Cut Edge Corrosion Treatment for Metal Roofs
Cut edge corrosion is a common form of deterioration affecting profiled metal roof sheets, typically occurring where protective coatings have broken down at exposed edges.
Targeted treatment can be an effective method of extending service life where corrosion remains confined to accessible areas and the underlying sheet remains in serviceable condition.
Where deterioration extends beyond exposed edges or into lap joints, further assessment may be required to determine the appropriate approach.
What Is Cut Edge Corrosion?
Cut edge corrosion is a defect commonly found on profiled steel roof systems. It is also typically the first defect identified by dilapidations surveyors during roof inspections. This is well understood within our team due to our background in building surveying.
How Roof Sheets Are Made
Roof sheets are produced from large coils of steel that are usually coated with a Plastisol finish before being formed into the profiled shapes used on roofs and cladding.
During manufacture, a protective zinc layer—known as the galvanisation layer—is applied to the bare steel. The sheets are then cut to size with guillotines, but this process leaves the edges exposed.
How Corrosion Begins on Metal Roofs
Soon after installation, the Plastisol coating starts to peel away from the steel and weathering quickly leads to rusting of the substrate.
For this reason, we recommend regular maintenance or roof inspections, because early detection of the defect reduces remediation cost.
If left untreated, corrosion becomes more advanced and the sheet ends may eventually rot through.
Rust and chemical corrosion can lead to roof leaks and internal damage.
Our roofing surveyors have seen many examples where corrosion has progressed beyond the fixing lines. This usually results in whole roof sheets having to be replaced rather than repaired.
It’s best to treat the cut edges as soon as the coating begins to peel, because early action reduces the cost of the repair.
Prompt treatment also helps ensure that costly dilapidations claims at the end of a commercial property lease are avoided or mitigated.
Causes & Signs of Edge Corrosion
Edge corrosion is usually caused by long-term weather and UV exposure.
Deterioration of unprotected cut edges can be accelerated in aggressive environments such as industrial or coastal areas.
When maintenance is poor and inspections infrequent, minor issues can develop into severe corrosion, resulting in significant structural damage to the roof.
We can demonstrate that sealing overlapping roof sheets with sealants and mastics can exacerbate cut edge corrosion in many cases.
We have examples where sealant applied to overlapping roof sheet joints seems to have trapped moisture between sheets.
Trapped moisture causes the metal roof sheets to rot from beneath and this becomes untreatable.
Corrosion can occur within metal roof laps without visible external signs.
Learn more about “underside corrosion“
In some cases, corrosion may develop progressively at lap joints, leading to partial or complete loss of the upper sheet.
👉 See: Progression of Underside Corrosion at Mid-Lap (Case Study)
In extreme cases, where the top and bottom sheets are affected by cut edge corrosion, this can cause a structural defect.
👉 See: Extreme Case of Underside Corrosion at Mid-Lap (Case Study)
In contrast, not all instances of moisture within lap joints lead to corrosion or material loss.
👉 See example: moisture retention at sealed mid-lap without underside corrosion
Edge Peel Of Factory Finish
Assessing the extent of cut edge corrosion
Cut edge corrosion typically initiates at exposed edges, but the extent of deterioration can vary significantly depending on age, environment, and previous treatment.
While corrosion may appear limited to the cut edge itself, consideration should be given to whether deterioration extends beyond the exposed edge into laps or underlying sheets.
The effectiveness of any treatment will depend on whether the condition is confined to accessible areas, or whether it has progressed beyond the visible surface.
When is cut edge corrosion treatment appropriate?
Cut edge corrosion treatment is typically appropriate where:
- corrosion is confined to exposed edges
- the original finsish to the roof sheets is largely intact
- there is no evidence of sheet loss within laps or joints
- the condition can be fully accessed and treated
When should a broader approach be considered?
A more extensive refurbishment approach may be required where:
- corrosion extends beyond the cut edge into the fixing line or underlying sheets
- there is evidence of deterioration of the original finish to the roof sheets
- other defects are present on the roof
- previous treatments or sealing may have altered moisture behaviour
- there is any indication that the structural integrity of the sheet may be compromised
In these situations, the condition of the roof should be assessed more broadly to determine whether a full coating system, or in more severe cases, roof replacement, is the appropriate remedial strategy.
