A profiled steel sheet is a thin steel core wrapped in a metallic layer, usually zinc, with paint on top. The wrapping protects the steel for as long as it is unbroken. A cut breaks it. Wherever a sheet has been sliced to length, notched round a pipe, or punched for a fixing, the bare steel core is left showing in a line only a millimetre or two thick. That line is the cut edge, and it is where corrosion tends to begin.
This guide covers the fault, not the product. It explains how edge rust forms, where to look for it on a unit or barn, which repairs hold and which only hide the problem, and when the sheet is past saving. For the sheets themselves, see our guide to metal roof sheets for commercial units. The wider picture of re-covering and repair sits on our commercial roofing page.
What a cut edge is, and why bare steel rusts first
Coated roofing steel is built in layers. At the centre is the steel, which carries the load. Over it sits a thin film of zinc, applied in the factory, and over the zinc goes a primer and a coloured topcoat. Rain meeting the surface meets paint first. Only at a cut or a scratch can water reach the zinc and the steel together.
Zinc does a useful second job. It corrodes in preference to steel, so a small exposed edge is partly protected by the zinc beside it, which slowly gives itself up to keep the steel sound. That sacrificial effect is the reason a freshly cut sheet does not rust overnight. It is also why the damage creeps. As the zinc beside the edge is used up, the paint above it loses its grip, lifts, and lets water in a little further along. The affected band widens from the edge inward.
Creep under the paint
Surveyors call that spreading band creep. You can see it as a line of blistered or flaking paint running parallel to the cut, often with a thin brown stain on the face below. The steel under the blister is already rusting, because rust takes up more space than the metal it came from and pushes the coating off the sheet as it grows.
Site cuts and swarf
A sheet cut on site with an abrasive disc is less forgiving than one cut in the works. The disc burns the coating back from the line and throws hot steel particles that settle on the sheet and rust within days. Many rust spots on a young roof are scattered swarf from the day it was built.
Where edge rust starts on an industrial roof
A roof is full of edges. Some are exposed to sitting water and some are not, and the difference decides which ones fail first.
End laps and side laps
Running up the slope, each sheet is laid so that its lower edge sits over the top edge of the sheet below. If the lap is clean, it sheds water. If debris, moss or windblown grit collects in it, the lap holds moisture against both edges, and steel that never dries rusts steadily. Low-pitched roofs suffer most, because water leaves them slowly.
Eaves, gutters and valleys
The bottom edge of each sheet overhangs the gutter. Water drips from that edge all year, and any fault in the coating there gets a continuous soaking. Valley gutters and box gutters, where sheets are cut to meet a lined channel, add a second line of exposed edges to the same wet spot. Our answer on box gutters is worth reading beside this one if a gutter edge is your problem.
Penetrations and rooflight surrounds
Every flue, vent pipe, extractor and plant stand needs a hole cut in the sheet, and each hole has a freshly cut perimeter. The same goes for the sheets cut back around a rooflight. These edges are often sealed with a mastic bead that has aged and split, so they turn into small basins that hold water.
Fixing holes
A screw or a rivet makes a hole through the coating, and any hole that is overdriven, driven at an angle or later enlarged by movement leaves a ring of bare metal around it. Where a roof has been re-fixed several times, the old holes remain as additional openings.
Ridges, hips and cut-back verges
Sheets trimmed to meet a ridge flashing, a hip or a verge are cut on site to whatever angle the building demands. Those long cut edges are the ones most likely to have been finished without a touch-up coat.
Conditions that speed corrosion up
An edge does not rust at the same rate on every building. The cut is the starting point; the conditions around it set the pace.
Standing water and slow drying
Anything that keeps a cut edge wet for long periods works against it. Ponding behind a ridge upstand, leaves piled at a valley, a gutter overflowing across the sheet face, or a lap blocked with moss all do the same job. Where sheets run level with, or slightly below, a parapet or adjoining wall, rain and run-off arrive with nowhere to go.
Condensation from below
Corrosion does not always begin on the weather side. In a cold, single-skin shed, warm damp air from the building condenses on the underside of the sheets, and the water collects at the laps and fixings. Roofs over wash bays, food processing, plating shops or livestock housing are the usual victims. The underside of a roof that looks fine from outside can be rusting at every lap.
Dissimilar metals
Water running off a copper, lead or brass component and across a zinc-coated sheet carries traces of that metal with it and can speed up attack on the zinc. Trim, flashing and fixings need to be chosen to suit the sheet, not simply to fit.
