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Moisture Content: Use & Service Classes

Use Classes for Moisture Content and Relevant Service Classes

Changes to official standards has seen Hazard Classes replaced by five Use Classes in the updated EN 335:2013. See Factsheet on the different environments for each of the five Use Classes that can cause panel deterioration.

Classes 1, 2 and 3 apply to plywood and LVL will be added as soon as the existing LVL standards are updated. APA OSB panels are suitable for Use Classes 1 and 2.

For maximum biological durability, both Service Classes and Use Classes must be considered when specifying a wood-based panel. Our guide below covers the Use Classes applicable to APA panels.

, as given in Eurocode 5, are used for assigning  strength values and calculating deformation in load bearing situations.  They are determined by the moisture content of the panel corresponding to the environmental humidity and temperature during service. See below.

For Use Class 1 situations – refer to Service Class 1 or Service Class 2

  • Situations in which 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 the wood and wood-based products will not remain wet for long periods. Water will not accumulate.
  • Use Class 3.2: In this situation the wood and wood-based 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.

Use Classes for plywood in Annex B of EN 636:2012+A1:2015.When selecting the Use Class for plywood, 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.

For LVL – refer to the plywood text in EN 335-3.

For plywood and LVL, guidance on factors affecting durability can also be found in DD CEN/TS 1099.