The suitability of cut edge corrosion treatment depends not only on the extent of corrosion, but on the overall condition of the roof sheet finish.
In some cases, deterioration may extend beyond the cut edge into the lap or underlying sheet, where it may not be visible from above.
This may indicate that the roof is no longer suitable for isolated treatment, and that more extensive intervention may be required.
Where deterioration progresses within the lap, corrosion can develop out of sight and may result in loss of material within the joint.
👉 See example: severe underside corrosion at mid-lap
The appropriate approach will depend on whether corrosion is confined to exposed edges or has progressed beyond the visible surface.
👉 See: guidance on selecting an appropriate coating system
A Better Cut Edge Corrosion Treatment
Liquasil was born from a building surveying practice.
Our team designed Flexlap to outperform traditional treatments. Instead of masking the issue with expensive sealants or coatings, Flexlap attacks the root cause.
Simplification is key and that results in lower costs.
Why Flexlap Stands Out
Contractors tell us that it’s simpler, cheaper, more effective and easier to apply than others.
The Flexlap Installation Process
- Prepare the affected surface to ST3 standard, which effectively means back to bare, non-shiny metal
- Apply SWT Primer to prepareed areas at wet film thickness of 125 microns
- Overcoat with Liquasil Flexlap
When cut edge corrosion is in the early stages, we recommend the joints on overlapping roof sheets at the mid-lap areas are not sealed.
Liquasil’s Flexlap can provide significant cost savings against most other cut edge corrosion treatment systems.
Example Cut Edge Corrosion Treatment Projects
Can An Existing Cut Edge Corrosion Treatment Be Over-Coated?
Where cut edges have been treated in the past, it is likely that a system was used that sealed the mid-lap joints.
For this reason, overcoating is not recommended. Inspections routinely show that such systems can trap moisture and cause underside corrosion.
This is a far more serious defect than the original cut edge corrosion.
Older treatments were also applied without an anti-corrosion primer, so corrosion is often still be present beneath the coating.
Because each manufacturer uses different technologies, best practice is for existing treatments to be removed.
This allows the surface to be fully assessed and prepared to achieve a durable repair.
Adhesion between different systems cannot be guaranteed, so starting again is the only reliable approach.
Overcoating existing treatments is usually a false economy.
Sealing Overlapping Roof Sheets Can Cause A Latent Defect
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 your roof looks like this, the old treatment should be scraped back and the substrate examined.
We usually find that the rust breakthrough is due to water trapped behind the silicone sealant.
The sheet ends then become more corroded and often, the steel will be rotting away.
In severe cases, the bottom sheet of the overlap also becomes corroded, perforated or completely rotted, because the old treatment has effectively hidden the problem.
If you regularly look at roofs for dilapidations claims, make a point of looking closely at existing treatments, because your claims could be considerably higher than you think.
When in doubt, call one of our building surveyors for their recommendation, because sometimes, re-treatment is not the answer.
If You Really Have To Seal Overlapping Roof Sheets
How To Seal Overlapping Roof Sheets
There are occasions when it might be necessary to treat top and bottom sheets at the mid-laps and then seal them.
In such cases, we prefer to 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 and help prevent underside corrosion.
Sealing laps is carried out at your own risk because 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 have a product called Lapsil that’s made just for this purpose, but again, it’s at your risk and down to your preference.
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 for 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.
Next Steps
The appropriate approach to cut edge corrosion depends on the condition of the roof sheets, the extent of deterioration, and whether corrosion is confined to accessible areas.
Where corrosion remains limited to exposed edges, targeted treatment can provide an effective means of extending service life. Where deterioration extends beyond the visible surface, a broader refurbishment strategy may be required.
If you are assessing a roof or preparing a specification, our team can provide guidance based on the observed condition of the roof.
👉 Request project-specific guidance or speak to Liquasil