Which kind of iron is on your roof?
Galvanised iron has a pure zinc coating, and ZINCALUME an aluminium, zinc and magnesium one. Resene says new sheets of both carry a form oil that must come off before painting, and start to grow white rust (zinc oxide) after 4 to 6 months. Bare metal gets a primer before the topcoats.
COLORSTEEL is ZINCALUME steel that's pre-treated, primed and painted at the factory. NZ Steel says it can be overpainted, generally with a water-based acrylic, and its prep for a roof more than about a year old is a wash with detergent and water. The catch: its warranty doesn't cover failure of paint added later, and air-dried paint weathers differently from the oven-baked finish. If you're unsure what you have, NZ Steel prints the COLORSTEEL name on the underside of each genuine sheet.
Make sure it really is iron
Corrugated asbestos cement, sold as Super Six, has a similar wave to corrugated iron, and painted sheets can be hard to tell apart from the ground. WorkSafe lists fibrolite sheets, which usually contain asbestos, and badly corroded metal among the brittle roofs workers shouldn't walk on. On a house built before 1 January 2000, get doubtful roofing tested before it's wire-brushed or water-blasted. Resene also says pre-1971 roofs should be tested for lead.
How a roofer paints an iron roof
Get the salt and grime off
Weathered galvanised steel can take a firm wash: Resene rates waterblasting at 3000 psi as the best prep for it. COLORSTEEL is gentler, with NZ Steel capping water-blasting at 2900 psi.
Treat red rust, and only red rust
Resene's roofing guide says hard wire brushing or sanding should be limited to red-rusted areas so the galvanising around them isn't damaged.
Swap what's past saving
Rusted screws and holed sheets are replaced before painting, because paint can't bridge a hole. Failing roof screws explains why fixings often go first.
Prime to suit the metal
Resene matches primers to the problem: Galvo One for white rust, Rust-Arrest for red rust (not on roofs that collect drinking water), and a pre-coated steel primer on COLORSTEEL.
Topcoats by the data sheet
Coats depend on the surface. Wattyl's 15-year warranty needs 2 coats of Solagard Roof on COLORSTEEL or previously painted iron, and 3 on bare galvanised iron or ZINCALUME.
What makes the job bigger or smaller?
The profile. Resene's coverage advice adds 10.5% to the flat roof area for corrugated iron and 50% for trough section roofing.
Rust and repairs. A wash and spot-prime is a far smaller job than rusty laps, tired screws and sheets to swap.
Exposure and access. Dulux recommends a primer coat especially in coastal or geothermal areas, and WorkSafe expects system-rated edge protection with infill panels on roofs pitched over 25 degrees.
A roofer can tell you which of these apply once they've seen the roof. Tell us what's going on and we'll pass it to one checked roofer nearby.
What you can check from the ground
You can
- Use binoculars to look for orange rust at the laps, around screws and along the bottom ends of sheets
- Note any ceiling stains after rain, which point to holes rather than surface rust
- Find your COLORSTEEL warranty paperwork before agreeing to paint over it
Leave to a roofer
- Rust treatment and every coat of primer and paint
- Replacing screws, sheets and flashings
- Judging whether the steel is too far gone
If it were our iron roof
We'd paint galvanised or ZINCALUME iron once it's weathering but still solid, and we'd want the quote to name the primer as well as the topcoat. A COLORSTEEL roof that's only faded we'd leave alone while it's under warranty, since NZ Steel counts uniform fading and chalking as normal wear. Where daylight shows through in several places, we'd get a re-roof priced before spending anything on paint.
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.