A coated steel roof sheet is a sandwich. The steel core is covered in a thin layer of zinc, then a primer, then a painted or plastic-type finish. Together those layers keep water away from the steel for a very long time. The trouble starts where the sheet was cut. A cut leaves a bare edge, and a bare edge has no coating to protect it.
That is cut edge corrosion. It begins as a thin orange line at the end of a sheet, a notch round a rooflight or a hole where a fixing goes through. It then spreads sideways under the paint, lifts it in blisters and flakes, and eventually eats a hole. On a roof over stock, machinery or a shop floor, a small rusting edge can turn into a regular drip.
The fault is slow, and easy to see if you know where to look. The harder part is deciding when a treatment is worth doing and when the sheet has gone too far. Below we explain the cause, the places it shows first, what salt air changes, the treatment options and the point at which replacement becomes the sound choice.
Why a sawn edge rusts first
The factory coating works because it is continuous. Zinc does two jobs: it covers the steel like a skin, and where that skin is scratched it corrodes in preference to the steel beside it, so small damage seals itself for a while. Paint sits on top and slows everything down. Cut through all three layers and you expose a strip of steel only as thick as the sheet. In theory the zinc nearby still protects it. In practice a long-exposed edge in a wet position does not stay protected for ever.
Water makes the difference. Moisture is pulled into narrow gaps by capillary action, the same effect that draws tea up a biscuit, and the lap between two overlapping sheets is exactly such a gap. Wet, dirt and the odd drop of salt-laden rain sit in the lap, so the edge stays damp while the open face of the sheet dries in the sun. The edge rusts from underneath, where nobody can see it, and the rust is bulkier than the steel it came from. That swelling pushes the paint away and opens a channel for more water.
Cuts made on site
Factory-cut edges are usually protected before the sheet leaves the works. Edges cut on site with an angle grinder or a nibbler need care, because the heat and swarf damage the coating around the cut. Iron filings left on the roof surface rust and stain the sheets below them, which is why swarf is swept away at the end of a day. A roof that has been cut about without treatment of the new edges often shows rust at those very places a few seasons later.
None of this means coated steel is a poor choice. Properly detailed, it is a sound and economical covering for a unit. It means the cut edges and fixings are where we start our inspection.
Where it shows first on a unit or warehouse
Walk the perimeter of a sheeted building and the pattern repeats. Look first at the lower edge of each sheet at the eaves, where water lingers on its way to the gutter and leaves collect against the lip. Then check the end laps, where one sheet sits over another and moisture is trapped. Around every rooflight, ridge cap and flashing, the sheets have been cut to shape and the edges lie close to a seam. And at the verge, where the sheet meets the gable, a cut edge often faces driving rain.
Fixing holes come next. Every screw is a puncture, and a washer that has perished or a screw that has backed out lets water run down the thread onto the steel. Rust shows as small orange rings around the head. If the screw heads themselves are going, they have usually been wet for a long time.
Signs from the ground and from inside
You can often spot the problem without a ladder. From the yard, look for brown streaks running down the sheet from a lap or a fixing, for paint peeling in strips along an edge, and for rust-coloured staining on the wall or gutter below. Inside, torchlight on the underside of a single skin sheet shows pinpricks of daylight where rust has gone through, and orange stains on the lining or insulation show where drips have been running. Gutters hold clues as well: flakes of coating and rust in a box gutter or downpipe mean the sheets above are shedding material.
Do not go onto a rusted roof to check. Steel that has lost thickness at its edges and fixings can give way, and sheets in poor condition are fragile in the same way that old cement and rooflight sheets are. Our guide to fragile roof safety explains how access is planned.
What salt air and the Sussex coast change
Any metal roof near the sea meets more salt than one inland. The evidence we can point to comes from zinc roofing. The maker VM Zinc reports that salty air can react with an unpainted zinc surface, and that lasting staining within 1 km of the shore is likely on faces the rainwater never reaches, such as facades and soffits. HomeBuilding says much the same of marine settings: staining and corrosion of some metals is a possibility, and the boards under the eaves pick up salty deposits that rain cannot clean away.
