Rusted roof repairs, sheet by sheet

Rusted roof repairs start with working out how far the corrosion has gone. We check every lap, edge and fixing, then treat what can be saved and replace what cannot.

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Central Auckland based
Written scope before work starts
Photos of what we find

Where rust starts on a metal roof

Modern roofing steel is protected by a metallic coating and, on prefinished sheets, a paint layer on top. Rust appears when that protection wears through. It almost never fails evenly. It starts in a few predictable places and spreads from there.

Cut edges at the bottom of sheets, side and end laps, the area around each fixing, and anywhere water and dirt sit for long periods are the usual suspects. On commercial roofs, add the spots under air conditioning units, around flues and anywhere metal filings from drilling were left on the surface. Those small filings rust within days and stain the sheet around them.

Close view of old grey corrugated roof sheets where they overlap

The weak points we check

A repair only lasts if these are dealt with, not just the visible patch.

Cut edgesSheet ends at the gutter line are exposed steel once cut. They are often the first place rust shows.
LapsWater and salt are held between overlapping sheets. The rust is often worse inside the lap than it looks from outside.
FixingsOld screws and nails rust around the head and the hole, and a failed washer lets water sit against the steel.
Unwashed areasParts of the roof sheltered from rain, such as under overhangs or behind parapets, collect salt and grime that rain never rinses off.
Dissimilar metalsCopper, lead and some other metals can speed up corrosion of coated steel when water runs from one onto the other. Copper pipe run-off is a common cause.
Debris and pondingLeaves, dirt and standing water hold moisture against the coating. Gutters and low pitch areas suffer most.

Treat or replace. How we decide

We sort each affected area into one of three groups. Light surface rust, where the steel underneath is still solid, can be cleaned back, primed and painted with a roofing coating system. Moderate rust confined to the end of a sheet can sometimes be dealt with by cutting back and fitting a new section with a proper lap. Rust that has gone through the sheet, or is flaking in layers, means that sheet needs to come out.

We also look at how many sheets are in each group. A few replacement sheets in a sound roof is a sensible repair. Rust through across large parts of the roof means patching will become an ongoing cost, and we will tell you that honestly.

Corrugated metal roof close up under a cloudy sky

Rust condition and repair options

Condition	What you see	Usual repair
Light	Surface rust spots, coating faded or chalky, metal still firm	Clean back, prime and coat
Moderate	Rust concentrated at laps or sheet ends, no holes yet	Treat, replace fixings, or cut back and lap in new metal
Severe	Holes, flaking layers, soft metal, rust visible from inside	Replace the affected sheets

Doing rusted roof repairs properly

Replacement sheets are matched to the existing profile and, where possible, the same coating type. Sheets are lapped in the right direction for the prevailing weather, fixed with new screws and washers, and any filings are swept off before we leave. Leaving swarf on a new sheet is one of the quickest ways to start rust again.

Where rust has opened holes and water has been getting in, we often find wet insulation or damaged sarking below. We will show you photos and include it in the scope if it needs attention. If you have a leak but are not sure rust is the cause, our commercial roof leak detection process will confirm it first.

On large industrial roofs, rust work often goes hand in hand with other maintenance. Our warehouse roof repairs page covers how we plan work around stock and operations.

Slowing rust down after the repair

Washing areas of the roof that rain does not reach, keeping gutters and valleys clear, and checking around fixings and laps once or twice a year all help a metal roof last longer. This matters most on buildings near the coast, where salt builds up quickly. Finding light rust early keeps the repair at the clean and coat stage instead of replacing sheets.

Rusted roof repairs questions

Can rusted roof sheets be painted over?

Surface rust on an otherwise sound sheet can be treated and coated. The rust has to be removed back to clean metal, primed and then painted with a system made for roofing. Paint over flaking rust or holes does not last.

Why does rust start at the laps first?

Water and salt sit longer between overlapping sheets, and that area does not get rinsed by rain. The coating there stays wet for longer, so it breaks down sooner than on the open sheet.

How do I know if a sheet needs replacing?

If the metal is perforated, flaking in layers, or soft when pressed with a screwdriver, it is past treatment. We check the underside as well, because rust can be worse inside than it looks from above.

Can you match my existing roof sheets?

Often, if the profile is still made. Some older profiles and colours are no longer available, and in those cases we explain the options before ordering. New sheets will look brighter than weathered ones for a while.

Is rust worse near the coast in Auckland?

Yes. Salt carried on the wind settles on roofs and speeds up corrosion, especially on areas rain does not wash, such as under overhangs and at laps. Buildings close to either harbour or exposed to sea winds usually need closer attention.

Do rusted roof repairs need a building consent?

Replacing damaged sheets with the same or a similar product is usually maintenance and does not need consent under Schedule 1 of the Building Act. If you are changing the roofing material or its weight, check with Auckland Council first.