Cut edge corrosion on profiled metal roofs: treat, overclad or replace
Updated 23 October 2026 · SEO Dons Editorial
Cut edge corrosion is rust that starts where a coated steel roof sheet was cut, shows first at the end laps, and breaks down the paint from the edge. Caught early it can be treated in place. Once sheets are perforated, or corrosion has started on the underside, the choice moves to replacing sheets or covering the roof with a new outer skin. This guide covers how the damage is assessed, what a treatment specification should state, and where treatment stops being worth it. The other failure points on an industrial roof are covered in the industrial roof refurbishment guide.
Why does corrosion start at the cut edge, and why at the laps?
Coated roof steel is made as coil-coated strip: BSI describes BS EN 10169 as specifying coil coated steel flat products, with sheet cut from wide strip among the products covered. A cut exposes the steel at the edge. MCRMA's durability guidance says that on a profiled sheet the edge corrosion is inhibited by the sacrificial action of the protective metallic coating. It adds that factory cut edges are usually clean, while site-cut edges can be rougher, are more susceptible to corrosion, and would benefit from anticorrosion paint to delay the formation of edge corrosion.
The laps are where it shows. MCRMA says cut edge corrosion often breaks down the paint system prematurely at end lap positions, even after a period much less than the expected durability of the paint system, and links it to an incorrect choice of paint system for the location and/or the type of metallic coating on the steel. BS 5427, as quoted by MCRMA, notes that deterioration can be more pronounced at fixings, laps, edges and abrupt changes in profile. MCRMA's archived design guide (Technical Paper 6, June 2004) adds that abrasive wheel cutters must not be used on site, because they generate heat and damage the coating.
How is the damage assessed?
The survey needs to separate surface corrosion that treatment can arrest from damage that it cannot. LRWA, writing on coating profiled metal roofs, says the degree of cut end corrosion can vary enormously and that if it is too prevalent the sheet may need to be completely replaced.
| What is seen | What it suggests | Likely route |
|---|---|---|
| Rust line along lap edges, paint sound elsewhere | Surface corrosion at the cut edge | Cut-edge treatment |
| Rust staining weeping from an earlier repair, or blisters in the sheet above the lap | Corrosion from the underside, which LRWA says may be impossible to treat from above | Re-sheeting, or replacing the bottom edge of the sheet |
| Holes, or sheet unsound around fixings | Structural loss of section | Sheet replacement |
| Corrosion at most laps, eaves and site cuts | Treatment may be a repeat visit to the same defect | Compare with over-sheeting or replacement |
Access comes first. HSE lists corroded metal sheets among the surfaces likely to be fragile, and its approach is to avoid work on fragile surfaces where possible, for example by working from below on a mobile elevating work platform. The ACR's Green Book says even a simple inspection of a fragile roof is highly dangerous without adequate knowledge and planning. LRWA adds that where the underside warning signs are present, the area should not be approached because the sheet may give way under loading.
What does a treatment system involve?
LRWA's guidance sets out the usual sequence for treating cut end corrosion:
- Identify the existing sheet coating, because plastisol, PVF2 and other finishes can need different preparation and priming.
- Remove all rust by wire brush or grinding wheel, to bright metal if possible.
- Apply a rust inhibitor, then a metal primer.
- Apply a flexible reinforcement to allow for expansion and contraction between the upper and lower sheet, with bond breaks where the specification needs them.
- Apply the coating at the manufacturer's priming requirements and coverage rates.
LRWA also flags a trade-off. Some cut edge systems seal the joint, and if the repair is not breathable, condensation on the underside of the sheet can collect above the sealed detail and promote corrosion from the reverse side. Its advice is to clarify the objective: for a protective treatment of the cut edge, use a system that does not seal the joints; for full waterproofing, the reverse-side risk has to be addressed first.
What should the treatment specification state?
- The sheet and its coating, as found at survey, and the extent: which laps, eaves and site cuts are included.
- Preparation to the standard above, with an adhesion test to establish whether priming is required.
- The named system and its certificate. The BBA says a certificate relates to a specific product, and a specific organisation, for a particular use, and that it holds on condition that the product is installed and maintained exactly as detailed in it. The BBA certificates guide explains how to check one.
- Movement and breathability: the reinforcement or bond-break detail at laps and the condensation position.
- Sheets to be replaced first, so that no coating is applied over perforated metal. LRWA says sheets that are structurally unsound through total corrosion, particularly around fixings, must be replaced.
- Warranty terms. MCRMA lists conditions some manufacturer and insurance-backed warranties carry, including formal inspections within strict time frames, annual maintenance programmes and prompt written notification of damage.
- Access method and the fragile-roof precautions that go with it.
When is treatment no longer worth it?
Treatment is the wrong answer where the sheet has lost section, where underside corrosion is present, or where the same laps keep failing. LRWA's two ways out of underside corrosion are re-sheeting or replacing the bottom edge of the sheet. A wider failure points to a new outer skin. MCRMA's over-roofing guidance says the existing structure must be shown suitable and the perimeter details able to take the added height, that the existing roof and rooflights should be established as non-fragile because they form the working platform, and that eaves gutters and outlets should be designed to BS EN 12056-3. It suggests an additional design life of at least 25 years is likely for the remedial work. The over-sheeting guide sets out the method, and fixings and laps, a frequent companion of cut edge damage, are covered in the fixings and lap seals guide.
If you want contractors to price a treatment, a sheet replacement or an over-roof against the same survey, you can send an enquiry through the site's form.
Sources
- MCRMA, GD39 section 5, metals: steel and aluminium (June 2022), section 4, durability, GD38 over-roofing (January 2022), GD02 advice on industry warranties (November 2012) and Technical Paper 6 (June 2004, archived) (MCRMA, checked 22 October 2026)
- LRWA, Guidance Note 10, waterproofing of metal profiled sheet roofing and cladding (LRWA, issued February 2014, checked 22 October 2026)
- HSE, fragile surfaces (HSE, checked 22 October 2026)
- ACR, the Green Book (Advisory Committee for Roofsafety, checked 22 October 2026)
- BSI Knowledge, BS EN 10169:2022 (BSI, checked 22 October 2026)
- BBA, frequently asked questions and 5 facts about Agrément (BBA, checked 22 October 2026)
Common questions
Can rust on a roof sheet simply be painted over?
LRWA's sequence starts with removing all rust by wire brush or grinding wheel, to bright metal if possible, then a rust inhibitor and a metal primer, and only then a reinforced coating. Coating over loose rust is not what the guidance describes, and the preparation is most of the job.
How soon can cut edge corrosion appear on a roof?
MCRMA says it can break down the paint at end laps after a period much less than the expected durability of the paint system. It links that to the paint system chosen for the location and the type of metallic coating on the steel, which is why the sheet maker's advice matters when a new roof is specified.
Does treating the cut edges make a roof safe to walk on?
No. The treatment is a coating, not a repair to the strength of the sheet. HSE says all roofs should be treated as fragile until a competent person has confirmed they are not, and corroded metal sheets are on its list of likely fragile surfaces.
Who decides whether a sheet is treated or replaced?
The survey and the specification should, lap by lap. LRWA says the degree of cut end corrosion can vary enormously, so a schedule that names which sheets are treated and which are replaced is more useful than a single rate for the whole roof.