Timber Treatment Explained: What H1.2, H3.2 and H5 Mean for Your Build
Why timber treatment matters
Untreated timber is vulnerable to insect attack and decay, and in New Zealand homes that mostly means structural framing hidden inside walls, floors and roofs where a problem can go unnoticed for years. To stop that happening, the Building Code sets out which preservative treatment level different timber needs, based on how exposed it will be to moisture and pests once it's built in.
The hazard class system
Treatment levels are defined in the NZS 3640 timber preservation standard as "hazard classes." The higher the number, the greater the exposure the timber is designed to handle:
- H1.2 – the minimum treatment for enclosed, internal framing: wall framing, roof framing, roof sarking, enclosed subfloor framing and interior flooring.
- H3.1 – painted cladding, exterior trims, cavity battens and window reveals.
- H3.2 – timber that's exposed to the weather or to more moisture, such as exposed subfloor framing, deck joists and decking, unpainted cladding, weather-exposed wall and roof framing, and cantilevered floor joists.
- H4 – fence posts.
- H5 – timber in direct ground contact, including piles, poles, and in-ground veranda or deck posts.
H1.2 and H1.1 apply to timber species covered by NZS 3602, while H3.1 upward apply specifically to pine species.
Why H1.2 is the standard for framing
H1.2 is a boron-based treatment that can be used for almost all enclosed timber framing. It became the standard nationwide from 1 July 2011, when an update to the Acceptable Solution B2/AS1 (commonly referred to by the industry phrase "Pink is Tough," after the treatment's distinctive pink colour) replaced the previous mix of treatment options, including LOSP treatments that had health and handling concerns.
Under NZS 3640, H1.2 treatment requires complete sapwood penetration and a minimum preservative retention level, and the Building Code requires roof framing and load-bearing wall framing to last at least 50 years, since these elements are difficult to access or replace later.
What this means on site
- Check the colour and the brand. H1.2 boron timber is tinted pink; H3.1 is typically green or left untreated in colour. Every piece should also carry an end tag, strip brand or packet brand showing the treatment plant, preservative and hazard class.
- Keep framing dry where you can. Boron is water-soluble, so it can leach out of timber that's left wet for long periods. Minimising rain exposure to framing during the build protects the treatment as well as the timber itself.
- Match the class to the use. A joist that's exposed under a deck, for example, needs H3.2, not the lower H1.2 used for enclosed internal framing — using the wrong class in the wrong place is a common and avoidable defect.
- Don't burn treated offcuts. Older copper-based treatments contain chemicals that make burning offcuts unsafe, and cut ends of treated piles should be site-treated rather than placed untreated into the ground.
Ask the engineer
Q: Does every piece of framing in my house need to be treated to the same level?
No. The treatment level depends on where the timber sits and how exposed it is — enclosed internal framing, exposed subfloor or deck timber, and in-ground timber all have different requirements under NZS 3640.
Q: My house was built before 2011 — does that matter?
Treatment requirements and standards have changed over time, so timber used before the current B2/AS1 Acceptable Solution took effect may have been treated differently. If you're buying or renovating an older home, it's worth having the framing and any moisture-affected areas checked.
This article is general information, not site-specific engineering advice. If you have questions about the treatment or condition of timber in your own build, get in touch with Fil Engineers.
