Part 1 of this two-part review from Jack Norton, secretary of the Timber Preservers’ Association of Australia, looked at wood structure and natural durability; here, Part 2 addresses timber preservation.

With wood being the primary food source for the wood eating species of termites, having a good understanding of wood structure, durability and wood preservation can be valuable information in predicting termite foraging behaviour, likely levels of termite resistance and making the correct recommendations to customers.

What are the different treatment levels?

According to AS/NZS1604, any product claiming to be preservative treated must be branded as specified in the standard (Figure 1). An important part of the brand is the Hazard Class. This part of the brand indicates the level of treatment or protection that has been applied to protect the (sap)wood for a defined service condition. Unpenetrated heartwood relies on the species’ natural durability.

Figure 1: All timber sold as ‘treated’ must show the treatment plant number, the preservative used to treat the wood and the hazard class for which the wood has been treated

The higher the Hazard Class, the greater the concentration of preservative specified in the Standard. The type of preservative that may be used for a nominated Hazard Class is also specified in the Standard.

Scientifically conducted field trials (Figure 2, main image above) were used to set the levels of preservative concentration, and if wood is treated (and branded) to the levels specified in the Standard for a specific Hazard Class, it will perform as required.


As mentioned earlier, The AS/NZS 1604 specifies which preservative may be used for protection when the wood is treated to perform in a specific Hazard Class.

All timber preservative formulations used in Australia must be approved for use by the Australian Pesticides and Veterinary Medicines Authority (APVMA). The APVMA approves the label, which sets out how the preservative may be used.

Wood preservatives may be dissolved in water, oil or a light organic solvent, such as mineral turpentine. A wood preservative is classified into one of  these three main groups. The list below covers only those preservatives that are used to treat wood to the specifications in AS/NZS 1604.

Water based preservatives are used to treat timber for a wide variety of applications, both indoors and outdoors for residential, commercial, and industrial uses.

  • Copper Chrome Arsenate (CCA) turns the wood green.
  • Alkaline Copper Quaternary (ACQ) turns the wood a different shade of green.
  • Copper azole (CuAz) turns the wood a brown-green colour.

CCA, ACQ and CuAz preservatives react chemically with the wood, making them insoluble. This makes CCA, ACQ and CuAz suitable for use in situations where the treated wood may be exposed to leaching (wetting and rain). This set of preservatives is effective in protecting wood from attack by borers, termites and decay.

Boron-based preservatives are also water soluble, but do not become insoluble after treatment. Because of this, products that are protected with boron-based preservatives can only be used in situations where the wood does not get wet. Whilst boron-based preservatives have some ability to protect against rot or decay, this sort of protection is not usually needed because the boron-protected wood is used in dry situations (e.g. flooring) and rot does not occur in dry wood.

Figure 3: Wood treated to the highest hazard class for marine applications

Oil-borne preservatives are mainly used for heavy-duty construction and in the marine environment. The oil- borne preservatives approved for use in Australia are creosote and Pigment Emulsified Creosote (PEC). Oil-borne treated products include utility poles, rail sleepers and marine piles (Figure 3).

Light organic solvent preservatives

(LOSP) systems are used for products treated in their final shape and form. LOSP treated products include high value joinery such as balustrades and fascias. LOSP treatments are only suitable for products used out of ground contact, and are often sold with a primer coat of paint. As copper naphthenate is the only coloured LOSP treatment (green), other LOSP treatments may contain a tracer colour. The AS/NZS 1604 specified LOSPs include:

  • Tributyl tin naphthenate or TBTN. This is a fungicide (stops rot) and leaves the wood colourless. This preservative must include one of the termite protection insecticides listed in the next column.
  • Copper naphthenate or CuN. This product is being increasingly used in the US, turns the wood green and is a fungicide only. This preservative must also be used with an insecticide.
  • Tebuconazole/propiconazole (or teb/prop). Like TBTN, teb/prop is a colourless fungicide and must be used with an insecticide.
  • The synthetic pyrethroids; permethrin, deltamethrin, bifenthrin, and cypermethrin are used for termite and insect control and have no ability to stop decay. These preservatives are colourless and often have a dye or pigment added, e.g. blue framing.

Glue line additives do not fit into the three groups of preservative just described. The preservative is added to the glue when products such as plywood or laminated veneer lumber or chipboard are being manufactured. Preservatives in this group currently include two synthetic pyrethroids, as well as imidacloprid and zinc borate.

The preservatives approved to protect timber treated for a particular Hazard Class are listed in Technical Note 7, which is provided under
the Publications tab on the Timber Preservers’ Association of Australia (TPAA) web site. ( publications/)

Will treated timber last?

In Australia and New Zealand, wood preservation means protection against insects, termites, decay and marine borers. Wood preservatives do not protect wood against fire, chemicals, wear and tear and incorrect use.

Just as termite barriers are useless if breached, wood treated to H3 level will not perform in a H4 application.

Provided a piece of timber is treated to the levels of penetration and preservative concentration specified in AS1604, it will perform as required.

A modern treatment plant

Wood sold ‘as preservative treated’ is treated under controlled industrial conditions. Any brush on or locally applied surface spray protectant provides temporary protection only. Surface applied protectants do not penetrate deep into the wood and their use must be accompanied by regular inspection and reapplication.

Preservative treated timber is not a ‘no maintenance’ product. Treatments are designed to protect the wood against insects, termites, and decay, and whilst the chromium in copper chrome arsenate preservatives provides some weathering protection, any preservative treated wood exposed to the elements should be coated with a stain or paint for the wood to keep looking good.

There is no doubt that industrial wood protection extends the use of our timber resource and conserves our forests. Timber has the smallest environmental impact of all building products, however, as with all building products it must be installed and used correctly.

More information on preservative treated wood is available from the Timber Preservers’ Association of Australia.