BBA Approved cut edge corrosion treatment from Liquasil. Flexlap is our self-levelling silicone product for treating cut edge corrosion.

Cut Edge Corrosion Treatment From Liquasil

Flexlap was the first stand-alone treatment for cut edge corrosion to gain BBA Approval in the UK.

It was also the first to include an anti-corrosion primer used to prevent further rusting of the substrate.

Made for the specification market, the Flexlap system is usually a fraction of the price of its competitors, yet is far superior in terms of performance.

Flexlap BBA Certificate

What Is Cut Edge Corrosion?

Cut edge corrosion is a defect that is often present on profiled steel roofs.

Roof sheets are manufactured from large coils of steel that are coated, usually with a Plastisol finish, before being processed into the profiled shapes we see on roofs and cladding.

The coated sheets are shaped and cut to size on a large guillotine, leaving cut edges unprotected by the Plastisol coating and vulnerable to the elements.

Over a relatively short period of time, these unprotected edges begin to rust and the Plastisol starts to peel back from the steel surface.

The newly exposed steel is soon affected by corrosion which creeps up the roof sheet as the peeling of the Plastisol extends.

If left, the corrosion becomes more advanced and the sheet ends will eventually rot through.

If the corrosion extends beyond the fixing lines, the structural integrity of the roof sheet can be compromised and any affected roof sheets will need to be changed.

It is for this reason that cut edge corrosion should be treated as soon as it is evident.


How Steel Profiled Sheets Are Made

Edge Peel Of Factory Finish

image showing edge peel-back, the strat of cut edge corrosion on a metal roof.

A Better Cut Edge Corrosion Treatment

Cut edge corrosion is the most common defect we see on metal roofs in our building surveying practice.

Many other such treatments are overly complex and actually create a larger defect.

Liquasil has designed a simpler, cheaper, more effective and easier to apply system that actually works.

The system is called Flexlap. It comprises a marine grade epoxy primer that is overcoated with our unique, self-levelling, semi-gloss silicone coating.

In most cases, the joints on overlapping roof sheets at the mid-lap areas should not be sealed.

If the bottom most sheet at the mid-lap areas are not affected by corrosion, why spend money preparing and coating it?

Liquasil’s Flexlap can provide huge cost savings against most other cut edge corrosion treatment systems, as well as significantly lower installation costs and a more resilient treatment.

Can An Existing Cut Edge Corrosion Treatment Be Over-Coated?

Although cut edge corrosion is a common defect on steel profiled roofs, the treatment is not always as straightforward as it should be.

If cut edge corrosion has been treated historically, then re-treatment might be uneconomical or impractical for a number of reasons.

We are often asked if an existing cut edge corrosion treatment can be over-coated with our product, but in order for our system warranty to be provided, the old treatment needs to be removed and the cut edges of the roof sheets will need preparing back to bare metal.

Often, we find that an old cut edge corrosion treatment will be masking a more serious problem, especially if the mid-lap overlapping joint has been sealed with silicone.

There is also the issue of adhesion of our cut edge corrosion treatment to other systems. Because systems are rarely compatible, we do not normally recommend overcoating other treatments with Flexlap.

Here’s an image of a roof that has been treated with a cut edge corrosion that uses silicone to seal the overlapping roof sheets.

Rust has broken through the surface of the old treatment.

In cases like this, it is imperative that the old treatment is scraped back and the substrate examined.

Usually, we find that the rust breakthrough is due to water trapped behind the silicone sealant.

This results in more the sheet ends becoming more corroded and often, you will find that the steel is rotting away.

In severe cases, the bottom sheet of the overlap is also corroded, perforated or completely rotted, because the old treatment has effectively hidden the problem.

If you specify cut edge corrosion treatments on industrial premises, always look closely at existing treatments that are present, because it may not be possible to re-treat!

When in doubt, call one of our building surveyors for their recommendation, because sometimes, re-treatment is not the answer.

metal roof with cut edge corrosion

Sometimes, sealing overlapping roof sheets might be necessary

There are occasions when it might be necessary to treat top and bottom sheets at the mid-laps, but in these cases, we do not recommend sealants, as they split easily, trap moisture and cause a latent defect.

Instead, we utilise our own HP Butyl Tape, a geo-textile faced, 20 year waterproofing tape that is far stronger than sealants.

This won’t necessarily stop or prevent underside corrosion, but it might help reduce the amount of moisture within the overlaps.

Sealing laps is at your own risk, since water could always be present beneath, due to condensation and potential leaks from other areas in the roof.

But if you really prefer using a sealant

We won’t stop you. We have a product called Lapsil that’s made just for this purpose, but again, it’s at your risk and down to your preference.

How To Seal Overlapping Roof Sheets

Strengthening The Mid-Lap Seal

When considering sealing overlapping metal roof sheets, the size of the gap between the upper and lower sheets needs to be considered.

When trafficked, the gap between overlapping roof sheets increases, but this effect can be reduced by stitching the sheets 100mm or so from the edge of the top sheet, ideally using stitcher screws, prior to treating the cut edge corrosion.

If rivets are used fro this purpose, only stainless, sealed head rivets with an incorporated washer should be used.

Liquasil does not recommend the sealing of overlapping metal roof sheets in most cases. Where sealing is deemed necessary, we strongly recommend that additional fixings or stitchers are incorporated to reduce inter-sheet movement due to foot traffic. Sealing overlapping roof sheets may trap moisture or condensation, which can cause under-side corrosion of the roof sheets. Careful consideration should therefore be given before sealing overlapping roof sheets, regardless of the sealing method.

Cleaning The Flexlap System

Flexlap is a soft silicone coating that in common with other cut edge corrosion treatments, is susceptible to environmental dirt pickup.

The unique composition of Flexlap makes it easier to clean than other cut edge corrosion treatments.

All that is required is a weak solution of washing up liquid and water. Simply wash the surface with a sponge or mop, rinsing regularly to avoid smearing dirt over adjacent surfaces.

Sometimes, underside corrosion will be present at mid-laps and this can result in unsightly rust staining being noticable below the treated areas. This is not a defect and will usually wash off with soap and water.

We do not recommend that jet washing equipment is used to clean Flexlap installations.

All installations should be examined annually for signs of surface damage and, if noted, the affected area should be cleaned, dried thoroughly and touched up with Flexlap.

Whilst Flexlap does not support fungal growth, it is possible for algae and soil to settle on the surface.

Washing will usually remove this, and Liquasil can provide an anti-fungicidal wash to help delay any recurrance.

Please note that Flexlap can only be touched with Liquasil products. Other products are not likely to adhere to the treated surface.

Roofing Contractors - Profit With Liquasil

Did you know that the supply of our BBA Approved cut edge corrosion treatment is restricted to experienced roofing and roof coating contractors that have completed our free product training?

Our systems are cheaper than the alternatives, yet out-perform them in so many ways.

Training is free. A small investment in time could easily be repaid countless times.


For a better, more considered solution for cut edge corrosion, there is no better system than Flexlap.

To learn more about it, please call 0121 709 5352 and speak directly to one of our building surveyors.