Cut edge corrosion, explained by the people who treat it

This site is run by Allbase, the company that develops and supplies the systems, surveys the roof and stands behind the repair. Every roof is graded before anything is quoted, and the work is carried out by expert contractors and guaranteed once it is done.

Most profiled and corrugated steel roofing in the UK is made with a galvanised zinc layer and a painted or plastisol finish. Those coatings protect the sheet surface, but when a sheet is cut to size the cut reveals a narrow strip of vulnerable steel.

How cut edge corrosion starts.

Failure

Paint delamination

The coating lifts away from the substrate and the exposed area widens each winter.

Failure

Cut edge corrosion at the overlap

Moisture held between the sheets attacks the unprotected cut.

Failure

Substrate exposed

Rust creeps back under the coating until bare steel is open to the weather.

Stage 01

Masking tape sets the line

Applied at sheet ends and overlaps so the finished line is clean and straight.

Stage 02

Mastic seals the overlap

Gun-applied so water cannot sit against the cut.

Stage 03

Basecoat worked into the profile

A corrosion-inhibiting film over the prepared cut and the mastic.

Stage 04

Topcoat to specification

Colour matched to the roof and finished to the system specification.

Four stages, one system.

The roof is sealed, protected and finished to the system specification. No strip-out, no sheet replacement.

Masking tape
Mastic
Basecoat
Topcoat

The three stages of cut edge corrosion. What to look for, and what each one takes to put right.

Every sheet on a metal roof was cut on site, and every cut left bare steel behind: at the eaves, along the end laps, at the verges and around the rooflights. Moisture gets in at the cut, the coating lifts back from it, and a thin rust line becomes lifting coating, then thinning steel, then holes. Most of these roofs went up between the late seventies and the early two-thousands, so the cuts have been weathering for decades while the sheets themselves are usually still sound. How far it has gone decides what the repair is.

Early

A rust line at the cut

What you see. A thin brown line along the cut of the sheet. Nothing running down the roof yet, so from the ground there is often nothing to see.

What it takes

Clean back, treat the corrosion, seal and coat. The quickest and cheapest stage to be at.

Advanced

Coating lifting, staining below

What you see. The coating has pulled back from the cut in a scalloped line, and rust is running down the sheets below. The stage most roofs are found at.

What it takes

Cut back the loose coating, treat the steel, then encapsulate and reinforce the cut before coating.

Critical

Holes or daylight

What you see. The sheet has corroded through in places and daylight shows at the cuts. Water is getting past the joint into the building.

What it takes

Replace the failed sheets first. Coating over perforation buys a season, not a decade, and the report says so.

Get a free roof assessment

Not sure which stage yours is at? The survey grades the roof, and the report says which.

Everything under one roof. From advanced systems and expert contractors to an extensive guarantee and ongoing maintenance, we've got you covered.

The supplier

The coatings and sealants come from the Allbase range, so the specification is written by the people who supply the product and matched to the substrate, the exposure and the condition found.

The surveyor

The whole roof is inspected on foot or by drone and written up with a grade per elevation, so nothing is quoted until the roof has been seen and nothing is replaced that can be repaired.

The expert contractor

Repairs are carried out by expert contractors vetted by Allbase, working to the written specification and inspected on completion, with a Basecare guarantee covering the material and the workmanship together.

Systems certified by

Expert contractors hold

CHAS, SafeContractor and Constructionline are SSIP recognised schemes, and operatives hold CSCS cards. IPAF and PASMA are required where access equipment is used, and NFRC membership and ISO 9001 and 45001 certification are held by many of the contractors in the network.

FAQs. The questions we get asked most.

What is cut edge corrosion?

Rust that starts where a coated steel sheet was cut: at the eaves, the laps, the verges and around rooflights. The cut leaves bare steel, moisture gets in, and the coating lifts back from it. The three stages above show how it progresses.

Can it be repaired, or does the roof need replacing?

Nearly always repaired. The corrosion is cleaned back, sealed and coated, and where the factory coating on the sheets has failed the whole roof is spray coated. Only sheets that have corroded through are replaced, and the survey report says which.

Where do you work?

Across the UK, through the expert contractor network. See the areas with a page of their own; the rest of the country is covered from them.

Who carries out the work?

Expert contractors vetted by Allbase, working to a written specification and inspected on completion. The systems are developed and supplied by Allbase, which also carries out the survey.

What does the guarantee cover?

Basecare covers the material and the workmanship together, in writing, provided the system is installed by an expert contractor and maintained on the agreed cycle.

How do I get started?

With a free assessment. A few details about the roof, and a specialist calls to talk it through. If it needs a closer look, a survey is arranged.