defines five use classes that represent different service situations to which wood and wood-based products can be exposed. It also indicates the biological agents relevant to each situation. Use classes have replaced hazard classes. This is not the same as indicating a product’s expected performance and how long specific wood and wood-based products might last in service.
- Classes 1, 2 and 3 apply to plywood. The same three classes also apply to LVL until new classes are available when the LVL standards are updated.
- Plywood Use Classes can also be found in Annex B of the plywood product standard (BS) EN 636:2012+A1:2015.
- For plywood and LVL, also check out DD CEN/TS 1099.
- Use Classes 1 and 2 apply to APA OSB panels.
For maximum durability, Use Classes must be considered together with the Service Classes in Eurocode 5, which assign strength values and help to calculate deformation in load-bearing situations. They are determined by the product’s moisture content corresponding to the environmental humidity and temperature during service.
For Use Class 1 situations – refer below to Service Class 1 or Service Class 2
- Where the wood or wood-based product is inside a construction, not exposed to the weather and wetting.
- Attack by disfiguring fungi or wood-destroying fungi is insignificant and always accidental.
- Attack by wood-boring insects, including termites, is possible although the frequency and importance of the insect occurrence depends on the geographical region.
For Use Class 2 situations – refer to Service Class 2 or Service Class 3
- The component could be subject to occasional wetting caused by e.g. condensation.
- The wood or wood-based product is under cover and not exposed to the weather, particularly rain and driven rain, but where occasional, but not persistent, wetting can occur. Condensation of water on the surface of wood and wood-based products may occur.
- Attack by disfiguring fungi and wood-destroying fungi is possible.
- Attack by wood-boring insects, including termites, is possible although the frequency and importance of the insect risk depends on the geographical region.
For Use Class 3 situations – refer to Service Class 3
- Minimum requirement for external use.
- Situations in which wood or wood-based product is above ground and exposed to the weather, particularly rain.
- Attack by disfiguring fungi and wood-destroying fungi is possible.
- Attack by wood-boring insects, including termites, is possible although the frequency and importance of the insect risk depends on the geographical region.
- Use Class 3.1: In this situation, products will not remain wet for long periods. Water will not accumulate.
- Use Class 3.2: When products will remain wet for long periods. Water may accumulate.
Service Class 1:
Moisture content in the material corresponding to a temperature of 20°C and the relative humidity of the surrounding air only exceeding 65% for a few weeks per year. The average moisture content in most softwoods will not exceed 12%.
Service Class 2:
Moisture content in the material corresponding to a temperature of 20°C and the relative humidity of the surrounding air only exceeding 85% for a few weeks per year. The average moisture content in most softwoods will not exceed 20%.
Service Class 3:
Characterised by climatic conditions leading to higher moisture content than in Service Class 2. Performance may be compromised if suitable preservative treatment and/or relevant surface and edge coating is not applied and if the panels are not properly maintained and installed.