Using treated timber

The Australian Standard for using preservative-treated wood (AS 1604.1: Specification for preservative treatment - Sawn and round timber) provides the treatment specifications for wood that needs to be protected from insects, termites and decay. The specifications set out in the standard apply to all preserved wood, whether treated in Australia or imported from overseas. Standards Australia also publishes a guide to the standards series dealing with wood and wood preservation.

Why wood is treated with preservative chemicals

Wood is treated with preservatives to protect it against deterioration when used in conditions where threats exist. Sapwood is found just below the bark in all trees and has little resistance to decay, borers or termites. It doesn't contain the chemical deposits that give heartwood its natural durability.

In most wood types the sapwood can be impregnated with chemicals that provide resistance to the potential biological hazards of placing the wood into service. Appropriate preservatives and treatment processes must be used to ensure the chemicals penetrate effectively and are retained by the wood.

Hazard levels and their accepted uses

Wood is treated to different levels, depending on the hazards to which it will be exposed. The different in-service hazard conditions have been classified with a hazard (H) class. The treated wood is branded with the relevant H class.

The higher levels have greater protection and all classes are suitable for use in the conditions described in the lower levels.

H1 is

• suitable for use in well-ventilated places where the wood is kept off the ground and completely protected from weather and wetting
• designed to reduce the likelihood of attack by insects other than termites

H2 is

• suitable for use in well-ventilated places where the wood is kept off the ground and completely protected from weather and wetting
• designed to reduce the likelihood of attack by insects including termites

H3 is

• suitable for use where the wood is kept off the ground but exposed to weather or periodic wetting
• designed to reduce the likelihood of attack by insects (including termites) and decay

H4 is

• appropriate for use where the wood is in contact with the ground or is continually damp
• designed to reduce the likelihood of attack by insects (including termites) and severe decay where a critically important end-use is involved

H5 is

• appropriate for use where the wood is in contact with the ground or fresh water
• designed to reduce the likelihood of attack by insects (including termites) and very severe decay

H6 is

• appropriate for use where the wood is in prolonged contact with sea water
• designed to reduce the likelihood of attack by marine borer and very severe decay.

Preserving wood

There are a number of methods for preserving wood to meet the target hazard classes. The appropriate level of chemical penetration and retention (concentration) is specified for each hazard class, and it is the responsibility of the person treating the wood to ensure that the AS 1604 specifications are met.

The retention of preservative in the penetration zone must be appropriate for the chemical and the required H class, as set out in AS/NZ 1605.

Selling preservative-treated wood

Wood (or wood articles) offered for sale as 'preservative-treated' or 'immunised' should be preserved with a treatment specified in the Standard and branded with a registered brand (including the relevant H class).

Wood preservers should brand all wood treated in their plant before it leaves the site.

Buying preservative-treated wood

When buying preservative-treated wood from any source, ensure it is properly branded and the H level class indicated is appropriate for the intended use of the wood.

Branding and certification

To comply with the AS 1604 Standards, brands may be any shape but they must incorporate the following with letters and figures not less than 4mm high:

• treatment plant registration number
• preservative identification number
• H level class.

Wood pieces larger than 15mm thick must be branded with a hammer or burn brand, a stamp, or a sticker. For smaller sizes, the brand may be on a label attached to wood pieces, bundles or bundle wrappings, or it may be on the delivery docket.

Wood pieces that may be exempt from the branding requirements include battens, fence palings, pieces with a cross-section 1,500mm² or less and pieces with a sawn thickness of less than 15mm.

Treatment certificates, where requested, should include:

• wood species and description
• hazard class
• preservative name
• penetration value
• retention value
• the wood preserver's name and identification number.

References

• Standards Australia (2005) AS 1604.1-2005. Specifications for preservative treatment - sawn and round timber. Australian Standard, distributed by SAI Global Limited.
• Standards Australia (2002) HB164-2002. Wood and wood preservation - A complete guide to the AS/NZS 1604 Standards series, distributed by SAI Global Limited.