Why iron roofs rust where they do
Most iron roofs in New Zealand are steel with a metallic coating, either zinc (galvanised) or an aluminium-zinc alloy like ZINCALUME. NZ Steel explains that on galvanised steel the zinc corrodes first to protect the steel underneath. Red rust is the sign that protection has run out in that spot.
So rust starts where the coating and paint get the hardest time: at the laps where sheets overlap, around nail and screw holes, at cut edges, and under flashings where water lingers.
Parts the rain doesn't wash, like overhangs, unlined soffits and canopies, collect salt and dust and corrode faster, especially near the sea. On a building within 500 m of the Auckland harbour, BRANZ found sheltered spots collected about 3 to 8 times more salt than exposed ones.
The culprits people don't expect
Swarf. The tiny steel filings thrown off by drilling and cutting rust quickly and leave orange spots on the roof. The metal roofing Code of Practice says cladding should be cut with shears, not friction or high-speed blades, partly because those throw hot swarf. Dimond says swarf should be swept or hosed off at least at the end of each day, and NZ Steel's COLORSTEEL warranty excludes damage from swarf left on the roof.
Copper and lead. NZ Steel says copper isn't compatible with galvanised, ZINCALUME or COLORSTEEL roofing, especially where water flows from copper onto them. Its simple rule: you can run water from zinc onto copper, but not from copper onto zinc. Lead flashings, or water running off lead, cause corrosion too.
Black pencil marks. NZMRM says a lead pencil should never be used to mark coated or pre-painted steel, because the carbon promotes corrosion that leaves a permanent mark.
Geothermal air. In Rotorua, BRANZ found aluminium-zinc coated steel showed only small rust spots after 2 years at its geothermal test sites, but heavy rust close to unprotected cut edges.
Reading the rust from the ground
| What it usually means | Usual fix | |
|---|---|---|
| White, powdery deposits on bare metal | The zinc coating weathering, normal on its own | Keep an eye on it, wash unwashed areas |
| Small orange spots across a newer roof | Often swarf left after installation | Ask the installer to clean and assess it |
| Rust streaks below nails or screws | Fixings or washers failing | Replace the fixings, treat and paint around them |
| Rust along the laps | Water sitting between sheets | Treat if the steel is firm, replace sheets if it's flaking |
| Rust under eaves and overhangs | Unwashed area collecting salt | Clean, treat and repaint, then wash it regularly |
| Holes or flaking steel | The steel is gone | Replace the sheets, or re-roof if it's widespread |
Rusted iron isn't safe to stand on
WorkSafe lists badly corroded metal roofing as a brittle roof that workers shouldn't walk on, alongside fibrolite and clear roofing. Stay off it, and keep kids and pets off it too. If the roof went on before 1971, Resene says it should be tested for lead paint before anyone sands or wire-brushes it. And in a home built before 2000, other roofing such as fibrolite or Super Six sheets may contain asbestos, so don't cut, drill or water-blast anything you're unsure about.
Treat, replace sheets or re-roof?
Treat and paint when the rust is light and the steel underneath is solid. 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, and prepared spots must be primed promptly, because overnight dew lifts the paint edges. Our page on painting an iron roof covers the full job.
Replace sheets when rust has gone through on a handful of sheets, often near a valley, a flue or the spouting (gutters). The new sheets need to be compatible with the old roofing and anything they touch.
Re-roof when rust is widespread. Resene itself says that if a roof is badly corroded, replacing the whole roof or individual sheets is often a better option than painting. Our guide to repainting or re-roofing an iron roof helps weigh it up.
Does replacing rusty iron need a consent?
Usually not, if it's like for like and the old iron has done its time. MBIE's own example of exempt work is replacing 20-year-old profiled metal roofing that has lasted more than 15 years with a comparable product. Wairoa District Council says the same about new corrugated iron replacing old with no change to the purlins.
The catch is roofing that failed early. The Schedule 1 exemption doesn't cover replacing something that failed the Building Code's durability rules, so iron that rusted through within 15 years can need a consent to replace. Changing the purlins or the roofing system needs one too. Ask your roofer, and check with the council if you're unsure.
How to check your roof's rust without going up
Pick a dry, bright day
Rust and streaks are easiest to see in good light. Use binoculars from the street, the back of the section or an upstairs window.
Work across the roof in order
Look at the laps, the lines of fixings, the ridge, the valleys, and around chimneys, flues and aerials.
Look underneath the edges
From the ground, check the underside of the eaves, the spouting and any canopies. These unwashed spots often rust first.
Check inside the roof space
With a torch, look for pinpricks of daylight, rust flakes on the insulation and water marks on the timber. Step only on the joists.
Check the downpipe outlets
Orange staining or rust flakes washing out are a sign something above is breaking down.
Take dated photos
Photos from the same spots each year show whether the rust is spreading, and they help a roofer quote.
What you can do, and what to leave
You can
- Look over the roof with binoculars and keep dated photos
- Hose salt off soffits and gutter undersides you can reach from the ground
- Make sure copper pipes or lead flashings don't drain onto the roof
- Keep trees trimmed back so leaves don't sit in the spouting
- Check the roof space with a torch after heavy rain
Leave to a roofer
- Anything on the roof, rusty or not
- Rust treatment, priming and painting
- Replacing rusted sheets and failed fixings
- Testing old paint for lead before preparation
- A re-roof, and any consent it needs
Questions to ask the roofer about rust
- Is the steel still sound under the rust, or has it gone through?
- Can you send me photos of the laps, fixings and worst sheets?
- If you replace sheets, will the new ones be compatible with what's already there?
- What exposure zone is my house in, and does that change the product you'd use?
- How will you cut sheets and clear swarf?
- Could any of this roof contain lead paint or asbestos, and how will you check?
- Does this job need a building consent?
Our advice on a rusty iron roof
Catch it early. Light rust treated and painted this summer is a modest job, and the same rust left for a few years can mean new sheets. If rust has gone through in more than a few places, we'd stop treating spots, get photos from a roofer and price a re-roof. And on a newer roof with orange spots, we'd ask the installer about swarf before assuming the worst.
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.