Picking the wrong treatment level for your timber can mean rot, structural failure, or a failed building inspection — yet many homeowners (and even some builders) aren't entirely sure where H3.2 ends and H4 begins. If you're planning a deck, pergola, retaining wall, or any outdoor structure, getting this right from the start will save you significant time and money down the track.
What Does the 'H' Number Actually Mean?
The H-rating system is New Zealand's hazard classification for treated timber, governed by Standards NZ and referenced throughout the NZ Building Code. The higher the number, the more aggressive the treatment — and the harsher the environment the timber is designed to withstand.
Treatment is typically achieved using preservative chemicals (such as copper azole or LOSP) that are pressure-impregnated into the timber. The treatment level determines how much preservative is retained in the wood and how deep it penetrates. BRANZ provides detailed guidance on timber durability and treatment selection for NZ conditions.
H3.2 Treated Timber: Overview
H3.2 is designed for timber used above ground in exposed exterior situations — places where the timber will get wet but won't be in direct contact with the ground or water. Think:
- Deck framing and decking boards (above ground)
- Pergola rafters and beams
- Exterior cladding framing
- Fascia and barge boards
- Exterior joinery framing
H3.2 timber is treated to resist fungal decay and insect attack (including borer) in conditions where it's regularly exposed to rain and moisture but has the opportunity to dry out. It is not suitable for ground contact or situations where it will be continuously wet.
H4 Treated Timber: Overview
H4 is a significantly heavier treatment level, designed for timber that will be in direct contact with the ground or fresh water. The preservative retention is much higher, giving it the durability to resist decay even when constantly damp. Typical uses include:
- Fence posts set in the ground
- Retaining wall piles and posts
- Deck posts embedded in concrete or soil
- Garden sleepers in contact with soil
- Sub-floor framing in contact with the ground
Because ground contact creates a permanently moist, oxygen-rich environment that fungi and insects love, H4 timber needs that extra protection. Using H3.2 in a ground-contact situation is a common and costly mistake — it will deteriorate far sooner than expected.
Head-to-Head Comparison
| Criteria | H3.2 | H4 |
|---|---|---|
| Intended use | Above-ground exterior, exposed to weather | Ground contact or fresh water contact |
| Typical applications | Deck boards, pergola framing, fascia | Fence posts, retaining wall piles, deck posts |
| Preservative retention | Moderate | High |
| Durability in ground | Not suitable | Designed for it |
| Weight | Lighter | Heavier (more chemical retention) |
| Workability | Easy to cut, nail, screw | Slightly harder; use appropriate fixings |
| Cost (rough guide) | Lower cost per lineal metre | Noticeably more expensive |
| NZ Building Code compliance | Required for above-ground exterior framing | Required wherever ground contact occurs |
| DIY suitability | Good — standard tools, wear gloves | Good — but always wear PPE when cutting |
| Colour/appearance | Usually green or brown tint | Often darker green or brown |
Cost Considerations
As a rough guide only, H4 timber typically costs 20–40% more than equivalent H3.2 sections, reflecting the higher chemical loading. Prices vary by species, section size, and supplier — check current pricing at Bunnings, Mitre 10, or PlaceMakers.
It can be tempting to use H3.2 throughout a project to save money, but if that timber ends up in ground contact, you'll be replacing it far sooner. The cost of remediation — pulling up a deck, replacing rotted posts, re-doing a retaining wall — will far outweigh any initial saving.
NZ Building Code and Compliance
Under the NZ Building Code, timber used in structural applications must meet the durability requirements set out in Clause B2 (Durability). For most structural timber in exterior or ground-contact situations, this means specifying the correct H-rating.
If you're building a deck, retaining wall, or any structure requiring a building consent, your plans will need to specify the correct treatment levels. A Licensed Building Practitioner (LBP) must carry out or supervise restricted building work — find one at LBP. Using the wrong treatment level can result in a failed inspection or, worse, a structure that becomes unsafe over time.
Safety When Working With Treated Timber
Safety note: Treated timber contains preservative chemicals. Always wear gloves, safety glasses, and a dust mask when cutting or sanding. Never burn treated timber offcuts — the smoke is toxic. Dispose of offcuts at an approved facility. Refer to WorkSafe NZ for safe handling guidance.
Our Recommendation
Here's a straightforward guide for common NZ scenarios:
- Building a deck with posts set in the ground? Use H4 for the posts and H3.2 for the above-ground framing and decking boards.
- Putting up a pergola on a concrete pad? H3.2 is generally sufficient for all framing, provided posts are not embedded in soil.
- Building a retaining wall? Use H4 (or H5 for marine environments) for all piles and ground-contact members.
- Installing fence posts? Always use H4 — fence posts are in ground contact by definition.
- Exterior cladding framing? H3.2 is the standard requirement.
When in doubt, go one level up. The extra cost of H4 where H3.2 might technically suffice is small compared to the cost of early replacement. If your project requires building consent or involves structural work, engage a qualified local builder — you can find experienced tradespeople in your area at tradedirectory.co.nz.
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