What you're looking at
NZ Steel introduced ZINCALUME in 1994. Its coating is 55% aluminium, up to 3% magnesium and the rest zinc, and NZ Steel says it lasts significantly longer than a galvanised coating. A new roof is bright, then darkens as its thin resin coat weathers, which NZ Steel says is only a visual change.
Galvanised steel, which NZ Steel sells as GALVSTEEL, has a coating of zinc alone. Where the surface is scratched, the zinc corrodes first to protect the steel. Kiwis still call it galvanised iron or corrugated iron, though it's been steel for a long time.
ZINCALUME or galvanised?
| ZINCALUME | Galvanised (GALVSTEEL) | |
|---|---|---|
| Coating | Aluminium, zinc and a little magnesium | Zinc |
| NZ Steel's coastal guidance | Not recommended in Severe, Very Severe or Extremely Severe sites | Suits moderate environments outdoors |
| E2/AS1, unpainted on an exposed roof | Exposure zones B and C only | Exposure zones B and C only |
| Roof upkeep | Rain washing | Rain washing |
| Spouting, downpipes and fascia | Hand-washed every 3 months | Hand-washed every 3 months |
| Painting | Prime first; NZ Steel won't cover paint-related failures | If it's painted, NZ Steel says do it straight after installation, with two topcoats |
Where it suits in New Zealand
NZ Steel puts ZINCALUME in its Mild and Moderate categories. Moderate typically starts 500 to 1000 metres from breaking surf, so a beachfront or harbour-edge house needs a different product, such as COLORSTEEL.
The Acceptable Solution uses NZS 3604 exposure zones, a different scale, and accepts bare ZINCALUME or galvanised steel on an exposed roof only in zones B and C. Keep the two systems apart and have your roofer check your address against each.
Geothermal air is harder on it than salt. BRANZ found aluminium-zinc coated steel appears to corrode faster in strong geothermal environments than marine ones, with heavy rusting beside unprotected cut edges. In much of Rotorua, or near the Wairākei field north of Taupō, ask about other options first. Wind and snow change the profile and fixings rather than the coating.
What it can't touch
This is where bare steel roofs go wrong. NZ Steel says lead isn't compatible with ZINCALUME, even as run-off from a lead flashing, and copper is worse, especially water dripping from copper pipes. Stainless steel fasteners shouldn't be used with it at all.
NZ Steel's rule of thumb is that you can run water down but not uphill: zinc onto copper is fine, copper onto zinc isn't. Unpainted galvanised spouting has its own trap, drip-spot corrosion, when water runs onto it from an inert surface such as a painted roof, glass or glazed tiles.
How long it lasts, and the upkeep
NZ Steel's warranty guide gives ZINCALUME roofing 25 years in a Mild site and 20 in Moderate, and 10 years for spouting, downpipes and fascia. With its recommended maintenance, it says the Building Code's 15-year durability will be met or exceeded.
The open roof relies on rain. Spouting, downpipes and fascia need washing every 3 months with water and a soft brush, or a water-blaster at no more than 2900 psi. If you collect drinking water, NZ Steel says the first 25 mm of rain off a new roof must be thrown away.
Painting it later
Both coatings take paint. Resene says new metal carries a form oil that has to come off, and white rust starts forming after 4 to 6 months of weathering; it and Wattyl both call for a primer before the topcoats. NZ Steel won't cover failures caused by paint added later, and points people who want colour to COLORSTEEL. If yours is already rusting, read rusty iron roof.
Ask before it goes on
- Which NZ Steel category and which NZS 3604 exposure zone is my address in?
- Do any lead flashings or copper pipes drain onto the roof, and what will you do about them?
- Will the flashings and ridge be ZINCALUME too, and what fasteners and washers are you using?
- Does the spouting suit the roof above it?
Yours, and the roofer's
You can
- Hose spouting, downpipes and fascia from the ground every 3 months
- Find where copper pipes and overflows drain, and tell your roofer
- Disconnect a rainwater tank before any roof washing or painting
Leave to a roofer
- Replacing or isolating lead flashings
- Any work on sheets, flashings or fixings
- Priming and painting
Our take on bare steel
Inland, away from geothermal ground, and fond of the plain silver look? ZINCALUME is a sensible roof. We'd sort out every lead flashing and copper pipe before the first sheet goes on, because those are what quietly eat it. Near the coast or a steaming vent, we'd skip bare steel and look at pre-painted options instead.
Want a roofer to take a look?
Tell us the job and your postcode. We match it with a Find My Roofer certified roofer who specialises in that kind of work, and they call you to quote. Your number goes to them alone.