A profiled metal roof looks like a single skin of colour, but the colour is only a thin coating on steel. Wherever a sheet has been cut, the coating stops and bare steel is left at the edge. Moisture sits against that line, rust begins, and over the seasons it creeps in beneath the paint. That is cut-edge corrosion. It is the most common reason a sound-looking metal-clad unit starts to stain, flake and eventually leak, and it often appears first at laps, at the eaves and around rooflights, where water lingers longest. The early stages are slow and treatable. The later stages are not, and a sheet that has rusted through is fragile in more than one sense. This guide explains the cause, the warning signs, the treatments from a simple edge repair to re-sheeting, and how the choice is made. It is written for the owner or manager of a metal-clad industrial unit, barn or garage.
Why cut edges rust first
Most profiled roof sheets are steel with a zinc layer and a paint or plastisol finish on top. In the factory the whole coil is coated before it is formed and cut. Every edge made after that point, whether at the factory or on site, exposes a sliver of steel and the layers above it. The exposed band is tiny, but it is the one place where water can reach the metal directly.
Once rust forms at the edge it swells, lifts the coating and opens a wider path for moisture. The corrosion then spreads sideways under the paint, away from the cut. So a roof may seem intact from the car park while a stripe of rust runs along every lap. By the time brown staining shows, the damage is usually already beneath the surface.
Insulated composite panels, which sandwich a foam or mineral core between two metal skins, have the same weakness. A cut end exposes the steel and, with some panels, the core, so the seal at the end of the panel matters as much as the paint.
Edges are not all equally at risk. The worst are those that stay wet or hold dirt: sheet ends at the eaves, laps where the overlap traps a thin film of water, edges that sit in a gutter, and cut openings for rooflights, flues and vents. Anything that holds leaves, moss or bird droppings against the metal makes it worse, because the debris stays damp long after the rain has stopped.
What it looks like and where to check
You do not need to climb to spot the first signs. From the ground, or from a first-floor window, look along the roof for these:
- Orange or brown staining along the lines where sheets overlap.
- Streaks running down the face of the sheet below a cut edge or a fixing.
- Blistered, bubbled or peeling paint near the eaves.
- Rust-coloured water marks on the wall or the ground under the gutter.
- Chalky white powder next to the rust, which is zinc breaking down.
Inside, look at the underside of the sheets on a dry day. Rust, pinpricks of daylight or stained insulation behind the liner tell you more than the top surface can. A drip that appears only in heavy rain often points to a failed lap, while a steady one points to a sheet that has lost its metal.
Fixings tell their own story. A screw head ringed with rust, or a sealing washer that has perished and split, lets water run down the shank into the purlin below. In time the timber or steel beneath decays too, so a roof that looks only mildly stained can hide fixings that no longer hold. Checking a sample of them by hand is a routine part of a survey, and it often decides between a local repair and a wider one.
The places we examine most closely on a survey are the eaves and the gutter line, the laps and sidelaps, every penetration through the roof and each rooflight. Rooflights matter twice over, because the sheets around them are cut, and because the rooflight itself may be fragile. Our safety guide on fragile roof safety explains why nobody should walk across a corroded sheet to look at it.
Stages of damage, and what each one means
Corrosion moves through recognisable stages, and the right treatment depends on where a roof has got to.
Surface staining
The coating is intact over most of the sheet, and the rust is a thin line at the cut. The steel is still strong. This is the cheapest point at which to act, because the edge can be cleaned and sealed.
Creep under the paint
The coating has lifted along the edge and the rust has crept inwards. Metal is being lost, though the sheet is still sound. Treatment here means cutting back the loose coating, removing rust and applying a protective system to a wider band.
Section loss
The steel itself is thinning. It may flake, form scale or show pits. A sheet in this condition can often still keep out the rain, but the fixings are losing their grip and the sheet will not carry a person.
Perforation
Holes have formed. Water now enters directly, the insulation below is getting wet and the sheet is no longer a repair candidate. The last two stages are when owners usually call, and they are the reason an early look pays off.
