The basics

Timber is a fantastic building material, particularly due to its sustainability. However, one of the potential downsides of using timber in construction and demolition is its susceptibility to rot, pests and moisture - particularly in New Zealand because of our wet climate.

To overcome these issues, timber is treated with a range of chemicals, including copper chrome arsenate (CCA), copper azole, LOSP solvent treatment, or boron, which changes the way it can be handled at the end of its life. Once treated timber is given a hazard classification which determines what it should be used for, how it can be handled, and how it can be disposed of.

Key timber framing rules

The minimum treatment required for most framing is H1.2 (boron treatment). However, if you’re building cantilevered floor joists, deck joists, bearers, or framing exposed to weather, you must use H3.2.

You can only use untreated Douglas Fir if the building’s design is deemed low risk under the New Zealand Building Code.

Putting it into practice

First things first – you can usually tell the treatment type by the timber’s colour.

• H1.2 (boron) has a pink tint
• H3.1/H3.2 is often green or has no tint (check stamp)

Because it’s not always obvious, it’s really important you check the timber you’re using for its hazard class mark (e.g. H3.2) and its treatment plant number. 

Any quality assurance marks are optional, but always good for that extra peace of mind.

It’s important that any timber stored on site is covered (so it’s dry) and off the ground. If you need help ensuring your timber stays dry, talk to your Mitre 10 Trade rep about our reusable timber covers.

Make sure you always treat cut ends with a brush-on preservative, and that you use the correct fasteners - galvanised, stainless steel (304+), or silicone bronze. This is especially important in coastal or exposed areas.

Quick Tips

• Check the spec: Always confirm the hazard class matches the plans.
• Site treat: Brush on preservative after cutting or drilling.
• Look for hazard class marks: Don't guess, read the stamps.
• Dispose of safely: Treated off-cuts must go to landfill or industrial incineration — never burn them household fires.

Extra for experts

Know your Hazard Classes (NZS 3640 Standard)

Main Treatment Types of Kopper products below: