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Treatments – wood preservation

Durability can be achieved with wood preservation techniques.

Chemicals are commonly used to protect timber from biological degradation agents, e.g. by fungi or insect attack. Read Hazards that can effect wood PDF.

Typically pine species such as radiata pine are non-durable without chemical preservation.

The environment or conditions that a timber product will be exposed to may be critical to durability. For example, exposure to rain and ultraviolet rays is a harsher environment than timber framing in a dry condition within an enclosed wall.

When it is not possible to prevent degrade by keeping timber dry through building design or protective coating, then a chemical treatment is necessary.

The type of wood preservative treatment employed is determined by the end-use application and specific biological hazards applicable to that situation. These are defined in hazard classes (see the table below, and Hazard Classes tab at bottom).

NB: The code has changed as of April 2011 to use H1.2 for Timber Treatment for enclosed framing. Read the brochure Pink is Tough – H1.2 Frames it Right here.

All other website information is being updated gradually.

CommoditiesAustralia / New ZealandAmericaAfricaEuropeJapan
Framing and flooring lumberH1/H2H1H11K1/K2
Stillplates or bottom platesH2/H3H3H22K2/K3
Windows, barge / fascia boardsH3H2H33K3
Decking, fence boardsH3H3H33K3
Fence posts, garden edging and landscapingH4H4H44K4
Wood foundations, transmission polesH5H5H54K5

Reference: NZ Pine User’s Guide courtesy of NZ Pine Manufacturer’s Association

Hazard Classes
NB: The code has changed as of April 2011 to use H1.2 for Timber Treatment for enclosed framing. Read the brochure Pink is Tough – H1.2 Frames it Right here.

In New Zealand, roundwood (posts and poles), sawn lumber, and plywood are treated to the following six hazard class levels. Preservative treatment requirements are generally equivalent to or exceed those of other countries which have formal wood preservation standards.

  • H1 – Sawn lumber used in situations continuously protected from the weather. The purpose of preservative treatment is to protect against attack by wood-boring insects. Boron is the main preservative used and treatment would comply with all relevant standards for insect protection.
  • H2 – Sawn lumber and plywood used in interior situations where there is a slight risk of decay and a risk of termite attack. CCA and LOSP are the main preservatives used. Treatment to this hazard class is solely for lumber and plywood which will be exported to Australia.
  • H3 – Sawn lumber and plywood which will be used in exposed exterior situations but not in contact with the ground. CCA and LOSP are the main preservatives used.
  • H4 – Sawn lumber, roundwood and plywood used in ground contact in non-critical situations. CCA and creosote are used in New Zealand for wood in this category.
  • H5 – Sawn lumber, roundwood and plywood used in ground contact with extreme decay hazard or critical enduse requires greater protection – mainly for house foundation piles and transmission poles. CCA and creosote are approved for this use. Preservative retentions are 33% higher than those of Hazard Class H4.
  • H6 – Sawn lumber and roundwood used in a marine environment. Only CCA is used and the main New Zealand pine commodity treated is marine piles.


Synthetic pyrethroids

Cypermethrin, deltamethrin and permethrin are approved actives for protection from wood borers (hazard class H1.1 and H1.2). These actives may also be used for treated to H2 (for export) and where required for H3.1.


Boron preservative are water based ‘non fixed’ preservatives. The borates are recognised as being effective against both wood borers, termites and decay fungi at appropriate concentrations.

The borate concentration required for each hazard class increases with the biological hazard; H1.1, H1.2, H3.1.

The H3.1 borate treated timber is required to have a primer coating before dispatch from the treatment facility.

This provides additional protection for the treated product up to and during construction. Weatherboard, fascia and joinery products treated to H3.1 are required to have a 3-coat paint system.

Some enclosed framing uses are also designated as H3.1. The Department of Building and Housing is considering whether H3.1 borate treated timber can be used without a paint system for critical framing applications.

