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External performance

Sheet materials | Factors affecting external 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

Remember to edge seal sheet material

External use and the need for Edge Sealing

Many of the problems which eventually beset wood based panel products in service could be avoided if the edges were effectively sealed against moisture before being installed. Unfortunately, all too often the faces of the board are decorated and protected whilst the edges, often hidden from view, are neglected. This can lead to premature failure of decorative treatments, the board itself, or both. 

Water is absorbed much faster into the edges of an of an unprotected board than into the faces. The reason is that, whether the board be plywood or another wood based product, much of the timber exposed at the edges can be end grain. 

Wood structure may be likened to close-packed tubes along which water can pass easily but between which it cannot easily permeate. Therefore, water is absorbed into the end grain of timber much faster than it is across the grain. This difference can be from several times to several thousand times faster depending on the species of timber.

The effect of such water penetration through the edges can cause any or all of the following faults:

  • swelling of the board edges
  • discolouration and watermarking
  • failure of surface coatings
  • mould and stain development
  • decay
  • delamination or disintegration

For most applications wood-based panel manufacturers recommend that any exterior grade panel used externally should be fully painted prior to installation, this includes both faces and importantly all edges, including those cut on-site. This will help to minimise moisture ingress and provide more balance to the boards, for example to prevent thinner boards, such as used for soffits, warping in service. There are a variety of different paint finishes available and guidance on their suitability and use should be sought from the paint/coating  manufacturer for the particular wood-based panel type and hazard class. It is possible to obtain reasonable performance from a conventional paint finish on most panels, without a paper overlay. For plywood a good face grade must be used, BS EN 635-4 gives guidance on which surface grades are suitable for coating under each service class.