If you've ever stood in the timber aisle at your local hardware store wondering why some framing timber is green and some is brown — or why the price jumps so much between grades — you're not alone. Timber treatment levels in New Zealand follow a hazard class system, and choosing the wrong one can lead to rot, structural failure, or a failed building inspection. Here's what you need to know.
What Is Timber Treatment and Why Does It Matter?
Timber treatment is the process of preserving wood against biological attack — things like fungi, rot, and wood-boring insects. In New Zealand's damp climate, untreated timber used in the wrong location can deteriorate surprisingly quickly. The NZ Building Code requires that timber used in construction meets minimum durability standards, and the hazard class system is how we define those standards.
Treatment levels are set out under NZS 3640, the New Zealand standard for chemical preservation of round and sawn timber. The higher the hazard class number, the harsher the environment the timber is designed to withstand.
The Hazard Class System at a Glance
| Hazard Class | Typical Use | Exposure | Common Treatment |
|---|---|---|---|
| H1.2 | Interior framing, above ground, dry | Insect risk only | Light boron |
| H3.1 | Exterior, above ground, protected | Moderate weather | LOSP or CCA |
| H3.2 | Exterior, above ground, exposed | Regular wetting | CCA or copper azole |
| H4 | Ground contact, not critical | Soil/freshwater | CCA |
| H5 | Ground contact, critical structural | Severe/prolonged | High-retention CCA |
For most homeowners, the three classes you'll encounter most often are H1.2, H3, and H5.
H1.2 — Interior Framing Timber
H1.2 is the standard treatment for interior wall framing, ceiling joists, and roof trusses — anywhere the timber will stay dry and is protected from the weather. The treatment is typically a light boron-based preservative, which protects against borer and termites but offers minimal resistance to moisture or rot.
You'll recognise H1.2 timber by its light colour — it often looks almost untreated. It's the most affordable option and perfectly suitable for interior structural use under NZS 3604, the standard that governs light timber-framed buildings in NZ.
Do not use H1.2 outside. Even under a covered deck or pergola, moisture levels can be high enough to cause rapid deterioration.
H3 — Exterior Above-Ground Timber
H3 is the go-to treatment for timber that will be exposed to the weather but isn't in contact with the ground. It comes in two sub-classes:
- H3.1 — for exterior timber that is sheltered or protected from direct rain (e.g., fascia boards under wide eaves, decking joists under a covered structure)
- H3.2 — for exterior timber fully exposed to the weather (e.g., open deck framing, pergola rafters, fence rails)
H3 timber is typically treated with LOSP (Light Organic Solvent Preservative) or CCA (Copper Chrome Arsenate) and is usually a light green or tan colour. It's suitable for:
- Deck framing and joists (above ground)
- Pergola and carport structures
- Exterior cladding battens
- Fascia and barge boards
- Fence rails and posts above ground
For most DIY deck and outdoor structure projects, H3.2 is the minimum you should be reaching for. BRANZ provides excellent guidance on specifying the right timber for exterior applications.
H5 — Ground Contact, Critical Structural Use
H5 is the heavy-duty end of the residential spectrum. It's designed for timber that will be in direct contact with the ground or embedded in concrete, where moisture exposure is prolonged and the consequences of failure are serious. Think:
- Fence posts set in the ground
- Retaining wall posts and piles
- House piles (subfloor framing support)
- Landscaping sleepers in contact with soil
H5 timber is treated with a high retention of CCA or similar preservative, giving it a noticeably darker green colour and a heavier feel. It costs significantly more than H3, but in ground-contact situations, it's a non-negotiable — using H3 in the ground will result in rot within a few years.
Note: There is also H4, which sits between H3 and H5 and is used for non-critical ground contact (such as garden edging or landscaping sleepers where structural failure isn't a safety risk). H5 is specified where structural integrity matters.
Working Safely With Treated Timber
CCA-treated timber contains copper, chromium, and arsenic compounds. While the timber itself is safe to handle and use, you should take precautions when cutting or sanding it:
- Always wear a P2 dust mask when cutting treated timber
- Wear gloves and wash hands before eating or drinking
- Never burn treated timber offcuts — the smoke is toxic
- Dispose of offcuts at an approved waste facility
Check WorkSafe NZ for full guidance on safe handling of treated timber products.
Choosing the Right Treatment Level for Your Project
A quick rule of thumb:
- Inside and dry → H1.2
- Outside, above ground → H3.1 (sheltered) or H3.2 (exposed)
- In the ground or concrete → H5
Always check the building consent requirements for your project. Structural work like decks, retaining walls, and subfloor framing will be inspected, and using the wrong hazard class can result in a failed inspection and costly rework.
Timber from Bunnings, Mitre 10, and PlaceMakers is clearly labelled with its hazard class — look for the stamp or tag on the end of the board.
When to Call a Professional
While selecting and cutting treated timber is well within a capable homeowner's ability, certain structural work requires a Licensed Building Practitioner (LBP). Under the NZ Building Code, restricted building work — including structural framing, foundations, and anything requiring a building consent — must be carried out or supervised by an LBP.
If you're unsure whether your project needs a consent, check with your local council or visit MBIE Building Performance. For complex decks, retaining walls over 1.5 metres, or subfloor work, it's well worth getting a qualified carpenter involved. You can find licensed local carpenters on tradedirectory.co.nz.