CCA – Copper chromium arsenate

CCA is the only preservative that can be used for all hazard classes. This preservative has a long history of use. It remains the preferred most cost effective preservative for the H3.2 to H6 hazard classes.

Although solutions of CCA are highly toxic, once the solution is in the wood, complex chemical reactions occur which firmly bind CCA to the wood, making it exceedingly resistant to washing out. Processes have been developed to accelerate this fixation process to minimise or even eliminate the possibility of environmental contamination associated with the use of CCA. However, where environmental or health legislation has forced restrictions on lumber treated with CCA, there are alternative formulations which are ideally suited for treatment of New Zealand pine. These include amoniacal copper quaternaries (ACQ), copper azoles, copper HDO and copper dimethyldiocarbonate (DMDC).

Copper azole and Alkaline Copper Quaternary

Copper azole(CuAz) and copper quaternary (ACQ), like copper-chromium-arsenic (CCA) preservatives, are water based and being very resistant to leaching, are particularly suitable for use in ground contact situations, or where the treated timber is constantly exposed to the weather.

These preservative types offer an alternative to CCA for some applications and particularly for exported timber where there may be restrictions on the types of preservative treatment able to be used.


Creosote is used for treating railway cross-ties and electric power transmission poles. Creosote treatment of sawn New Zealand pine is particularly effective because deep penetration of the heartwood can be achieved.

LOSP – Light Organic Solvent Preservative

The term LOSP refers to preservative actives applied in a hydrocarbon solvent (“white spirit”). LOSPs are used for the treatment of fully machined componentry and fabricated commodities.Their main advantage is that, unlike water-borne preservatives, they cause no swelling of the wood during treatment and require no secondary air or kiln drying after treatment.

Synthetic pyrethroids, IPBC (iodocarb), tributyl tin (TBT) compounds, copper naphthenate and azoles (propiconazole and tebuconazole) are all preservative actives used in LOSP formulations.

They are frequently formulated with water repellent additives (waxes and resins), which may also serve to “bind” the actives to the wood to help prevent redistribution when the carrier solvent evaporates.

The LOSP preservatives are used for hazard classes H1.1, H1.2 and H3.1 although approvals are specific to certain actives or combinations of actives. Copper naphthenate is the single LOSP preservative active approved for use to H3.2.

The combination of propiconazole, tebuconazole and permethrin is gaining wide acceptance as a replacement for TBT treatments.


The approval of a new active, or extension or change to an existing approval requires presentation of biological efficacy data relevant to its use within a specified Hazard Class(es).

The type of data required and recommended test procedures are outlined in the Australasian Wood Preservation Committee (AWPC) Protocols for Assessment of Wood Preservatives.

This document can be accessed from the TPAA website.

The chemical formulations used as wood preservatives are first required to be approved substances under the New Zealand HSNO Act 1996.

The hazardous properties and proposed uses and lifecycle are taken into account when the controls for the use of the substance are written.

This may restrict how and where the substance is used. Environmental Exposure Limits (EEL) and Workplace Exposure Standards (WES) may also be set.

It is the chemical supplier’s responsibility to ensure the wood preservative formulation is covered by a substance approval under the HSNO Act and will comply with NZS 3640 specification for that active or formulation type.

There is also an implied requirement under the Building Code regulations that other broader fitness for purpose requirements will be met by the treated timber to be sold for a particular end-use application.

Examples of technical information that would be expected to be available are the paintability and gluabiltiy of timber, corrosion of fittings or fasteners, and also the health and safety information for safe handling during handling, storage and use, and for disposal of material.


Where a wood preservative formulation is to be used to treat timber to comply with NZS 3640:2003 and the New Zealand Building Code, then the active ingredients must be listed in the preservative standard and be of an approved formulation type.

The processes used to apply the timber preservative to timber are not specified in the Preservative Standard NZS 3640:2003.

Borate preservatives may be applied by dip, spray or vacuum/pressure processes.

All other preservative types, LOSP and water-based CCA, copper azole and copper quaternary are applied in vacuum/pressure treatment vessels.