How cut-edge corrosion starts, what a surveyor looks for, how a repair coating and over-cladding compare, and the asbestos question to settle before anyone touches an older sheet.
A coated steel roof sheet looks like a single material, but it is a thin steel core with a protective layer on each face. The weak point is the edge. Wherever a sheet has been cut, at a lap, a rooflight, a ridge or an eave, the bare steel is exposed along the line of the cut. Water sits there, the steel rusts, and the rust creeps under the coating, lifting it in a line that follows the edge. This is cut-edge corrosion, and once it reaches the laps and fixings, a warehouse, workshop or farm building roof starts to leak.
It is slow at first, then quicker. A roof may seem sound from below for years, then show a line of brown stains and flaking paint at every lap. At that point the owner faces a choice that is often put as repair against replacement, but there is a middle route in over-cladding. This guide sets the options out in neutral terms, covers the asbestos check that comes first on older buildings, and says what a survey should tell you. The wider work is described on our commercial roofing page, and the short answer on what cut-edge corrosion is covers the basics.
How the damage starts and spreads
Profiled sheets are shaped into ridges and troughs so they span between purlins, the horizontal rails that carry the roof. Where two sheets overlap, the cut edge of one lies close to the face of the other. Water drawn into that narrow gap by capillary action stays there for long periods, because it dries slowly. The bare steel at the cut edge starts to corrode first, and the rust spreads sideways under the coating, which blisters and lifts.
A surveyor looks for the signs at the places where edges are exposed:
- Brown staining along side laps and end laps.
- Paint blistering or flaking in a line parallel to a cut.
- Rust around fixings and washers, where holes were drilled through the sheet.
- Corrosion at rooflight edges and where sheets have been cut to fit flashings, since those are places where a sheet is often trimmed on site and the bare edge is left to the weather.
- Pitting in the troughs, where water collects.
Fixing holes matter as much as edges. Each fastener passes through the sheet, and once a washer has perished, rain runs down the shank. A sheet that is rusty only at the edges and fixings is in a different state from one where the rust has eaten through the face. That difference is what decides between repair and cladding. Weather plays its part as well: Met Office averages for Manston give about 613 mm of rain a year and 110.05 days with 1 mm or more, so an East Kent roof spends long stretches wet.
Asbestos cement comes first
Before any repair or over-cladding is planned on an older roof, find out what the sheets are made of. Health and Safety Executive advice is to assume asbestos is present in any building from before the year 2000. Its guidance A14 classes all asbestos cement roofs as fragile, so nobody should walk or lean on one, whatever it looks like from below.
Cut-edge corrosion belongs to coated steel, but a building can have a mixture: a steel-clad extension on an older asbestos cement shed, or asbestos cement rooflights in a metal roof. A survey for asbestos tells you what is there, and the same HSE rules apply to every option from then on.
HSE treats the removal of asbestos cement, such as roof sheeting, as non-licensed work where the sheets are lifted with care and taken off whole rather than broken up. Where the sheet will be badly damaged or broken up, the work moves into the notifiable non-licensed category. Asbestos insulating board, which can be found in soffits, is a different material and can be licensable. A roofer who is not licensed passes licensed work to a licensed firm.
Drilling, screwing or cutting into asbestos cement sheet breaks it and releases fibres. That is a reason to survey before choosing between coating and cladding, not after.
Waste is wrapped twice and labelled, then held in a skip that locks; an open skip is never right for it. Our answer on asbestos surveys before roof work explains the sequence.
Repair coatings in neutral terms
A repair coating treats the existing sheets in place. The usual sequence is the same whatever the product. The roof is cleaned, loose paint and rust are removed back to sound metal, a primer goes on, and a coating is applied over the whole surface or over the affected laps. Fixings and sealants at laps are renewed, and any sheet that has rusted through is replaced.
This approach suits a roof where the damage is mostly at edges and the sheets themselves are still sound. It keeps the existing roof in place, adds little weight, and can be done in sections, which suits a building that stays in use. The limits are plain too. A coating cannot restore steel that has gone, and rust left under the paint will lift it again. How long a coating lasts depends on the condition of the substrate and on the product, so ask for the maker's stated figure and the conditions it assumes. A warranty is not a lifespan.
