Plywood is made up of several 'plies', or layers of wood veneer, glued together and bonded in a high temperature press.

The direction of the grain in each layer runs at right angles to the layer above and below it.

This keeps the board very stable in terms of its overall dimensions, since swelling and shrinking tends to occur across the grain in timber, and the in-between veneers help to resist any movement.

Glue bonds and veneer grades

The glues used in plywood vary depending on the strength and durability required.

They range from Type A, the strongest and most weather resistant bond, to Type D, the weakest and least durable bond.

There are also four grades of veneer quality.

These range from A Grade, which has a high-quality appearance free from knot holes, splits and other defects, through to D Grade, which contains various imperfections on the surface.

Plywood as an underlay product

The underlay sheets used for resilient floor coverings are generally Type A bond with AC veneers. (A grade face, C grade back).

This allows the blemish-free face to be installed face up, providing a very smooth substrate.

Underlays for floor coverings that don't require a very smooth substrate (e.g. engineered products) often have lower grade surface veneers, such as CD (with the C grade face turned up).

There are various sheet sizes and thicknesses available. The smallest size commonly used for underlay is 1200 x 1200 with a thickness of 5.5 mm.

However, some installers like to use larger sizes for particular jobs, and sometimes thicker sheets if they want to improve the structural integrity of the floor.

Once the boards reach a thickness of 12 mm or more, they are generally supplied in the form of structural ply flooring, with tongues and grooves along the long edges.