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Treated Timber

Purpose of Treatment

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

Water-borne preservatives (e.g. Copper Chrome Arsenate (CCA), Alkaline Copper Quaternary (ACQ), Copper azole) - which are carried into the wood mixed in water. They have a wide variety of applications, both indoors and outdoors, for residential, commercial, and industrial structures.

Light organic solvent-borne preservatives (commonly called LOSPs) - which are carried into the wood mixed in a light organic solvent such as white spirit. They are used in high value joinery and similar products and are treated generally in their final form and shape and must only be used out of ground contact. The actives in LOSPs include tributyl tin naphthenate (TBTN), azoles and the synthetic pyrethroids (e.g. permethrin).

Envelope treatments - (e.g. blue pine) which are synthetic pyrethroids (e.g. bifenthrin, permethrin) dissolved in water or oil such as linseed oil and are applied by spraying or dipping to cover the timber in the preservative. Used primarily in framing timber South of the Tropic of Capricorn.

Oil-borne preservatives (e.g. pigment emulsified creosote (PEC)) which are carried into the wood as oil or mixed in oil. Used primarily for heavy duty construction and in the marine environment including utility poles, rail sleepers and marine piles.

Levels of Treatment - Hazard Levels

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

Hazard Level
Specific Service Conditions
Biological Hazard
Typical Uses
Inside, above groundCompletely protected from the weather and well ventilated and protected from termitesLyctid BorerFraming, flooring, furniture, interior joinery
Inside, above groundProtected from wetting, Nil leachingBorers and termitesFraming, flooring, etc., used in dry situations
Inside, above groundProtected from wetting, Nil leachingBorers and termitesFraming (envelope treatment) used in dry situations south of the Tropic of Capricorn only
Inside, above groundProtected from wetting, Nil leachingBorers and termitesLVL/Plywood (glue-line treatment) used in dry situations south of the Tropic of Capricorn only
Outside, above groundSubject to periodic moderate wetting and leachingModerate decay, borers and termitesWeatherboard, fascia, pergola posts (above ground), window joinery, framing and decking
Outside, above groundProducts predominantly in vertical exposed situations and intended to have the supplementary paint coat system that is regularly maintainedModerate decay, borers and termitesFascia, bargeboards, exterior cladding, window joinery, door joinery and non-laminated verandah posts
Outside, in-ground contactSubject to severe wetting and leachingSevere decay, borers and termitesFence posts, greenhouses, pergola posts (in-ground) and landscaping timbers
Outside, in-ground contact, contact with or in fresh waterSubject to extreme wetting and leaching and/or where the critical use requires a higher degree of protectionVery severe decay, borers and termitesRetaining walls, piling, house stumps, building poles, cooling tower fill
Marine watersSubject to prolonged immersion in sea waterMarine wood borers and decayBoat hulls, marine piles, jetty cross bracing
  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. (See: CCA Treated Timber )

Technical Report No 5, Treated Timber in Australia : CCA and the alternatives
(PDF, 1.7 MB)
Australian Timber Importers Federation
Australian Pesticides and Veterinary Medicines Authority
Information on Arsenic Timber Treatments by the Australian government authority responsible for the assessment and registration of pesticides and veterinary medicines and for their regulation up to and including the point of retail sale.
Timber Preservers Association of Australia
The Timber Preservers Association of Australia (TPAA) represents the nation's timber preservation industry. It is an organisation comprising timber treaters, suppliers of preservatives, research organisations, as well as individuals and bodies having an interest in timber preservation.