Access is the next question. A roof with corroded sheets may be fragile as well, so work needs edge protection, crawling boards or a platform from below; see our guide to commercial roof coatings and the note on fragile roof safety. The price depends on the area, the condition and the access, and the survey sets the written price. A short trial patch on one lap, checked after a few weeks, shows whether the preparation has held before the whole roof is committed.
Over-cladding in neutral terms
Over-cladding fits a new weathering layer over the old roof instead of stripping it. In outline, spacer bars or support brackets are fixed through the old sheets to the purlins, and new profiled sheets are laid over them. The old roof stays in place beneath, which keeps the building weathertight while the work goes on, and avoids lifting sheets that could be fragile.
It is a candidate when the old sheets are widely corroded but still able to carry the new layer, or when asbestos cement sheets are present and the owner prefers to leave them undisturbed under a sealed cover. That second case needs an asbestos survey and a method that does not drill or damage the old sheets, which is a specialist question. Our guide to over-cladding industrial asbestos sheets covers it.
Over-cladding brings extra points to settle:
- Weight: the Planning Portal puts the approval threshold at a new covering weighing 15 per cent or more over the existing one, so check with building control.
- Fire performance: the new covering must not behave significantly differently in a fire.
- Insulation and condensation, which the new build-up changes.
- Rooflights and flashings, which need raising to suit the thicker roof.
Where a building is later stripped, the hidden layer must be handled as it was found. That is a reason to record the build-up with photos and a note of the sheet type, so a future owner or contractor knows what lies beneath the new roof.
Comparing the two routes
Neither route is the better buy in every case. The table sets out what each suits, in general terms.
| Question | Repair coating | Over-cladding |
|---|---|---|
| Sheets sound? | Needed | Fair is enough |
| Added weight | Very little | A full layer |
| Asbestos cement? | Rarely suits | Can leave it undisturbed |
| Roof in use? | Section by section | Mostly weathertight throughout |
| Approval check | Seldom | Weight and fire |
A third route is replacing the sheets, which suits a roof where corrosion has reached the structure or the insulation. Our note on profiled roof sheet replacement explains it.
A fair survey says which route the building suits and why, with photos. It should also say what would change the answer, for example if corrosion is found under the first sheet lifted. Be wary of a firm that offers only one option before seeing the roof.
Cost cannot be read from a rate. Per-square-metre rates for commercial roofs vary too much with access, condition and area to be useful, so each route is priced once the survey is in, and the price covers the whole job. Ask for the specification in writing: the product, the number of coats and the thickness for a coating, or the sheet gauge and spacer details for cladding. Coating work scales with the area to be cleaned and treated, and cladding with the area to be covered and the height of the build-up, and both with the access needed.
What to ask before you decide
A decision on a commercial roof often involves a landlord, a tenant, an insurer and a lender, so write the answers down. These questions help.
- What are the sheets made of, and has an asbestos survey been done?
- Where is the corrosion worst, and can I see photos of the laps, fixings and rooflights?
- Is the steel itself sound under the rust?
- What happens to the building's use while the work is done?
- What access and edge protection will be used?
Insurers, lenders and landlords often want the findings in writing, so keep the survey photos and the specification together. Under the Construction (Design and Management) Regulations 2015, a client's duties on a project with only one contractor are carried out by that contractor, and it falls to the contractor to prepare the construction phase plan. For a commercial client, ask what the plan covers. Our guide to CDM regulations for roof work has more.
Scaffold on the highway needs a Kent County Council licence, applied for by the scaffolders with 5 working days' notice. The standard VAT rate is 20%, and GOV.UK confines the domestic reverse charge to VAT-registered businesses in the Construction Industry Scheme, so ask how a quote treats VAT. For a tenanted building, ask who holds the repairing obligation under the lease before choosing a route, since that usually settles who pays. Send photos of the laps, the fixings and the underside.
