Cut Edge Corrosion on Metal Roofs

Profiled metal roof and wall sheets are made from steel that carries a protective metallic layer and, on top of that, a paint-type coating. The layers protect the faces of the sheet well. The cut edge is different. Wherever a sheet has been sliced to length, trimmed around a rooflight or cut for a pipe, the steel core is exposed with nothing over it.

What exactly happens at the edge?

Moisture sits in the tiny gap where sheets overlap, and it cannot dry out. Over time the exposed steel corrodes, and the rust, which takes up more space than the metal it came from, pushes up under the coating. The paint blisters, cracks and flakes, and that gives water a larger area to attack. The process feeds itself, which is why a small orange line can become a wide band. Rust is also rarely confined to one sheet: the overlap puts two edges together, so one wet lap can set two sheets off at once.

Which edges are most at risk?

The lower ends of sheets that sit in a gutter hold water the longest, so they are common starting points. Side laps trap moisture and dirt. Cut-outs around rooflights, vents and flues are exposed edges made on site, where cutting tools can also burn the coating nearby. Ends of sheets in contact with leaf litter or moss are also vulnerable, and so are places where fixings have punched holes. If you see rust in these zones, our guide to replacing metal roof sheets shows what happens when a sheet is beyond saving.

There is no single distance, and anyone who quotes one without looking at the roof is guessing. Spread depends on the coating, the age of the roof, the setting and how much water the edge holds. What can be said is how the damage behaves.

Does it stay at the edge?

Not for long. Rust creeps under the coating, moving away from the edge across the face of the sheet. The coating depends on its grip on the steel, and once that grip is broken the damage travels in whatever direction water takes it, which on a sloping sheet is usually down the rib and along the lap. The visible stain at the edge often understates what has happened beneath the paint, so the surveyor taps, scrapes and probes rather than judging by colour.

What speeds it up?

Environments with industrial pollution, nearby salt or constant condensation inside the building all hasten corrosion. So do roofs where debris builds up in valleys and gutters. A roof that has not been cleaned for a long time can have sound sheets and failing edges, because the dirt keeps the laps wet.

Where does it end?

The final stage is perforation, when the steel rusts right through and water enters. Before that, the sheet loses strength, which matters for anyone stepping on it. Do not walk on a corroded sheeted roof, and read fragile roof safety before arranging any access. A roof that has reached this point is unlikely to be a candidate for treatment.

Treatment only works if the steel underneath is still sound. The assessment decides that, and it should happen before a price is written, not after the first sheet has been cleaned.

What is checked on the roof?

The roofer looks along every lap and cut edge, noting where rust is surface staining and where it has become scale or flaking. They scrape back loose coating to see what is beneath, and they test sheets near the worst areas for softness or holes. They also check fixings, sealants and the sheets under rooflights, since water that enters at one spot travels along the rib and shows up elsewhere. Gutters are inspected too, because blocked or corroded gutters keep the lower sheet ends wet.

What do the photos show?

We photograph the problem areas on your own roof and share the pictures, so you can see exactly where the rust is and how deep it goes. A good record usually marks the sheets that can be treated, the sheets that are borderline and the sheets that need replacing.

What about the age of the roof?

Age affects the answer, but condition decides it. A middle-aged roof with well-kept edges can take treatment, while a younger one with standing water may not. If the building dates from before 2000, HSE guidance says to assume asbestos is present, and a sheet that proves to be asbestos cement is always fragile. That needs identifying before anyone cleans, scrapes or drills it. Our page on overcladding an asbestos roof covers one route for those buildings.

Treatment aims to stop the corrosion and re-seal the edge so water stays out. It does not make old steel new. Its value is in buying a long period of service from a roof that is still fundamentally sound.

What are the usual steps?

The roof is cleaned first, removing dirt, moss and any loose coating. Rust is taken back by mechanical abrasion, using wire brushes, scrapers or powered tools, until sound metal or coating that is still holding firm is reached. The cleaned steel is treated with a rust-inhibiting primer, then edges and laps are sealed, often with a reinforcing tape or a flexible coating designed for the purpose. Finally a topcoat goes over the area, sometimes across the entire roof so the colour and finish match.

What makes it succeed or fail?

Preparation. A fine coating over loose rust fails quickly, because the rust keeps working beneath it. The weather has to be dry during application, the product has to suit the existing coating, and the instructions of the maker must be followed. Ask what the treatment product is, what surface preparation is specified, how many coats go on and what warranty applies. A product that suits a clean, recently coated sheet may not suit an old weathered one, and the maker's data sheet should say which surfaces it is meant for. Request a small trial patch on a typical sheet if there is any doubt. Our guide to commercial roof warranties explains how to read the answer.

Is treatment a one-off?

No. Treated edges still need periodic checks, because new damage can appear at laps and cut-outs. Add the roof to a regular inspection routine, and clear gutters and valleys so water leaves quickly. Photograph the treated laps when the work finishes, then photograph them again at each inspection, so any new rust is easy to compare against a clean starting point.

The choice turns on how much sound steel is left and what you expect from the building. It is rarely a matter of picking the cheaper option on the day.

When does treatment make sense?

It suits roofs where corrosion is confined to edges and laps, sheets are strong, fixings are sound and leaks are few. It also suits buildings where you want to avoid disruption, since the work happens from outside and the occupants can carry on. Treatment can be done in stages as budget allows, starting with the worst areas. It also keeps the existing sheets out of the waste stream, which matters if you are weighing up the environmental cost of a full strip, and it avoids new penetrations through a roof that is otherwise watertight.

When is re-sheeting the better buy?

If rust has gone through the sheets, if large areas have lost their coating, if the sheets bend or flex underfoot, or if leaks are widespread, treatment is money spent on a roof that cannot hold it. Replacing the sheets, or covering the roof with new sheets over a new support system, then gives a fresh start. Our guide to coating or replacement sets out that comparison in more detail.

What sits in between?

Some roofs call for a mix: replace the worst sheets, treat the rest and renew the gutter linings. A roofer who is willing to recommend that mix, instead of pushing a single answer, is usually reading the roof rather than selling a product. Ask for the reasoning sheet by sheet, so you can see which parts of the roof sit in each group and why, and so a later survey can check whether the untreated sheets have held up.

Once the method is chosen, the practical questions come next: access, timing, disruption and cost. Each can change the price more than the product itself does.

What drives the cost?

The size of the roof, the proportion that needs treatment, the height and ease of access, and how much of the work is cleaning rather than coating. Edge protection or a mobile platform may be needed, and rooflights add complication. Because every roof differs, the written price comes after a survey of yours. Ask whether it includes VAT, which is 20% at the standard rate, and whether any access equipment is in the total.

How is the work run?

On a working site, the roofer should agree where materials are stored, where waste goes and how staff and visitors are kept clear of areas below. Cleaning residue should not wash into gutters and drains unchecked. Tell the contractor about roof-mounted plant and cabling so it can be protected. Work in sections where practical, and agree what happens if rain stops the job part way through.

What should you ask for at the end?

Photos of each stage, the product names and batch details, and a note of what was treated and what was left. File them with the roof records for next time. Send us a few pictures of the rust and the sheet profile when you enquire, so we can see what kind of roof it is before the survey.