Timber is treated with a preservative to improve the timber's resistance to attack by wood destroying fungi and wood destroying insects. In other words, the timber's durability is enhanced to a level which is suitable for the intended use. However, such "treatment" does not afford the timber protection against weathering.

Both untreated timber and treated timber used externally should be protected from the affects of weathering by the application of a coating or oil.

Are all Treatments the same?

There is a variety of treatments available. Some treatments protect the timber against borers and/or termites; others protect the timber against insects, borers and decay. Different preservatives are used for different protection requirements or Hazard Levels. There are 6 Hazard Levels (see table below) which are based on the hazardousness of the exposure. Different Hazard Levels have different preservatives, different preservative penetration patterns and different preservative retention requirements. Hardwoods treated to H1 level will have all the sapwood penetrated. There is no need to penetrate the heartwood because lyctids do not attack the heartwood. For the rest of the levels, H2 to H6 inclusive, the penetration from the surface by the preservative increases and the amount of preservative in the treated envelope increases as the hazardousness of the exposure level increases.

Types of Preservatives

Oil-borne preservatives (e.g. creosote, PEC) which can themselves be oils and which are carried into the wood as an oil or mixed in an oil.

Water-borne preservatives (e.g. ACQ, Copper Azole) which are carried into the wood mixed in water.

Light organic solvent-borne preservatives - commonly called LOSPs (e.g. permethrin) - which are carried into the wood mixed in a light organic solvent.

Levels of Treatment - Hazard Levels

There are six levels of treatment. These are called hazard levels and relate to the hazard to which the timber is going to be exposed.

Hazard Class	Exposure	Service Conditions	Biological Hazard
H1	Inside above ground	Fully protected well ventilated	Borers only
H2	Inside above ground	Protected from wetting and leaching	Borers and termites
H3	Outside above ground	Moderate wetting and leaching	Decay, borers & termites
H4	Outside in ground	Severe wetting & leaching	Severe decay, borers & termites
H5	Ground contact	Extreme wetting, leaching &/or critical use	Very severe decay, borers & termites
H6	Marine waters Nth & Sth	Prolonged immersion in sea water	Marine wood borers & decay
H6SW	Marine waters Sth only	Prolonged immersion in sea water	Marine wood borers & decay

Notes:

1. Examples shown in this table are not exhaustive.

2. Not all preservatives are suitable for all hazard levels.

The sapwood of all species is non-durable because of its life-supporting starches and sugars. Therefore, for timber exposed to the weather or in ground contact, the sapwood is normally either removed or preservative treated.

The sapwood of the majority of species is comparatively easily penetrated by preservatives, whereas the heartwood of many species is very difficult to treat because of the changes that occur to the wood cells at the time of heartwood formation. The general rule of thumb is that only the sapwood of hardwoods can be penetrated where as the sapwood and some of the heartwood of most softwood species can be penetrated.

Most treatment methods for round timbers (posts, poles and piles) aim to provide a considerable loading of preservative in the ring of sapwood. Removal of bumps to improve the aesthetic appearance of the round timber should be avoided if the underlying heartwood is not of high durability. Because it is only the sapwood that is usually commercially treatable, only durable timbers should be used as sawn timbers that are going to be placed in a high-hazard situation. It is a common misconception that when ordering sawn or dressed hardwood for external use, the durability of the hardwood can be enhanced by having it treated. Remember that only the sapwood of hardwoods can be penetrated.

Australian Standards relating to treated timber and treated timber products (1604 series) set out minimum preservative penetration requirements and minimum preservative retention requirements.

In NSW and in Qld there is legislation which sets out minimum preservative penetration requirements and minimum preservative retention requirements. In most cases minimum requirements are the same in each legislation and in the Australian Standards.

The Australian Pesticides and Veterinary Chemical Authority (APVMA) requires that after 7th June 2006, timber may not be treated with CCA preservative if it is to be used for certain uses and the timber industry has imposed voluntary restrictions on the sale of timber treated with CCA for certain uses.