Walk the roof of an older steel-clad unit and you will often see the same pattern: orange-brown staining along the lower edge of a sheet, paint lifting in a thin line, and a bead of rust where one sheet laps over the next. That is cut edge corrosion. Profiled steel roofing is made from coated coil, and wherever the coil is cut the bare steel is exposed in a line only a fraction of a millimetre thick. Moisture finds that line, and the damage creeps sideways under the coating. It is slow and it is easy to miss from the ground, but left alone it ends in holes, leaks and sheets that no longer hold their fixings.
What the coating is, and why a cut edge is a weak point
A metal roof sheet is a steel core with a protective skin. The steel is first coated with zinc (galvanising) or a zinc-aluminium alloy, which protects it sacrificially: if the surface is scratched, the zinc corrodes in preference to the steel. On top of the metallic layer goes a primer and a coloured organic coating, usually a polyester paint or a plastisol (a thick PVC film) on sheets meant for harsher exposure. The whole build-up is far thinner than people expect, and each layer has a job.
When a sheet is sheared, guillotined or cut with a saw to length, that structure is sliced through. The edge shows a thin cross-section of bare steel, with zinc on both faces nearby. For a while the zinc protects the edge, because it sits in contact with it and corrodes first. Over time, with water and dirt lying on the edge, the zinc is used up. Once it is gone the steel itself rusts, and rust takes up more space than the metal it came from. That swelling pushes the paint film away from the sheet, opens a path for more water and carries the corrosion further across the face. The process is a slow, steady creep rather than a sudden failure, which is why it so often gets noticed late.
Where to look on a roof
The cut ends of sheets are the obvious spots, and on a single-skin roof those are at the eaves, the ridge and any side where sheets have been trimmed to fit. The lower cut edge at the eaves is the usual first casualty, because water running down the sheet collects there and lingers. Sheets that overhang into a gutter are exposed to wet from above and condensation from below.
Other places catch out the unwary:
- Side laps and end laps, where one sheet overlaps another and water is drawn in by capillary action.
- Around holes cut for vents, flues, pipes and plant, because every opening produces a new cut edge.
- Fixing holes where a screw has been driven off-line or over-tightened, which cracks the coating.
- Cut ends of flashings, ridge pieces and cladding trims, which are sometimes formed on site with a grinder.
- Rooflight edges, where a steel sheet meets a plastic panel and moisture sits in the joint.
- Box gutters and valley gutters, where sheet edges sit in standing water.
You can see most of this from a safe access point with a camera. The staining pattern gives a lot away: a line of rust at the bottom of a sheet means the edge has been wet for years, while blisters in the paint in the middle of a sheet show that corrosion has gone further under the coating than the surface suggests.
What speeds it up
Corrosion at a cut edge needs water and time, and anything that keeps the edge wet shortens the timeline. Poor drainage is the main cause. A sheet that overhangs the gutter by too little sends water behind the gutter and back along the underside, and a gutter clogged with leaves holds a puddle against the metal. Moss, bird droppings and silt on shallow-pitched roofs keep the surface damp for longer than a clean roof would stay wet.
The environment matters too. A unit near an industrial process that gives off fumes or steam, a food factory with condensation, or a livestock building with ammonia in the air will corrode metal much faster than a dry storage shed. Salt-laden air accelerates it, but north Hampshire is a long way from the coast, so here the commoner culprits are condensation, trapped debris and industrial exposure rather than sea spray. Mixing metals can also play a part. Where copper or lead runoff drips onto zinc-coated steel, or a different metal is fixed in contact with it, galvanic action eats the zinc faster.
Workmanship counts as well. Sheets cut on site with an abrasive disc instead of a nibbler or shear throw off hot particles that burn the coating around the cut and embed in the surface, where they rust as small brown spots. Swarf (the metal filings left behind by drilling and cutting) that is not swept off a roof will do the same. And a roof with low pitch simply lets water linger. All these can be avoided at installation, which is one reason older roofs and rushed repairs show the problem first.
