Preservative Treated Timber

The durability and service life of timber products can be enhanced by impregnating the cells with stable preservatives. This process is commonly referred to as 'treating', and is a cost efficient and reliable method of improving the timber's natural resistance to attack from insects and decay.
Why treat timber?

Treatment preservatives can improve the durability of timber by acting to repel wood destroying insects (such as termites, white ants and borers), and breaking down fungi spores that lead to wood rot.

In Australia, the use of framing treated for termite resistance is becoming a standard practice for builders who want to offer their customers peace of mind. The CSIRO estimates that termites cause more damage annually than fire and storms combined. So given that home insurance in Australia doesn't cover pest damage of any kind, and that the cost of treated framing is negligible when building a new home - it just makes sense.

In outdoor environments, Australian grown plantation pine can be cost efficiently treated and used as a sustainable alternative to imported exotic hardwood for many common projects - including carports, pergolas, decking structures and playgrounds.

Timber treatments can also significantly enhance the lifespan of timber used in in-ground applications - such as garden walls and landscaping projects.

However, treatments do not provide protection against the effects of weathering. All timber used in outdoor environments should be protected with the application and maintenance of a suitable coating system.

How is timber treated?

Treatment preservatives are usually impregnated into the timber cells via a liquid carrier agent - which can include water, solvents or oils such as linseed.

The impregnation process can vary depending on the timber species, preservatives, carrier agents and treatment hazard level - but can include dipping, spraying or vacuuming the carrier agent into the timber substrate in a high-pressure environment.

Timber treatments are primarily focused on protecting the sapwood. The sapwood of all species is considered non-durable (because of its life-supporting starches and sugars), and is comparatively easily penetrated by preservatives. It is not possible to effectively treat the heartwood of many timber species, as heartwood cells contain extractives that prevent the uptake of preservatives.

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


Treatment Types

LOSP (Light Organic Solvent Preservative) is commonly used in high value joinery and products treated in their final form and dimensions. LOSP treatments can only be used in above ground applications, providing up to Hazard Level 3 (H3). The active preservatives can include tributyl tin naphthenate (TBTN), azoles and synthetic pyrethroids.

ACQ (Alkaline Copper Quaternary) contains copper and a quaternary ammonium compound. Its colour can range from dark to brownish green, and it is applied using a pressure process as a water-borne preservative. It is free of arsenic and can provide up to Hazard Level 4 (H4). It can be painted or stained when dry.

Copper Azole is one of a new generation of preservative treatments that can be used in water-borne pressure treatment processes. It is a preservative that contains copper, boric acid and tebuconazole and is commonly used with softwood species as an arsenic-free alternative to CCA for treatments up to Hazard Level 4 (H4). It is odourless and can be stained or painted when finished.

CCA (Copper, Chromium and Arsenic) impregnates the timber with water and salts under pressure. Offering a high level of resistance to leaching, CCA can be used on timber exposed to the elements or used in-ground. It is free of odour but has a green appearance which may detract from its appearance. The Australian Pesticides and Veterinary Chemical Authority (APVMA) requires that CCA treated timber may not be used for public projects, childrens playgrounds, garden furniture, picnic tables, external seating, domestic decking or handrails.

Treatment Chart
Treatment Exposure Hazards Applications Product(s)
H1 Indoor, above ground, no wetting Lyctid Borer Framing, indoor joinery, etc
H2-F Indoor, above ground, no wetting Borers and termites Framing (South of the TOC) T2 Blue
H2-S Indoor, above ground, no wetting Borers and termites LVL / Plywood (South of the TOC)
H2 Indoor, above ground, no wetting Borers and termites Framing, indoor joinery, etc T2 Red, Beam 17
H3 Outdoor, above ground, periodic wetting Decay, borers and termites Outdoor structures, decking, etc T3 Green PlusLGLBeam 17
H4 Outdoor, in-ground, some wetting Severe decay, borers and termites In-ground posts, garden walls, etc
H5 Outdoor, in-ground, extreme wetting Extreme decay, borers and termites Retaining walls, building poles, etc
H6 Prolonged exposure to marine water Marine wood borers and decay Boat hulls, marine piles, etc

Discarding of treated timber at the end of its service life, and the handling of off-cuts, is best managed through appropriate recycling and disposal methods.

Check your local government legislation for the appropriate waste handling actions in your region.