Plywood was originally developed to provide panels with dimensional stability and good strength properties both along and across the panel. Plywood is an engineered wood product, made from straight, well-grown timber suitable for a wide range of applications. Plywood has an EN product standard covering its use in exterior conditions. To enhance the durability, appropriate coatings or preservative treatments are added to plywood. British Standard BS EN 636 refers to bond quality and surface appearance, the durability of the timber species used in the veneer and the veneer quality. In plywood, the higher mechanical (structural) properties are in the direction of the major axis, which is usually the longer side.
Plywood can maintain a high performance under a wide variety of environmental conditions. The high strength to weight ratios are influenced by the species used, softwood and hardwood, and also on the glues used in production. Plywood is generally made from veneers that are peeled from a log and are bonded together with an adhesive, with the grain of adjacent veneers at right angles to each other. The adhesive is cured by pressing the panel using heated platens.
As defined in BS EN 309,
- Plywood Wood-based panel consisting of an assembly of layers bonded together with the direction of the grain in adjacent layers, usually at right angles.
- Veneer is a thin sheet of wood not more than 7mm in thickness.
- Ply is either a single veneer, or two or more veneers joined edge to edge or end to end.
- Layer is either one ply or two or more plies, glued together with their grain direction parallel, or with another material.
- Multi-ply Plywood formed of more than three layers.
- urea-formaldehyde (UF) - only suitable for interior use.
- phenol-formaldehyde (PF) - has greater moisture resistance, suitable for use in humid or in exterior situations.
- melamine-urea-formaldehyde (MUF) - for exterior or even marine plywood.
- Structural plywood are used in, floor decking, flat roofing, wall sheathing, concrete formwork, external cladding.
- Marine plywood has a very high performance under severe exposure conditions, and used in ship or boat building.
- Utility plywood is intended for non-construction applications, for joinery, furniture and mostly for interior uses.
- Class 1 - for interior applications with no risk of wetting,
- Class 2 - for use in protected exterior applications,
- Class 3 - for use in unprotected external applications.