End-Lap Corrosion on Metal Roofs

Technical Defect Analysis

What This Defect Is

End-lap corrosion occurs at the eaves of profiled steel roof sheets, where the factory-cut edge of the upper sheet is exposed to the weather.

As the original protective coating breaks down, corrosion progresses along this cut edge and gradually travels back up the roof sheet profile, causing more damage.

As long as the corrosion is not extensive, perforated and extending beyond the fixing line, it should be possible to treat with a cut edge corrosion treatment.

However, if the sheet ends have perforated through and rotted away in places, it is best practice to change the roof sheet. Slither sheet repairs at the eaves rarely work for long, as they often trap moisture and cause more damage.

Causes

Exposure of the factory-cut edge at the eaves
Breakdown of Plastisol or polyester coatings
Weathering and UV degradation
Persistent moisture at the eaves
Capillary action where water tracks back under the coating

How to Identify

Curling or peeling of the coating at sheet ends
Rust staining (“gingering”) along the exposed edge
Flaking or delamination of original coatings
Corrosion advancing up the profile
Thinning or perforation of the sheet end in more severe cases

image showing advanced cut edge corrosion to profiled steel roof

Image showing typical end-lap corrosion to the valleys of a metal roof

Image showing typical end-lap corrosion to a valley on a steel profiled roof.

Risks if Untreated

Progressive corrosion of the sheet end
Loss of the protective coating over a larger area
Wind-uplift vulnerabilities due to weakened edges
Eventual perforation and material loss
Increased remedial cost as the damage progresses

Image showing extensive end-lap corrosion to a profiled metal roof sheet

Image showing typical end-lap corrosion to a valley on a steel profiled roof.

Condition Ratings

Early

Edge peelback, lifting finish with minor corrosion beneath.

Moderate

Visible rust with early corrosion along the cut edge.

Poor

Heavier rusting & extensive coating loss.

Severe

Advanced corrosion, possible perforations or sheet-end loss.

Recommended Liquasil Treatment

If you are treating cut edge corrosion in isolation, without fully recoating the metal roof, Liqusil’s stand-alone cut edge corrosion treatment, Flexlap is the solution. It’s BBA Approved, which means that it has been independently tested for fitness for purpose.

If you are specifying a full metal roof coating, cut edge corrosion is treated as standard within the Metalseal system.

Clean roof sheet ends to remove loose coating and contaminants.
Mechanically prepare the cut edge to ST3 standard (bare, non-shiny steel).
Follow the sheet profile in a straight line across troughs and crowns.
Prime prepared steel with Liquasil SWT Corrosion Primer.
Once dry, apply Liquasil Flexlap (or Metalseal 20/SF as part of a full-roof coating programme).
Apply Flexlap at a wet film thickness of 350–400 microns (Metalseal 20 may require two coats to achieve film thickness).

Suggested Specification Wording

“Throughly clean the sheet ends to an area at least 500mm beyond the highest point of corrosion. Prepare end-lap areas to ST3 Standard (bare, non-shiny metal) following the roof sheet profile across troughs, webs and crowns to the highest point of corrosion. Apply Liquasil SWT Corrosion Primer to all prepared areas and allow to dry. Apply Liquasil Flexlap (or Metalseal 20/SF for full-roof treatment) to form a uniform, fully bonded protective coating at a wet film thickness of approximately 350–400 microns.”

Frequently Asked Questions

Is end-lap corrosion serious?

Whether the defect is serious depends on the extent of the corrosion that is present. If heavy rust and perforation of the sheet extends beyond the first fixing line, this can cause structural weakness in those areas, making the roof sheet susceptible to wind uplift. In most cases we see, end-lap corrosion is often caught early enough to be treated.

When should end-lap corrosion be treated?

Ideally, as soon as the defect is noticed, it should be treated. If you are an industrial property tenant about to vacate the premises, your landlord’s surveyor will pick up on the defect and you will still have have to pay for it to be repaired, if you have a fully repairing and insuring lease.

Can I treat end-lap corrosion myself?

Theoretically there is no reason why you couldn’t treat cut edge corrosion yourself, but it is very labour intensive. The work is in the cleaming and surface preparation and remember, you’ll be working at height, so it’s best left to trained professionals. Liquasil does not supply products to anybody that has not undegone their product training.