Some Facts About Timber Treatments & Uses

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 ground Completely protected from the weather and well ventilated and protected from termites Lyctid Borer Framing, flooring, furniture, interior joinery
Inside, above ground Protected from wetting, Nil leaching Borers and termites Framing, flooring, etc., used in dry situations
Inside, above ground Protected from wetting, Nil leaching Borers and termites Framing (envelope treatment) used in dry situations south of the Tropic of Capricorn only
Inside, above ground Protected from wetting, Nil leaching Borers and termites LVL/Plywood (glue-line treatment) used in dry situations south of the Tropic of Capricorn only
Outside, above ground Subject to periodic moderate wetting and leaching Moderate decay, borers and termites Weatherboard, fascia, pergola posts (above ground), window joinery, framing and decking
Outside, above ground Products predominantly in vertical exposed situations and intended to have the supplementary paint coat system that is regularly maintained Moderate decay, borers and termites Fascia, bargeboards, exterior cladding, window joinery, door joinery and non-laminated verandah posts
Outside, in-ground contact Subject to severe wetting and leaching Severe decay, borers and termites Fence posts, greenhouses, pergola posts (in-ground) and landscaping timbers
Outside, in-ground contact, contact with or in fresh water Subject to extreme wetting and leaching and/or where the critical use requires a higher degree of protection Very severe decay, borers and termites Retaining walls, piling, house stumps, building poles, cooling tower fill
Marine waters Subject to prolonged immersion in sea water Marine wood borers and decay Boat hulls, marine piles, jetty cross bracing
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