information/FAQs

Preservative treatment of plywood

All timber products decay under certain conditions. For example, if the moisture content is high enough, the area confined and unventilated, and the temperature conducive, fungi can cause decay in most timber species. Direct contact with the ground is extremely conducive to decay.

Correct preservative treatment against fungal attack is essential for all plywood products that are permanently exposed to the weather, either painted or unpainted.

Most treatments are aimed at prevention of fungal growth and decay. Applications requiring treatment and recommended treatments are specified in Australian Standard AS1604: "Preservative Treatment of Sawn Timber, Veneer and Plywood".

There are several methods of treating plywood:

Impregnation of veneers prior to manufacture;
Including chemicals in the glue line at manufacture;
Pressure treatment of the manufactured plywood; and
Preservative treatment of surfaces after manufacture.

Machining, sawing and boring of the plywood should be completed before surface treating with brush-on preservatives.

Knowledge of the conditions under which plywood is to be used is essential if correct advice on the type of treatment required is to be obtained. The CSIRO, PAA and preservative application companies are all sources of information on recommended treatments.

Some preservative treated plywoods may be difficult to bond, particularly with phenolic or resorcinol adhesives. Painting over some preservative treated plywoods can also cause problems if there is incompatibility between the preservative and the paint. Most commercially available water-borne salt preservatives can be painted over.

When used in permanently fully exposed applications structural plywood must be preservative treated and surface finished with paint or water repellent.

Infestation with termites is not usually a problem for plywood provided the application does not involve ground contact, and other good building practices such as soil poisoning have been correctly carried out. Some plywood is subject to infestation by boring insects under certain conditions.

Reference: Plywood Association of Australia.