Preservative-treated wood is surface coated or pressure impregnated with chemicals that improve the resistance to damage that can result from biological deterioration (decay) due to the action of fungi, insects, and microorganisms. Preservative treatment offers a means for improving the resistance and extending the service life of those wood species which do not have sufficient natural resistance under certain in-use conditions. It is possible to extend the service life of untreated wood products by up to ten times through the use of preservative treatment.
Preservative-treated wood can be used for exterior structures that require resistance to fungal decay and termites, such as: bridges, utility poles, railway ties, docks, marinas, fences, gazebos, pergolas, playground equipment, and landscaping.
Four factors are necessary to sustain life for wood destroying fungi; a suitable food supply (wood fibre), a sustained minimum wood moisture content of about 20 percent (common for exterior use conditions), exposure to air, and a favourable temperature for growth (cold temperatures inhibit, but do not eliminate fungi growth). Preservative treatment is effective because it removes the food supply by making it poisonous to the fungi and wood destroying insects such as termites.
An effective wood preservative must have the ability to penetrate the wood, neutralize the food supply of fungi and insects, and be present in sufficient quantities in a non-leachable form. Effective preservatives will also kill existing fungi and insects that might already exist in the wood.
There are two basic methods of treating wood; with and without pressure. Non-pressure methods include the application of preservative by brushing, spraying or dipping the piece of wood. These superficial treatments do not result in deep penetration or large absorption of preservative and are typically restricted to field treatment during construction. Deeper and more thorough penetration is achieved by driving the preservative into the wood cells with pressure. Various combinations of pressure and vacuum are used to force adequate levels of chemical into the wood.
For a wood preservative to function effectively it must be applied under controlled conditions, to specifications known to ensure that the preservative-treated wood will perform under specific in-use conditions. The manufacture and application of wood preservatives are governed by the CSA O80 series of standards. CSA O80 provides information on the wood species that may be treated, the types of preservatives and the retention and penetration of preservative in the wood that must be achieved for the use category or application. To ensure that the specified degree of protection will be provided, a preservative-treated wood product may bear a stamp indicating the suitability for a specific use category.
Wood preservatives in Canada are governed by the Pest Control Products Act and must be registered with the Pest Management Regulatory Agency (PMRA) of Health Canada. Common types of wood preservatives that are used in Canada include chromated copper arsenate (CCA), alkaline copper quaternary (ACQ), copper azole (CA), micronized copper azole (MCA), borates, creosote, pentachlorophenol, copper naphthenate and zinc naphthenate.
Acid salts can lessen the strength of wood if they are present in large concentrations. The concentrations used in preservative-treated wood are sufficiently small so that they do not affect the strength properties under normal use conditions. In some cases, the specified strength and stiffness of wood is reduced due to incising of the wood during the pressure impregnation process (refer to CSA O86 for further information on structural design reduction factors).
Hot dipped galvanized or stainless steel fasteners and connection hardware are usually required to be used in conjunction with preservative-treated wood. There may be additional materials, such as polymer or ceramic coatings, or vinyl or plastic flashings that are suitable for use with preservative-treated wood products. The manufacturer should be consulted prior to specification of fasteners and connection hardware.
For further information, refer to the following resources:
Wood Preservation Canada
Canadian Wood Preservation Association
CSA O80 Series Wood preservation
CSA O86 Engineering design in wood
Pest Management Regulatory Agency of Health Canada
American Wood Protection Association