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NZ Building Code · Site ConditionsExposure & coastal zones: picking hardware that won’t rust
Exposure zones grade how corrosive the air is at your site so you fix it with hardware that lasts.
Exposure zones (NZS 3604 §4.2.3) grade how corrosive the air is at a site — mostly from airborne sea salt, and from geothermal gases in places like Rotorua. They set the corrosion resistance of every exposed fixing and fastener, which is the difference between stainless 316 lasting decades and galvanised nails bleeding rust in a few years. Get this wrong on the coast and you’re back fixing rust stains under warranty.
How the zones run
The zones run roughly from Zone B (sheltered inland) through to Zone D (severe, sea-spray). The closer you are to the sea, the tougher the hardware needs to be:
- Zone A — inland, 500m+: galvanised (HDG 600g/m²). Standard galv hardware is OK.
- Zone B — open sea, 500m–50km: HDG zinc-plated. Galv is adequate.
- Zone C — sea spray, 100–500m: HDG zinc or Type 304 stainless. Use premium galv or 304SS.
- Zone D — severe marine, <100m or geothermal: Type 316 stainless only.
Why Zone D is non-negotiable near the coast
In Zone D, all exposed fixings must be SS316. Galv will fail in 5–7 years near the coast, so there’s no shortcut — the stainless goes in from the start.
Watch the geothermal areas
Geothermal areas like Rotorua and Taupō often require Zone D hardware (SS316) regardless of distance from the sea. The H₂S in the air corrodes galvanised faster than salt spray does. Check the council’s hazard map before you order hardware.
Plain-English guide, not advice. This page helps you understand and navigate the rules — it is general information, not design, engineering or consent advice, and it does not reproduce the copyrighted tables of NZS 3604 or any Standard. Always check the current Standard or Acceptable Solution and your BCA, and use a suitably qualified LBP, engineer or QS where it matters.
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Common questions
What sets exposure zones apart from one another?
Exposure zones (NZS 3604 §4.2.3) grade how corrosive the air is at a site — mostly from airborne sea salt, and from geothermal gases in places like Rotorua. They run roughly from Zone B (sheltered inland) through to Zone D (severe, sea-spray), and set the corrosion resistance needed for every exposed fixing and fastener.
Which hardware do I use in each zone?
Zone A (inland, 500m+) takes galvanised (HDG 600g/m²). Zone B (open sea, 500m–50km) takes HDG zinc-plated. Zone C (sea spray, 100–500m) takes HDG zinc or Type 304 stainless. Zone D (severe marine, under 100m or geothermal) takes Type 316 stainless only.
Can I get away with galv near the coast?
No. In Zone D, all exposed fixings must be SS316 — galv will fail in 5–7 years near the coast. Get this wrong on the coast and you’re back fixing rust stains under warranty.
Do geothermal areas need special hardware?
Yes. Geothermal areas like Rotorua and Taupō often require Zone D hardware (SS316) regardless of distance from the sea, because the H₂S in the air corrodes galvanised faster than salt spray. Check the council’s hazard map.
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