Coated steel sheet is a layered product. A steel core carries the load, a metallic layer of zinc or similar protects it, and a polymer paint coat on top gives colour and weather resistance. Together they last for decades while the layers stay intact. The trouble is at the places where the layers stop: the cut edge of a sheet, the wall of a fixing hole, a scratch made by a boot or a tool.
That is cut edge corrosion, and it is the most common reason an industrial steel roof ages faster than it should. It is slow and quiet at first, then visible as rust-brown streaks and lifting paint, then as perforation. Catching it in the first stage changes what can be done. This guide explains the mechanism, the places to look, and the treatments in order from cleaning to replacement. It sits within our wider commercial roofing work for units, farms and workshops, so the links below lead on to the neighbouring topics.
How a cut edge starts to rust
When a coated sheet is guillotined, cut with a saw or punched for a fixing, the steel core is exposed at the cut. The zinc layer nearby gives some protection because it corrodes in preference to the steel, but that protection is limited and uses itself up. The paint coat just behind the edge begins to lift as corrosion products form beneath it, which opens a bigger gap, which exposes more steel.
Water makes it worse. Capillary action draws moisture into the tight space between overlapping sheets, where it sits long after the surface has dried. Dirt, bird droppings and leaf litter held in the laps hold moisture against the edge and add acids to it. In coastal areas, salt does the same, though Swindon is a long way from that problem. Industrial atmospheres, agricultural ammonia and the washing down of a building can all raise the risk.
The corrosion creeps inwards from the edge underneath the coating, so the visible damage is only a fraction of the real extent. Scraping at a blistered patch usually reveals a wider area of lifted paint than the eye first suggested.
Why does it show at the laps first?
Because that is where two cut edges, a trapped film of water and a fixing pattern all coincide. Side laps and end laps hold moisture longest, so they are the first place we look.
Where to look on your own roof
You do not need to climb to spot the early signs, and you should not climb if the roof has any fragile areas. From the ground or a safe vantage point, look for the following.
- Rust-coloured streaks running down from laps, ridges and sheet ends.
- Bubbled or lifted paint along cut edges, particularly at the eaves and around rooflight openings.
- Staining or rust dust on gutters, which is run-off from corroding sheets above.
- Brown spots around fixings where the sealing washer has perished.
- Light visible through the roof from inside, which means corrosion has gone right through.
From inside, the underside of the sheet shows its own warnings. Rust blooms around fixings, white deposits on insulation and drip marks on the floor all point to moisture in the wrong place. A liner panel that sags or bulges may be holding trapped water.
Penetrations are a favourite site. Where a pipe, flue or extract duct passes through a sheet, the hole is cut on site, and often cut roughly. Flashings that were never sealed properly let water in at the very place where the edge is least protected.
Causes that make corrosion worse
Cut edges explain where corrosion begins. Several other factors explain how quickly it spreads.
Poor drainage is the first. Roofs that hold water, whether because of a dip in the sheets or blocked outlets, keep their laps wet for longer. Debris is the second: leaves, moss and silt behind rooflights or in valleys stay damp and acidic. The third is trapped condensation, where warm moist air from inside meets the cold underside of the sheet and runs down to the edge. Poorly ventilated buildings, particularly those housing animals, damp processes or wet products, suffer disproportionately.
Mixing metals can also cause trouble. Where steel meets aluminium, copper or lead, moisture sets up a small electrical cell and the less noble metal corrodes. Rainwater running off copper pipework onto steel sheets or off a lead flashing over a coated sheet will attack the coating along the water's path.
Do scratches matter as much as cut edges?
Yes. Foot traffic, dragged tools and dropped equipment break the paint and zinc just as a saw does. Damage made during installation is common, and once weathering begins at a scratch it behaves exactly like a cut edge. This is why contractors on the roof should walk on the crests of profiled sheets, use boards, and avoid dragging materials across the surface.
Treatments: from cleaning to replacement
What can be done depends on how far the corrosion has gone. We think of the options as a ladder, starting with the least work and stepping up only when a lower rung will not hold.
Can early corrosion be treated in place?