Those pages describe zinc, not painted steel sheets, so read them as an explanation of how salt behaves rather than a rate for any other metal. The principle still carries across in plain terms. Salt settles on the roof, rain washes the exposed surfaces, and the places that stay dirty are the undersides of sheets, the insides of laps, the gaps beneath cappings and the sheltered side of a building. Those are precisely the places where a cut edge is already at risk, so a coastal site gives the problem more to work with.
What that means in practice
On the seafront and around the Adur estuary, the sheltered faces and laps are where we expect trouble to show first, and we inspect them closely. Rinsing is a habit worth borrowing. VM Zinc advises washing zinc with clean water twice a year or more to keep salt build-up down, and a gentle freshwater rinse of the undersides and sheltered edges of a coated steel roof, done on the day you clear the gutters, lifts the same salt film before it settles into an edge.
Inland sites are not immune. Dirt, leaves, bird droppings and industrial fallout hold moisture against an edge just as salt does, so the coast speeds the process up and does not create it. Our guide to types of commercial roof explains which sheet and panel types are in play.
Treating cut edge corrosion and metal roof rust
The right treatment depends on how far the rust has gone, and the survey decides it. We sort what we find into three grades.
Surface rust is orange staining and light blistering with the steel still solid. It is the best case. The edge is wire-brushed or ground back to bright metal, the dust cleaned off, a rust-inhibiting primer worked in and a compatible topcoat applied over a wide enough band to seal the edge. Compatibility matters: the new paint must suit the old coating, or it will lift within a season. Deeper rust, where the steel is flaking or pitted but not yet perforated, needs the same preparation and often a stripe coat or an edge sealant on top, and in some cases a full coating of the sheet, which is where a metal roof coating comes in.
When treatment stops being enough
Once a sheet is perforated, or the rust runs along the lap and under the fixings, treatment cannot give back the missing steel. Patching it hides the problem for a winter. The real options are to replace the worst sheets, covered in our guide to roof sheet replacement, or to overclad the whole roof if the structure and the deck suit it. A coating over widespread perforation is money wasted, and we will say so when the photographs show it.
Costs depend on how many edges are affected, the access needed, the coating system and whether sheets must come out. A figure given from a description over the phone would be a guess. We survey first and photograph each defect, then send one total price in writing. Our whole range of industrial roof work is on the commercial roofing page.
Keeping rust from coming back
Prevention is mostly housekeeping, and it starts at the gutters. Leaves and moss sitting against the lower edge of a sheet keep it wet for weeks, so clearing outlets, box gutters and the sheet edges at the eaves costs the least of any rust treatment. Birds add droppings that hold moisture and attack coatings, and trees overhanging a roof drop debris that rots into the laps.
Next comes the fixing detail. Perished washers, backed-out screws and cracked sealant at the laps and ridge let water behind the edge, so replacing them as they appear stops a small leak becoming a corroded sheet. Any new cut, whether for a flue, a vent or a new rooflight, should have its edges primed and painted at once, not left bare for a later visit that never happens.
Maintenance on a rhythm
We suggest an inspection once or twice a year, with gutters cleared and a walk round the laps, eaves and fixings. When sheets have reached the end of the line, the replacement route above applies, and the asbestos question comes next. If the roof has older cement sheets or you are not sure what the sheets are made of, read about the industrial asbestos roof before anyone drills, cuts or scrapes. HSE advises assuming asbestos is present in premises built or refurbished before 2000, and drilling a fixing hole is a form of work on the material.
A hand-over set of photographs shows the edges as they were treated, so any later change is easy to spot, and it gives whoever manages the building a dated record to check against once the winter gales and salt air have done their worst. If you manage a unit, send us a few pictures of the eaves, the laps and any rusting edges, along with the age of the building, and we can say what we would check first.
