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Plywood - Getting it right (January 2007).

David Sulman of the Scottish Timber Trade Association provides an update on recent changes to plywood specification and descriptions.

Plywood is a ubiquitous and highly versatile product, available in many forms, which makes it suitable for a wide range of end uses. Plywood was arguably the first engineered wood product, made by bonding thin sheets of wood together, with the grain of each sheet laid at opposing angles, known as cross-banding, which improves the strength and stability of the product. The earliest occurrence of plywood was in ancient Egypt around 3500 BC, when wooden goods were produced from sawn veneers glued together in a crosswise pattern. Today, plywood is manufactured by peeling logs on a rotary lathe, the inventor of which was Immanuel Nobel, a 19th Century Swedish inventor, engineer, architect, investor and industrialist, whose brother Alfred, founded the Nobel Prizes. In commercial terms, plywood came into its own from the 1930’s onwards, following advances in adhesive technology.

In general terms, plywood for structural uses is typically produced from softwood species such as Douglas fir, Spruce, Pine, Fir etc and hardwoods such as Birch. Plywood for decorative use utilises hardwood species from temperate and tropical sources. Various coatings or treatments can be applied to plywood to enhance its physical characteristics and performance, including durability, fire resistance, abrasion resistance etc.

Certain types of plywood 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.

For many years, there have been relatively few changes to the terminology applied to plywood, however, there have recently been some fundamental changes which specifiers and users of plywood products should be aware.

In terms of the use of plywood products in the construction sector, the harmonised standard for wood based panels, EN 13986, requires that plywood must comply with one of the three performance classes within EN636 and suppliers must be able to provide evidence to substantiate such claims for their products.

The key changes are;

• If you have previously specified or purchased WBP (weather and boil-proof) or Exterior quality plywood for construction purposes, you should now specify plywood to comply with EN636, Class 3.

• If you have previously purchased Interior or MR (moisture resistant) plywood for construction purposes, you should now specify plywood to comply with EN 636, Class 1.

• A new intermediate category has been introduced, EN 636 Class 2, which is suitable for construction uses in protected exterior or humid service environments.

The requirements for each Class can be summarised as follows. (Note: This summary is provided for information only, reference should be made to the Standard for further and more detailed information).

Plywood for use in dry conditions: EN 636-1: Plywood to be used in conditions characterised by a moisture content in the material corresponding to a temperature of 20oC and a relative humidity of the surrounding air only exceeding 65% for a few weeks per year. These conditions correspond with Service Class 1 according to EN 1995-1. Boards of this type are suitable for use in Biological Hazard Class 1 of EN 335-3. End uses include warm roofs, intermediate floors, timber frame internal and party walls.

Plywood for use in humid conditions: EN 636-2: Plywood to be used in conditions characterised by a moisture content in the material corresponding to a temperature of 20oC and a relative humidity of the surrounding air only exceeding 85% for a few weeks per year. These conditions correspond with Service Class 2 according to EN 1995-1. Boards of this type are suitable for use in Biological Hazard Class 1 and 2 of EN 335-3. Note: Plywood of this type is appropriate for protected external applications, but is also capable of resisting exposure to weather for short periods, i.e. when exposed during construction. It is also suitable for interior applications where the service moisture condition is raised above the humidity level of dry conditions. End uses include cold roofs, ground floors and timber frame external walls.

Plywood for use in exterior conditions: EN 636-3: Plywood to be used in climatic conditions leading to higher moisture contents than in service Class 2. These conditions correspond with Service Class 3 according to EN 1995-1-1. Boards of this type are suitable for use in Biological Hazard Classes 1, 2 and 3 of EN 335-3. Note: Plywood of this type is capable of withstanding exposure to weathering conditions and liquid water, or water vapour in a damp, but ventilated location, under consideration of Section 8.2 of EN 636. End uses include fully exposed service conditions.

There have also been changes to the bond quality classifications of plywood. Plywood bond quality is divided into three classes, according to EN 636-1, EN 636-2 and EN 636-3, based upon moisture resistance.

• Class 1: Dry conditions: This bonding class is appropriate for normal interior climate.

• Class 2: Humid conditions: This bonding class is appropriate for protected external applications, but is capable of resisting weather exposure for short periods, e.g. when exposed during construction. It is also suitable for interior situations where the service moisture content is raised above the Class 1 level.

• Class 3: Exterior conditions: This bonding class is designed for exposure to weather over sustained periods.

It must be noted that the durability of plywood (i.e. its resistance to decay etc) depends not only upon the bonding performance level, but also on other factors, especially the durability rating of the species used in its manufacture.

It is a matter of considerable importance that where appropriately specified plywood (in common with other wood based panels) is used in humid or exterior conditions, it is imperative that the product receives an appropriate protective finish, with particular attention being paid to the sealing of all faces and edges, to minimise moisture induced movement in service.

Plywood products continue to provide cost effective solutions to construction industry needs, with a long history of satisfactory performance in a wide range of end uses and service environments, including structural and non-structural applications. Guidance on the specification and use of plywoods can be obtained from specialist suppliers of plywood products and timber industry associations, such as the Scottish Timber Trade Association.

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