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Plywood Specifications & Use Classes

Certain types of wood-based panels can be used in demanding exterior situations. However to provide the desired performance and service life they must be correctly specified, detailed, installed and maintained

Exterior use covers a wide range of situations from full exposure, where the panels may be in contact with salt or fresh water to more protected situations where there is a risk of occasional wetting, such as soffits under porches and lorry decks.

Just because a product has an exterior glueline this does not mean it is suitable for an exterior use!

EN 636: 2012 (Plywood Specifications)

Specification covers the performance of a panel and takes into account the glue used to bond the panel together and its overall performance under varying conditions.

EN 636-3: Exterior Conditions

For use in unprotected external applications, as defined in EN 335-3 Use Class 3, where the moisture content will frequently be above 20%

EN 636-2: Humid Conditions:

For use in protected exterior applications as defined in EN 335-3 Use Class 2, with a moisture content corresponding to environmental conditions of 20°C and 85% relative humidity

EN 636-1: Dry Conditions

For interior applications with no risk of wetting, defined in EN 335-3 Use Class 1, with a moisture content corresponding to environmental conditions of 20°C and 65% relative humidity

  1. RH = Relative humidity: level not to be exceeded for more than a few weeks each year
  2. EMC = Maximum equilibrium moisture content
    Note: In similar conditions the EMC of panel products typically will be lower

EN 335: 2013 (Use Classes)

Use classes deal with the risk of biological attack in the form of fungi or insects. They are summarised below starting with the most stringent conditions:

EN 335: Use Class 3

Not covered and not in contact with the ground. Either continually exposed to the weather or protected from the weather but subject to frequent wetting

  • Moisture content:
    • Frequently above 20%
  • Risk of fungal attack:
    • Often liable to attack
    • Disfigurement of decorative panels can also occur as a result of surface moulds and staining fungi
  • Risk of insect attack:
    • Attack is possible, its frequency and importance depends upon the geographical region. Attack by beetles can also depend upon veneer species and thickness

EN 335: Use Class 2

Under cover and fully protected from the weather but where high environmental humidity can lead to occasional but not persistent wetting

  • Moisture content:
    • Can occasionally attain or exceed that which would result from exposure to a temperature of 20ºC and RH of 90%
  • Risk of fungal attack:
    • Moisture content can occasionally increase to a level which can allow growth of wood-destroying fungi
    • Disfigurement of decorative panels can also occur as a result of surface moulds and staining fungi
  • Risk of insect attack:
    • Attack is possible, its frequency and importance depends upon the geographical region. Attack by beetles can also depend upon veneer species and thickness

EN 335: Use Class 1

Under cover, fully protected from weather and not exposed to wetting

  • Moisture content:
    • No higher than that resulting from exposure to a temperature of 20° C and relative humidity of 65%
  • Risk of fungal attack:
    • Regarded as being dry, and thus the risk of attack by surface moulds or by staining or wood-destroying fungi is insignificant
  • Risk of insect attack:
    • Attack is possible, its frequency and importance depends upon the geographical region. Attack by beetles can also depend upon veneer species and thickness

Factors affecting performance

The factors of exterior exposure that can have a significant effect on the performance of wood-based panels include:

  • Rain (particularly wind-driven) and where drying-out is impeded, such as in ground contact
  • Fluctuations in relative humidity and consequently moisture contents within the panel
  • Sunlight, particularly on south-facing aspects and dark coloured surfaces
  • Fungal organisms and insect attack
  • Frost and freeze thaw action
  • Chemical pollution such as salt

Correct design & workmanship and maintenance can help to minimise the effects of some of these factors.

Wood-based panel products, because of their nature will perform differently to large sections of solid timber of the same species, and differently to each other, especially in an external environment.