The stage also tells us how far the problem has spread. A few isolated laps at the eaves point to a local fix. Rust along every lap across several bays points to a roof whose coating has run its course, which is a conversation about re-sheeting rather than patching.
Treatments, from a brush to new sheets
The treatments sit on a ladder, and the best choice is the lowest rung that will last.
Clean, treat and seal the edge. The rust is removed back to bright metal with a wire brush or a mechanical tool, loose paint is cut back to a sound edge, and the bare steel is primed and coated with a paint system made for metal roofs. Where a lap is open, a sealant or tape is added. This is the right treatment for surface staining and light creep, provided the edge is dry and the work is done in suitable weather.
Replace individual sheets and fixings. Where a few sheets have lost too much metal, they can be replaced, and corroded fixings and sealing washers renewed at the same time. The new sheet should match the profile and gauge of the old, or it will not lap properly.
Apply a coating over the whole roof. A full coating can slow weathering on a roof that is still sound but ageing. It is no cure for sheets that have lost their strength, and it needs a clean, rust-free base to bond to. Our note on commercial roof coatings sets out what a coating can and cannot do.
Overlay with a new sheet. A new profiled sheet can sometimes be fixed over the old, which saves stripping and disposal. It adds weight and it relies on the existing purlins (the horizontal beams the sheets are fixed to) and fixings being sound.
Strip and re-sheet. Where section loss runs across most of the roof, new sheets are the sensible answer. It is also the moment to renew the insulation, the flashings at the ridge and the rooflights, so that the new sheets are not fixed beside tired details that will fail next. If the old sheets are asbestos cement rather than steel, they follow different rules, dealt with in our asbestos page for industrial roofs.
How long metal sheets last, and what shortens it
Published lifespans for coated steel sheet vary with the finish and the setting. The supplier Cladco, selling PVC plastisol coated sheet, describes 15 to 25 years to first maintenance and an overall life in excess of 40 years in typical environments. MyBuilder's garage roof guide (2026) says metal roofs last 40 to 70 years, while the trade body CORC says 50 years or more. All of those assume correct installation and upkeep. Treat them as typical ranges that rise or fall with exposure, workmanship and upkeep, never as a promise.
What shortens a life on the ground is rarely mysterious. Coastal air and industrial emissions attack coatings faster. Standing water, blocked gutters and moss hold moisture against the edges. Site cutting without sealing the new edges, and drilling through the sheet in the wrong place for a new fixing or service, create fresh exposed steel. Contact between different metals, such as a copper pipe above a steel sheet, can speed corrosion where water runs from one to the other.
Installation habits matter as well. Sheets cut on the roof with an abrasive disc throw hot particles onto the surface, and those particles can embed in the coating and rust in spots well away from any edge. Cutting with shears or a nibbler avoids that, and sealing each new edge before the sheet goes down costs very little compared with treating rust later.
Maintenance is therefore the cheapest lever an owner has. Clearing gutters and valleys, sweeping debris from laps and checking the edges each year, ideally from a safe position, catches trouble at the first stage. A planned approach is covered by our guide to commercial roof maintenance.
Deciding between repair and re-sheeting
The decision turns on three questions: how much metal is left in the sheets, how widespread the rust is, and what the roof will be asked to do in the next ten years or so. A building with a short lease or a planned redevelopment may justify a limited repair. A building that will stay in use for a long time is usually better served by a lasting fix. The questions are set out further in deciding whether a tired commercial roof is worth repairing, and the pricing side is in our guide to commercial roof costs.
On a survey we look at the roof from several angles. We examine the edges and laps at close range, test the fixings, check inside for stained insulation and daylight, and photograph the roof so you can see exactly what we saw. The written price that follows is fixed, and it covers the option that fits the condition we found. Where a repair works, the repair is what goes on paper. The detail of our wider commercial service is on the commercial roofing page.