Surface rust or structural damage? How we assess it
Not all corrosion is equal, and the decision on treatment turns on how far it has gone. We look at the roof in three stages. First, is the coating intact and the rust only at the very edge? Second, has the rust lifted the paint and moved across the face, but the steel still has its thickness? Third, has the metal turned to flaky scale or perforated, so that light or water gets through? A screwdriver tip pressed gently against the rusted area tells you a lot: sound metal under the stain resists, while corroded steel crumbles or flakes.
The wider system matters as well. Fixings corrode too, and if the screws or washers holding the sheet are rusted through, the sheet is no longer secure even if its face looks reasonable. The purlins (the horizontal steel or timber members that carry the sheets) and the fixings into them also need checking where leaks have run for years. Our survey photographs each of these so you can see the damage on the monitor rather than relying on a description. Where we find asbestos-cement sheets or other suspect material mixed into the roof, we stop and treat it differently, and our guide to asbestos roofs on commercial buildings explains how that changes the work.
Treatment: what can be saved and what cannot
Where the corrosion is confined to the edge and the sheet is otherwise sound, treatment can extend its life considerably. The method is straightforward. The area is cleaned, loose rust and flaking paint are removed by hand or with a mechanical brush, and the bare metal is treated with a rust converter or inhibitor where the product allows it. A zinc-rich or other compatible primer goes on next, followed by a top coat that overlaps onto sound paint. Sealing the cut edge itself with a flexible edge sealant stops water returning to the same spot. Lap sealing and replacement of failed fixings and washers completes the job. Our guide to commercial roof coatings covers the full coating route for whole roofs.
There are limits. A coating cannot restore steel that has already been lost, and painting over scale buries the problem rather than curing it. Where the metal has perforated, or the rust has gone across a large part of the sheet, replacement of individual sheets, or of a whole slope, is the honest answer. Another option for a roof that is structurally sound but worn is overcladding with a new sheeted layer, which keeps the old roof as a base and leaves the building watertight while you carry on trading. If you are weighing these choices, the broader industrial roof sheeting guide covers how sheeted roofs are built and replaced.
| What we find | What it means | Usual response |
|---|---|---|
| Light staining at the cut edge, paint intact | Early stage, zinc still working | Clean, seal the edge, monitor |
| Lifted paint and spreading rust, steel still thick | Corrosion under the coating | Strip back, prime and recoat, seal laps |
| Flaking scale or pinholes | Section loss; sheet failing | Replace the sheet or the slope |
| Rusted fixings and washers | Sheet no longer held securely | Replace fixings, re-seal, check the purlin |
| Rust at rooflight junctions | Water trapped between steel and plastic | Re-seal or replace the rooflight set |
Preventing it from returning
A treated edge only stays treated if the cause is dealt with. That means clearing gutters and outlets, checking that sheets overhang far enough into the gutter, removing debris and moss from laps and valleys, and fixing any leak that is putting water onto the roof from inside. Where the problem is condensation under the roof, improving ventilation or fitting a liner may do more than any paint.
For site work, cut ends should be made with the right tools, deburred, and touched in with compatible paint before the sheet goes up. Cut edges on trims and flashings need sealing at the time, not later. We also recommend a regular inspection routine, and our guide to commercial roof maintenance sets out what to check and how often. A roof that gets a twice-yearly look and a quick clean seldom reaches the stage where sheets have to be replaced.
Getting it looked at
If you manage a metal-clad building and you can see rust lines at the eaves, start with a survey. We climb up, photograph the edges, laps, fixings and rooflights, and tell you which parts are worth saving and which are past it. The next step is a quote with fixed figures for treatment or replacement, whichever the roof needs. Many callers expect a recommendation to replace the whole roof, and we are happy to say when a repair will do. Details of the work we do on business premises are on our commercial roofing in Basingstoke page, and you can reach the office on 01256 213956 or by WhatsApp with a few pictures of the staining.
