Timber treatments preserves timber but can also be hazardous to health.

On this page:

• types of timber treatment
• hazard classes
• LOSP preservative
• CCA preservative
• alternative timber preservatives
• durability of fastenings
• alternatives to treated timber.

Timber treatment prevents deterioration by insect or fungal attack and preserves the quality of the timber.

Unfortunately, the chemicals that are used to preserve timber are also hazardous to people.

Precautions must be taken by anyone working with treated timber including those who carry out the treatment and those who work with treated timb

Types of timber treatment

The level of treatment selected must be appropriate for the intended use of the timber. The most commonly used preservatives for timber treatments are:

• Boron compounds
• CCA compounds (copper chromium arsenic)
• LOSP (light organic solvent preservative) – commonly tributyl tin oxide (TBTO).

Hazard classes

All fine wood dust is hazardous – protection to eyes, respiratory system and skin must be provided and worn.

Hazard class	Exposure	Preservative options	Health and safety issues
Untreated	Protection from weather and ground conditions		• Wear mask to prevent inhalation of dusts when cutting or sanding
H1.1	Protection from weather and ground conditions	Boron	• Wear mask to prevent inhalation of dusts when cutting or sanding
		LOSP with permethrin	• Wear mask to prevent inhalation of dusts when cutting or sanding • Solvent exposure • Skin irritant
H1.2	Protection from weather and ground conditions but exposure to moisture possible	Boron	• Wear mask to prevent inhalation of dusts when cutting or sanding
		LOSP with IPBC chemicals	• Wear mask to prevent inhalation of dusts when cutting or sanding • Solvent exposure • Skin irritant
H3.1	Exposure to weather; above ground only	LOSP with TBTO (tributyltin oxide), TBTN (tributyltin naphthenate), propiconazole + tebuconazole or IBPC (iodo propynyl butyl carbamate) treatment chemicals	• Wear mask to prevent inhalation of dusts when cutting or sanding • Solvent exposure • Skin irritant
		Copper-based	• Potentially carcinogenic if containing chrome and arsenic (CCA) • Avoid skin contact use e.g. handrails, play equipment • Wear mask to prevent inhalation of dusts when cutting or sanding
H3.2	Exposure to weather, above ground; or protection from weather and risk of moisture entrapment	Copper-based	• Potentially carcinogenic if containing chrome and arsenic (CCA) • Wear mask to prevent inhalation of dusts when cutting or sanding
H4	Exposure to weather in ground or freshwater; permanently wet	Copper-based	• Potentially carcinogenic if containing chrome and arsenic (CCA) • Wear mask to prevent inhalation of dusts when cutting or sanding
H5	Exposure to weather in ground or freshwater; permanently wet	Copper-based	• Potentially carcinogenic if containing chrome and arsenic (CCA) • Wear mask to prevent inhalation of dusts when cutting
H6	In ground or seawater; permanently wet	Copper-based	• Potentially carcinogenic if containing chrome and arsenic (CCA) • Wear mask to prevent inhalation of dusts when cutting

 

LOSP preservative

Preservation involves the use of solvents to impregnate timber with chemicals such as TBTO (tributyltin oxide), TBTN (tributyltin napthenate), propiconazole + tebuconazole or IBPC (iodo propynyl butyl carbamate), permethrin, azoles and copper naphthenate. The solvents give off gas (evaporate) for some time after treatment.

Tributyl tin oxide (TBTO), a commonly used preservative with LOSP, is a strong skin irritant, so skin and eye protection must be worn in conjunction with its use.

Use and handling

• Store in a well-ventilated, covered area.
• Remove wrapping to allow vapour to evaporate.
• Wear gloves and long sleeves when handling timber.
• Wear safety glasses and a filter mask when cutting and sanding.
• Work in a well-ventilated environment and carry out cutting, sanding and so on outside.
• Avoid working with solvent-damp timber.
• Always wash hands after handling LOSP timber.
• Wash clothes separately if there is sawdust on them.

Environmental effects

Solvents leach out of timber easily, so LOSP timber should only be used to H3.1 level, i.e. above ground use only.

CCA preservative

CCA treated timber (also commonly called tanalised timber) contains copper, chromium and arsenic, which is a toxic chemical mixture but is relatively inert once fixed in the timber. However, it is not recommended in skin contact use such as children’s playground equipment and hand rails. Treatments such as ACQ and copper azole do not have the chrome and the arsenic but the higher levels of copper increase the corrosion potential with steel and galvanised steel fixings.

Use and handling

• Carry out all cutting, sanding and so on outside.
• Wear a filter mask, safety glasses and gloves when cutting and sawing.
• Take particular care when the timber surface is wet or has crystalline chemical deposits on it.
• Clean up (timber scraps, sawdust) thoroughly afterwards.
• Dispose of waste to an approved (municipal) waste disposal area.
• Do not compost or mulch waste.
• Do not burn waste.
• Wash your hands before eating, drinking or smoking.
• Wash exposed areas of your body after working with treated timber.
• Wash work clothes separately from other clothes.

Environmental effects

Small quantities of chemicals can leach out over time so it should not be used where it may come into contact with public drinking water.

Alternative timber preservatives

Other timber preservatives that are marketed as more environmentally friendly alternatives include:

• copper azole-based (CuAz)
• alkaline or ammoniacal copper quarternary-based (ACQ).

These treatments are approved for used for H3.1, H3.2, H4 and H5 hazard treatment levels. They contain higher levels of copper than the CCA preservatives, which mean they are more corrosive to metals. Care must be taken when selecting fixings, and consideration must be given to run-off from timber onto other materials.

Durability of fastenings

Zinc-based and stainless steel fastenings are permitted with CCA treated timber under NZS 3604 Table 4.3. The durability of fastenings with alternative treatments such as CuAz and ACQ is not included in NZS 3604 or the NZBC compliance document E2/AS1.

BRANZ has carried out tests on the durability of fastenings with CuAz and ACQ treated timber, with the following results:

• 316 stainless steel fastenings were generally unaffected by use with CuAz and ACQ treated timber.
• Mild steel and galvanised mild steel fastenings had a relatively rapid rate of corrosion with CuAz and ACQ treated timber – typical rates were 2–5 times faster corrosion, but up to 20 times faster in extreme cases.

Recommendations

BRANZ recommends the following fasteners with CuAz and ACQ treated timber:

• 304 or 316 stainless steel.
• Durable equivalent fasteners such as silicon bronze.

Alternatives to treated timber

Alternatives to using treated timber may include substitution with other materials where possible. For example:

• use concrete piles, strip foundations, recycled hardwood (e.g. jarrah telephone poles) or concrete block retaining walls instead of piles or posts
• use heartwood timbers such as cypress or eucalyptus for weatherboards, decking, and so on instead of LOSP treated timber
• use Douglas fir, Lawson cypress or kiln-dried radiata pine instead of boric-treated timber where the in-service moisture content will always be 18% or less.