Rust along the sheet ends is rarely just cosmetic
Cut edge corrosion is the rust that forms where profiled steel roof sheets were cut to length, at the factory or on site. Cutting exposes bare metal at the very edge of the sheet, the one spot the original coating system never protected. On most metal-roofed buildings around Truro that means the eaves line above the gutter and the overlaps between sheets, exactly where rainwater lingers longest before it drains away.
It starts as a thin brown line. Left alone, the rust creeps back beneath the factory finish, lifting and peeling it as it advances. A tidemark at the gutter edge becomes a band of flaking steel, and in time the sheet end loses the thickness it needs to hold a fixing or shed water cleanly.
Why Cornish conditions speed it up
Truro sits within a few miles of salt water in almost every direction, and the air carries that salt well inland. Salt holds moisture against the metal, so a cut edge that would dry quickly in a drier county stays damp here for days at a time. Combine that with wind-driven winter rain and the mild, humid weather Cornwall is known for, and the corrosion cycle rarely gets a pause.
The local building stock adds to the problem. Plenty of the steel-framed units on the industrial and trading estates around the city, and the agricultural sheds in the countryside beyond, were roofed in plastisol-coated steel decades ago. Those coatings have largely done their job, but the cut edges were always the weak point, and on roofs of that age many are now actively corroding.
Treat it early and the numbers stay small
Caught in its early stages, cut edge corrosion is a localised repair. The affected edges are mechanically prepared back to sound metal, treated with a rust-inhibiting system, and sealed with a flexible corrosion-resistant coating, with lap joints bridged where needed. The rest of the roof stays untouched and the building stays in use throughout.
Wait, and the job changes character. Once rust has travelled deep into the laps or perforated the sheet ends, you are pricing sheet replacement, access for strip-and-refit, and possibly internal repairs from water that has already found a way in. The earlier the edges are stabilised, the smaller the job, which is why an inspection at the first sign of staining is worth far more than it costs. Signs you can often spot from ground level:
- Brown staining or streaks along the gutter line
- Coating peeling or blistering at sheet ends
- Visible rust at side and end laps
- Damp patches or drips inside near the eaves
- Debris-filled gutters holding water against the sheets
When treatment is the wrong answer
We survey before we quote, and sometimes the survey says no. If sheets are perforated, if corrosion has eaten through both layers of a lap, or if the underside of the roof is rusting from condensation as fast as the topside is from rain, a coating will only hide a failing roof. In those cases sheet replacement or an over-roof is the honest recommendation, and that is what we will tell you, with photographs to show why. We would rather lose a coating job than treat a roof that cannot be saved.
From edge repair to a coated roof
Where the sheets are sound, cut edge treatment often makes most sense as part of a full roof coating. The edges and laps get the corrosion treatment, then the whole roof is cleaned and recoated, sealing fixings and refreshing the weathered surface in a single programme of work. For an ageing metal roof this close to the Cornish coast, that is usually the difference between repeating spot repairs every few years and resetting the whole roof in one visit.
If you have noticed rust lines on a metal roof in or around Truro, ask us to take a look. The survey is straightforward, the report is plain English, and the advice stands whether or not you use us for the work.
