What that rust at the sheet ends is telling you
Across the warehouses, logistics sheds and industrial estates that ring London, plenty of coated metal roofs are sound across the face but rusting at the edges. This is cut edge corrosion, and it comes from the way the sheets are made rather than any fault in the build. Profiled steel sheets are cut to length after coating, so the cut exposes bare steel at every sheet end and side lap. The coated surface is well protected; the raw edge never was, because no coating wraps a line created after the coating is applied.
From that exposed steel, rust forms and then creeps back under the coating, lifting it off the metal as the corrosion advances. The brown stain visible from the loading yard is only the part that has already reached the surface; the damage usually runs further underneath.
City air and ageing industrial stock
The metal roofs that matter here are mostly on outer-London logistics parks and the older industrial estates rather than the centre. The drivers are the age of much of that steel-clad stock and a damp, sometimes grimy urban atmosphere that keeps the sheets from drying cleanly. Pollutants and persistent moisture sit in the overlaps, where capillary action holds water in the joint, and decades of thermal movement as the sheets heat and cool work the corrosion front up from the gutter line. Shaded, north-facing laps that stay damp are often where it shows first.
The case for treating it early
While the rust is shallow, treatment is a localised, low-disruption job. The affected edges are mechanically cleaned back to sound metal, then treated, primed and sealed with a flexible coating system built to move with the sheet. The rest of the roof stays in use and the building keeps operating throughout. Once a sheet perforates, that option is gone, because no coating recovers a rusted-through sheet. You then face replacement: access equipment, stripping out, new sheets, and disruption inside a building that often runs to a tight schedule. The gulf in cost and downtime between an edge treatment and a re-sheet is exactly why early action pays.
Signs to check before they spread
- Brown staining along the eaves, visible from ground level
- Coating lifting or curling at the sheet overlaps
- Rust rings around the fixings near sheet ends
- Rust flakes or coating fragments in the gutters
- Damp patches or drips inside the building under the laps
Each of these flags a problem rather than measuring it. The reliable answer needs a survey: getting onto the roof, opening the worst laps where it is safe, and establishing how far the corrosion has run beneath the coating.
Our honest position when a roof is finished
We survey before we quote, and sometimes the survey costs us the easy job. If sheets are perforated, if corrosion has travelled a long way under the coating, or if the underside is rusting where no treatment can reach, we tell you straight. Coating over a failed sheet is wasted money that lifts again within a season. The honest options then are replacing the worst sheets and treating the rest, or an overlay if the roof as a whole has run its course. Where the edges are failing and the faces are chalking and fading too, dealing with the cut edges as part of a full roof coating is usually better value, protecting the whole surface in one visit. We are based in the South-East and work across England, including roofs in and around London, and every recommendation follows from what the survey actually finds.