Yes, when it is limited to the coating and surface rust. The area is cleaned of dirt and loose paint, rust is removed back to sound metal, and a rust-inhibiting primer and compatible topcoat are applied. Cut edges and fixing heads receive a sealing treatment. This is a repair for the areas found, and it works where the steel underneath is still thick and sound.
When is a coating system the answer?
When corrosion is widespread but shallow. A full-roof cleaning, repair and coating process can extend the life of the sheets considerably, provided the product is chosen to suit the substrate. Our guide to commercial roof coatings explains what a coating can and cannot do.
When must sheets be replaced?
When the steel has lost section, when holes have formed, or when sheets are too weak to take foot traffic. Individual sheets can be swapped if the damage is local. When corrosion runs across whole slopes, replacement of the covering or an over-clad is more sensible. See our guide to roof over-cladding for when that works. In every case the decision is made from photographs of your own roof, and our advice leans towards the lightest repair that will actually hold.
What does a repair on a lap actually involve?
The lap is opened up, the trapped dirt and corrosion products are cleared, the edges are treated and sealed, and the fixings are replaced with new ones fitted with fresh washers. Where an end lap has been squeezed too tight, a new sealant bead and a slightly longer overlap are used to stop water creeping back in. It is slow, careful work, and worth the time because the lap is where the next failure would begin.
Safety around corroded sheets
A corroded steel roof is not only a leak risk. Sheets that have lost metal along their laps can lose their strength, and fixings that have rusted through lose their grip. A roof that looks solid may not hold a person. Sheets near rooflights are of particular concern, because the combination of weathered plastic panels and corroded neighbours produces a mix of fragile and nearly fragile surfaces.
Nobody should walk on a roof in this condition without proper access equipment, and that includes staff, delivery drivers and anyone sent to fix a gutter. Our guide to fragile roofs and safety sets out the precautions we take and the ones you should expect from any contractor.
If a roof has rust showing at the laps, assume it is not safe to walk on until it has been checked by someone with the right equipment.
Where we see signs of significant corrosion, we make the roof safe first, which can mean boarding walkways, marking fragile areas and recommending that access is restricted until the repair is done. After that, we explain what we found and what it means for the roof's remaining life.
Insulation, condensation and the underside of the sheet
On insulated roofs, corrosion often starts from the inside. A built-up system with a liner, insulation and outer sheet depends on its seals to keep moist air away from the cold outer skin. If the seals fail around fixings or at laps in the liner, water vapour moves into the build-up and condenses on the underside of the outer sheet. The sheet corrodes from the inside outwards, and the first sign you see may be a drip rather than a rust stain.
This is why a repair that treats only the outside can disappoint. The sheet may be sealed and coated, yet the source of moisture continues unaltered. A sound survey asks where the moisture is coming from, not only where it is going. Sometimes the answer is a faulty vapour control layer. Sometimes it is a building use that produces more humidity than the roof was designed to handle. Our guide to commercial roof insulation covers how those layers are meant to work.
Gutters are the second hidden factor. Gutters that overflow, leak at joints or sit lower than the sheet edge soak the eaves, where cut edges are most numerous. Relining or replacing failing gutters, as described in our answer on industrial gutter lining, often slows corrosion at the lowest laps considerably.
Prevention and what to ask a contractor
The simplest corrosion treatment is not letting it start. Keep laps, valleys and gutters free of debris. Wash down roofs that collect deposits. Seal cut edges and holes at installation, and touch up any scratches at once. Insist on sealing washers at every fixing and check them when the roof is inspected. A regular inspection programme turns small defects into small jobs, a point developed in our guide to planned roof maintenance.
When you ask a contractor to price a repair, ask four things. What is the condition of the steel beneath the coating? How far does the damage extend beyond what is visible? What method will treat the cut edges? And how will the work be done without making the roof unsafe? Answers that are vague about the metal itself are a warning.
We survey the roof, take photographs of what we find, and write up the options with the smallest effective repair first. A written quote arrives after the survey, never before it. Repairs we carry out come with a 10-year workmanship guarantee, and we tidy the roof and yard before leaving.
One last point for tenants and landlords: corrosion is often arguable. Was it wear, a design fault, poor installation or neglect? Dated photographs of the roof before and after any work, kept in the roof file, are the best evidence you can hold when the question is asked.