Telling edge corrosion from surface weathering
Not every discoloured sheet is rotting. Before any treatment is priced, the first task is to separate three things that look alike from the ground.
| What you see | What it usually is | What it needs |
|---|---|---|
| Faded, chalky paint across the whole sheet | Weathering of the topcoat | Cleaning, then a decision on recoating at the next planned visit |
| Orange-brown staining below a fixing or flashing | Rust at a hole or a trim edge | Local treatment and a new seal |
| Blistering or lifting paint along a cut or lap | Active creep from the edge | Cut back, treat and recoat the edge band |
| Pinprick holes or soft, flaking metal | Section loss in the steel | Patch or replace the sheet |
The screwdriver test
On site we tap and probe suspect areas. Surface rust is loose and powdery and comes off with a wire brush, leaving sound grey metal. Section loss is different: the steel flakes in layers, the edge crumbles, or a screwdriver tip pushes through. The two get different repairs, and one mistaken for the other is how roofs get painted over and fail within a season.
Look at the underside too
Where the building allows, we check the inside face from a mezzanine or gantry. Rust that has reached the underside at a lap line tells us the steel is further gone than the top surface suggests.
Treatment systems that hold, in the order they are applied
A lasting repair is about preparation. The coating that goes on last matters less than what is under it. The sequence below is how we set out an edge repair on a roof whose steel is still good.
Step one: clear and clean
Debris is removed from laps, valleys and gutters, and the sheet is washed so that the real condition of the coating can be seen. Moss and dirt left in place under a new coat will push it off again.
Step two: cut back and abrade
Loose, blistered paint is scraped back until the edge of sound coating is reached. Rust is wire-brushed or ground off to bright metal where that can be done safely. A feathered edge between sound paint and bare metal, rather than a hard step, gives the new coat something to grip.
Step three: treat and prime
Any remaining rust is treated with a rust converter or inhibitor, then the bare band is primed with a zinc-rich primer, which restores a layer of sacrificial protection. Skipping the primer and painting straight over a converter is one of the shortcuts that costs a roof its repair.
Step four: seal laps and fixings
Failed mastic is removed, not covered. New sealant is applied to clean, dry laps and around fixings, using a product that stays flexible, because a rigid seal across a lap that moves with the temperature will crack.
Step five: build the topcoat
An edge-specific coating, often a reinforced paste or a liquid system, is applied over the primed band, extending a short way onto sound paint on each side. Where the whole roof is weathered, the edge repair can be the first stage of a full recoat. We cover that question in our answer on liquid roof coatings.
A treatment is only as good as the dry window it is applied in. Primer and sealant laid on damp steel look fine on the day and let go in the first cold snap.
When treatment stops being the answer
Treatment suits a roof where the steel is intact and the damage is confined to the coating at the edges. Past a certain point, it is the wrong spend.
Section loss and holes
Once rust has eaten through the sheet, coatings only hide the problem. A small hole can be patched with a fixed plate or a replacement panel section. Widespread perforation along laps means the sheet has lost its strength, and fixings begin to pull through.
Fixings that no longer hold
Rust at a fixing hole grows the hole. Sheets that rattle, lift at the ridge or show elongated holes around screws have a fixing problem as well as a coating one. Re-fixing into a corroded sheet seldom works.
Roofs with edge rust everywhere
If every lap across a whole slope shows the problem, the sheeting has likely run its course. The better route is usually a clear choice between recovering, overcladding or replacing, and our guide to fragile roofs and safe access matters here, because heavily corroded sheets must be treated as fragile before anyone walks on them.
Old sheets that may contain asbestos
Older cement-based sheeting is a different material with its own rules. Cement-based sheeting is not metal and needs a different approach. We identify an unknown sheet before touching it, and HSE guidance on asbestos cement sets out what applies if it is carefully removed or substantially broken.
Cost and quoting
Sheet and repair prices vary too much by roof for a national figure to mean much, so we price edge treatment on survey. For context, MyBuilder's 2026 guide puts a roof inspection and report at about £150 to £500, depending on whether it uses a standard visit, a drone or thermal imaging. Once we have surveyed, the quote arrives covering access, preparation, primer, coating, labour and disposal.
Keeping a metal roof from rusting at the edges again
Prevention here is dull work done on a schedule. Because edge rust grows from places that stay wet, most of the job is keeping those places dry and visible.
Clear the laps and valleys
Moss, leaves and grit are removed from laps, valley gutters and the area behind any upstand. It is the most useful task on the list, and the least costly.
Treat site cuts straight away
When anyone cuts into a sheet to fit a flue, an extractor or a solar mounting, the new edge needs a primer and a topcoat the same day. Many costly corrosion problems begin with a contractor who cut a hole and left.
Fit a programme to the building
Where a roof has rusting edges, the sensible approach is to treat the worst, record the rest, and look again at set intervals. Our guide to built-up felt roofs covers the other common flat-roof system on estates where sheet and felt buildings sit side by side.
Our work carries a 10-year workmanship guarantee, written down. We photograph your own roof when we survey it, and leave the site tidy. Ring 01453 933613, send a WhatsApp, or fill in the enquiry form with a few pictures of the worst laps and edges.