Using wood treated with preservative

A diagram showing sapwood-heartwood combinations in sawn wood

Sawn wood can have any number of sapwood-heartwood combinations.

There is an Australian standard for using preservative-treated wood. The Australian Standard AS 1604.1 - Specifications for preservative treatment - Sawn and round timber, provides the treatment specifications for wood that needs protection from attack by 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 attack is possible. The sapwood, present in all trees, is just below the bark and has little resistance to decay, borers or termites. Sapwood doesn´t contain the chemical deposits that give heartwood its natural durability.

In most wood types, the sapwood can be impregnated with wood preservative chemicals that provide resistance to the biological hazards likely to be experienced when the wood is placed into service.

Appropriate preservatives and treatment processes must be used so that chemicals penetrate effectively and are retained adequately in the wood.

Hazard levels and conditions in which wood can be used

Wood is treated to different levels, depending on the in-service hazard to which it is to 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.

  • Hazard class - H1: Suitable for use in a well-ventilated place where the wood is kept off the ground and completely protected from weather and wetting. The treatment is designed to reduce the likelihood of attack by insects other than termites.
  • Hazard class - H2: Suitable for use as H1-treated wood and for use in a well-ventilated place where the wood is kept off the ground and completely protected from weather and wetting. This treatment is designed to reduce the likelihood of attack by insects including termites.
  • Hazard class - H3: Suitable for use as H2-treated wood and appropriate for use in a place where the wood is kept off the ground but is exposed to weather or periodic wetting i.e. outside above ground. This treatment is designed to reduce the likelihood of attack by insects, including termites, and decay.
  • Hazard class - H4: Suitable for use as H3-treated wood and appropriate for use in a place where the wood is in contact with the ground or is continually damp. This treatment is designed to reduce the likelihood of attack by insects, including termites, and severe decay where a critically important end-use is involved.
  • Hazard class - H5: Suitable for use as H4-treated wood and appropriate for use in a place where the wood is in contact with the ground or fresh water. This treatment is designed to reduce the likelihood of attack by insects, including termites, and very severe decay.
  • Hazard class - H6: Suitable for use as H5-treated wood and appropriate for use in a place where the wood is in prolonged contact with sea water. The treatment is 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 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 carrying out the treatment process to ensure that the specifications in the AS 1604 series of standards are met.

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

Selling preservative-treated wood

Wood (or wood articles) offered for sale or sold as 'preservative-treated' or 'immunised' should be treated with a preservative 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 treatment plant before it leaves the site.

Buying preservative-treated wood

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

Brands and branding

To comply with the AS 1604 series of standards, brands may be any shape but they must incorporate the following with letters and figures not less than 4 mm 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 1500 mm2 or less and pieces less than 15 mm sawn thickness.

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.

Last updated 05 October 2010