What is cut edge corrosion, and why does it spread?

Profiled metal cladding and roof sheets come with a factory coating on both faces. When a sheet is cut to length, the cut edge has no coating on it: it is bare steel. On a wall or a roof those cut edges sit at the laps where one sheet overlaps the next, and at the eaves and gutters where water collects. Water sits against the bare edge, the steel rusts, and the rust works its way under the factory coating.

Rust takes up more space than the steel it came from, so as it grows it pushes the coating up from underneath. That lifted coating lets more water in, which rusts more steel, which lifts more coating. It is a loop, and it only runs one way. What starts as a brown line along a lap becomes flaking paint, then a rusted band, then a hole. On a roof that means leaks. On cladding it means panels that look far worse than the rest of the building and eventually fail.

It also matters for warranties. Corrosion left untreated can void the warranty on the cladding itself, which is another reason to deal with it while it is still small.

The fix is to get back to sound metal and keep water off it. We strip the corroded edge back, treat the bare steel, and seal every exposed edge so the rust stops returning. Where the roof or elevation needs it, a protective coating system then goes over the whole surface, which protects the treated edges and brings the colour back at the same time. Roof coating systems carry manufacturer warranties of fifteen years and up.

As soon as you see it. A brown line along the sheet laps is the cheapest version of this problem you will ever have. An annual roof inspection catches it early; we carry those out with a written report so you know what needs doing and when.