Roof Rust Treatment NZ: Treat, Paint or Replace?

Quick answerLight surface rust on a metal roof can usually be treated and painted: the rust and loose paint are removed, the roof is washed, bare and rusted spots are primed and two topcoats go on. Once rust has pitted deeply, flaked in layers or eaten through the sheet, paint will not hold it back, and replacing the affected sheets or the whole roof is the better answer.

Weathered metal roof being washed before rust treatment and painting

Rust on a roof rarely appears all at once. It starts as a few orange spots around fixings or along the laps, spreads under failing paint and, left alone, works its way through the steel. Caught early, roof rust treatment and a good paint system can add years to the roof. Caught late, the same money is better spent on new steel.

This guide explains why metal roofs rust, how treatment and painting work, the signs that rust has gone too far, why the hidden parts of a roof rust first near the sea, and what to expect from a painted roof afterwards. Our roof painting service page covers the full painting process.

Why is my metal roof rusting?

Because the layers that protect the steel have worn through. Roofing steel is coated with zinc or a zinc and aluminium alloy, and pre-painted roofing adds a paint layer on top. Weather, salt and dirt slowly wear those layers away. Once the metallic coating is used up in a spot, the steel underneath starts to rust.

Some places go first. Laps where sheets overlap hold moisture, fixings and cut edges expose the steel, and scratches from foot traffic or dropped tools break the coating. Older galvanised roofs often show white, powdery corrosion before red rust appears, which is the zinc itself corroding. Areas the rain never rinses, such as under flashings and around flues, also corrode faster because salt and grime stay on the surface.

Can a rusty roof be treated and painted?

Yes, if the rust is on the surface and the steel is still solid. Paint makers set out a clear sequence for an old galvanised roof with flaking paint and red rust. Resene's system, for example, starts with killing moss and mould, scrubbing or water blasting the roof, and scraping off loose paint and corrosion. Bare areas are then primed with a primer made for galvanised steel, lightly rusted areas get a second coat of primer, and two coats of roof paint go on top.

Preparation is where a roof rust treatment succeeds or fails. Paint over loose rust, salt or chalky old paint and it will lift again, often within a few seasons. Near the sea, salt needs rinsing off between coats so it is not sealed under the new paint. Rusted screws should be replaced, not painted, and any badly corroded flashing replaced before painting starts.

One safety point: decramastic and some older roofs can contain asbestos, and WorkSafe warns against water blasting them before they are checked. Test first if you are not sure what your roof is made of.

When is rust too far gone to paint?

When the steel itself has lost strength. Resene's own advice is that if a roof is badly corroded, replacement is often a better option, either the whole roof or, in some cases, individual sheets. Paint protects sound steel; it cannot rebuild metal that has rusted away.

The table below is a practical guide to what each stage usually needs. A close look from the roof, and from the ceiling space where you can see the underside of the sheets, decides which stage you are at. Where only a few sheets are gone, roof repairs that swap those sheets can let the rest of the roof be painted. Where rust runs through most sheets, a roof replacement ends the cycle.

What you see	What it usually means	Typical answer
White powder or light orange spotting	Coating wearing, steel sound	Wash, treat, prime and paint
Rust around screws and along laps	Local corrosion at weak points	New screws, spot treatment, then paint
Flaking rust in layers, deep pitting	Steel losing thickness	Replace affected sheets, paint the rest if sound
Holes, rust visible from the ceiling space	Steel rusted through	Replace sheets or the whole roof

Paint it or replace it?

Cheeky's Roofers checks how far the rust has gone and gives you a written quote with the paint and replacement options side by side.

0800 766 342 Book Online Get a quote

Why do roofs rust first under eaves and near the sea?

Because rain does not reach those surfaces, so salt and dirt build up and keep them damp. New Zealand Steel calls these unwashed areas and lists soffits, the undersides of gutters, fascias, wall cladding under eaves and sheltered areas around garage doors among them. On the open roof, rain does the washing; in these spots, nobody does unless the owner does.

Salt makes it worse. Under NZS 3604, the high corrosion zone takes in land within 500 metres of the sea, and that includes harbours. Much of Wellington's harbour edge and south coast is likely to fall in the higher zones, from Island Bay and Lyall Bay to Eastbourne and the Petone foreshore, though it is decided address by address, so check yours on BRANZ Maps.

If you live in those areas, look under the eaves and along the gutter line before you judge the roof by its top surface. That is often where rust shows up first. Our page for roofers in the Eastern and Southern Suburbs covers the coastal suburbs we work in.

How long does a painted rusty roof last?

It depends on four things, so be wary of anyone who quotes a fixed number of years. The first is how much rust there was and how well it was removed. The second is the paint system and whether it was applied in the right conditions. The third is the environment: a sheltered inland roof is far easier on paint than one facing salt spray. The fourth is maintenance afterwards.

New Zealand Steel notes that continued maintenance and over-painting will greatly extend the life of steel roofing. In practice that means washing the unwashed areas regularly, keeping gutters clear so water does not sit against the roof edge, and touching up scratches before they rust. A painted roof that is looked after will outlast an identical one that is ignored.

Can I treat roof rust myself?

Small jobs at ground level, such as washing soffits and touching up a fascia, are reasonable DIY. Work on the roof itself is a different matter. WorkSafe's guidance expects edge protection or scaffolding for roof work, metal roofs are slippery when wet or dewy, and water blasting from a ladder is a common way to get hurt.

There is also the quality question. Treatment only works if the rust is fully removed and the right primer is used, and a missed patch under fresh paint will keep spreading out of sight. If you do want to wash the roof yourself, our roof cleaning service page explains what we use and why.

When should you call a roofer about rust?

As soon as you see red rust, because the cost of treating it rises as it spreads. Spots around a few screws are a small job; rust through several sheets is a much bigger one.

Cheeky's Roofers inspects the roof and the ceiling space, tells you honestly whether paint will hold or whether sheets need replacing, and puts the options side by side in a written quote. The measure and quote are free. You can book online and we will arrange a time that suits.

Frequently Asked Questions

Is rust converter enough on a roof?

Rust converter can help on small, sound areas, but it is not a substitute for removing loose rust and priming with a product made for galvanised steel. On a roof, follow the paint maker's full system so the primer and topcoats bond properly and the warranty on the paint is not affected.

Can I paint new COLORSTEEL to stop it rusting?

New pre-painted steel does not need painting to protect it, and painting it too early can cause adhesion problems. Paint makers advise that new pre-painted steel is lightly sanded or left to weather before painting. Regular washing of unwashed areas is what protects it.

Why are my roofing screws rusting before the roof?

Screws are a small target with a thin coating, and the washer area holds moisture. If the screws are a lower durability class than the roofing, or the roof is near the sea, they can rust well before the sheets. Replacing them with screws rated for the environment is a simple fix.

Does lichen cause rust on a metal roof?

Lichen and moss hold moisture against the coating and can damage it as they grow, which gives corrosion a head start. Treat the growth and wash the roof before painting, and keep on top of it afterwards, especially on shaded faces that stay damp.

Will my insurance pay for a rusty roof?

Rust is normally treated as gradual deterioration, which house insurance policies commonly exclude. Damage from a sudden event, such as a storm lifting rusted sheets, may be assessed differently, but the insurer will look at the roof's condition. Check your own policy wording.