The short answer
Cut-edge corrosion is rust that begins at the cut or drilled edge of a coated steel roof sheet, where the protective coating no longer covers the steel. It starts at sheet ends, laps, rooflight surrounds and fixing holes, creeps inwards under the paint and eventually breaks through. Light cases can be cleaned and treated, while advanced ones mean replacing the sheets.
What cut-edge corrosion is
Profiled metal roofing and cladding is made from steel sheet that carries a protective coating on both faces, typically a metallic layer topped with paint. While that coating is unbroken, the steel inside is sealed from air and water. The system works because the coating is continuous.
Cutting breaks the continuity. When a sheet is cut to length, notched round a rooflight or drilled for a fixing, the steel core is exposed at the edge. Moisture reaches it, and rust begins. Because the edge is a thin line, the early signs are easy to miss, but the process then spreads.
Rust takes up more space than steel. As it forms it lifts the paint, and moisture can then travel under the coating from the edge, so the corrosion creeps inwards along the face of the sheet. The paint blisters and flakes, and bare steel is revealed.
The term cut-edge corrosion describes this particular pattern, where the damage is traced to the edge of a cut. It is different from general weathering of the coating across the whole sheet, and from corrosion caused by contact with other metals or by chemicals. Telling them apart matters, because the treatment is different.
How fast it spreads depends on the environment, the coating and the detail, so each roof differs.
Where it starts on a profiled sheet roof
A sheet roof has many cut edges, and some places suffer more than others. A survey walks the roof and looks at each in turn.
Sheet ends at the eaves and ridge are the most exposed. The cut end sits where water collects and runs off, and the edge is in constant contact with moisture. Overlaps, where one sheet laps another, trap water and dirt in the joint and keep the edge damp.
Cut-outs around rooflights, flues and plant penetrations are a second group. They are cut on site, often by hand, and the edges may be rough or left unsealed.
Fixing holes are the third. Every fixing that passes through a sheet makes a hole, and a fixing that is overdriven or loose lets water in around its edge. Corrosion starts around the washer and spreads outwards.
Look also at the gutter line, since the sheet end that overhangs it often sits in standing water, and at the junctions with walls and flashings.
Streaks of brown rust below the lap lines, paint lifting along the sheet ends and holes forming around fixings are the usual signs. Our guide to an industrial unit roof leaking covers how leaks from such points are traced.
Treat it or replace the sheets
The choice depends on how far the corrosion has gone, and the survey has to establish that.
Where the damage is light, confined to the paint and the surface of the steel, it can often be treated. The roofer removes loose paint and rust back to sound metal, applies a rust-inhibiting primer to the exposed steel and then a compatible finishing coat. Edges can be sealed with a coating, and fixings that are loose or failing are renewed. The aim is to restore the barrier and slow further decay.
Treatment has limits. It works only on steel that is still sound, and it needs the surface to be dry and the weather suitable. Done properly, it extends the life of a roof for a period, but it does not make the sheet new, and it needs to be checked in later years.
Where rust has eaten through the steel, holes have formed or the sheet is thin and flaky at the edges, treatment is no longer adequate and the sheets have to be replaced. A roof with widespread corrosion across many sheets may be better recoated or overclad, or replaced as a whole.
Our answer on metal roof sheet replacement covers the replacement route, and the one on liquid roof coatings covers recoating.
What we look at, and what you receive
On a survey, we walk the roof using safe access, as many sheet roofs are fragile, and photograph the edges, laps and fixings in each area. We record the extent of corrosion, whether it is at the surface or has gone through, and any signs of leakage below.
We look at the cause as well as the effect. Standing water at the eaves, a blocked gutter, poorly sealed cut-outs and over-tightened fixings all feed the problem, and a repair that treats the rust without fixing the cause will not last.
If the building is old, we also look for materials that might contain asbestos before anything is cut or scraped, and recommend a survey where that is needed. Scraping or sanding an unidentified sheet is not a safe way to find out.
The report sets out what was found, with images, and offers the options: treatment, partial replacement or full renewal. A written price follows for whichever route you choose, based on the evidence.
How long a treatment lasts depends on the roof and its setting. The commercial roofing hub lists the services we offer for industrial and commercial buildings.
