Cut edge corrosion is one of the most common defects affecting profiled metal roofs. While the defect itself is well understood, the economic implications of how it is treated are often overlooked.
In many specifications, treatments include additional steps—such as sealing overlaps or coating non-affected areas—which can significantly increase cost without necessarily improving performance.
This guidance examines:
- how cut edge corrosion develops
- how it is typically treated
- where unnecessary cost can be introduced
- and what building surveyors should consider when specifying remedial works
What Is Cut Edge Corrosion?
Cut edge corrosion occurs where the factory-applied coating at the sheet edge deteriorates, exposing the underlying galvanised steel to moisture, oxygen, and environmental pollutants.
Over time this leads to;
- coating peel-back
- surface corrosion
- progressive loss of protection at sheet ends and overlaps
Where It Occurs on a Roof
- Eaves / end laps – this is commonly referred to “End-Lap Corrosion.”
- Mid-lap overlaps – most commonly expressed as “Mid-Lap Corrosion.”
- Areas of high exposure or water run-off
Key observation:
In most cases, corrosion is confined to the top sheet only, while the underside or lower sheet remains intact.
What is rarely discussed is underside corrosion, which if allowed to progress in extreme cases, can lead to complete loss of structural integrity of the roof sheets.
Unlike cut edge corrosion, which is visible and treatable, underside corrosion often develops unseen and in many cases, cannot be effectively repaired once established.
The most serious corrosion is often the least visible—and in some cases, introduced by the treatment itself
How Cut Edge Corrosion Is Typically Treated
Traditional specifications for the treatment of this defect often include:
- Abrasion and preparation of affected areas
- Application of corrosion primer
- Coating of both top and bottom sheets
- Sealing of overlapping sheets using silicone sealant or a mastic
This approach is often adopted as standard practice, rather than being assessed on a case-by-case basis.
Where Additional Cost Is Introduced
1. Treating Non-Affected Areas
Where only the top sheet is corroded, coating both sheets:
- doubles material usage
- increases labour time & cost
- adds no direct benefit to unaffected areas
2. Sealing Overlapping Sheets
Sealing mid-laps typically involves:
- application of silicone or other sealant along each overlapping horizontal roof joint
- additional preparation and finishing time
This can result in:
- significant additional material cost
- increased installation time
- higher overall project cost
The Risk of Unnecessary Interventions
Where overlaps are sealed without clear justification, potential issues include:
- movement-induced splitting of sealant
- capillary ingress of water
- trapping of moisture within the lap
Over time, this can lead to:
- concealed underside corrosion
- deterioration of previously unaffected areas
- more extensive future repairs
- possible loss of structural integrity to the roof
Where sealing is introduced, particularly in areas subject to movement or incomplete bonding, conditions can arise where water becomes trapped within the lap.
While corrosion at end laps and mid-laps is typically visible and can be assessed during inspection, it is important to consider what may be occurring beneath the surface.
Where additional materials or sealing measures are introduced at lap joints, the way in which moisture behaves within the roof construction can change. In particular, where movement, incomplete bonding, or minor defects occur, conditions may arise that allow water to enter the lap but not readily escape.
It is in these circumstances that a more serious form of deterioration can develop—one that is often not identified until significant damage has already occurred.
What Do Independent References Say?
The British Steel Colorcoat® Inspection and Maintenance Manual recommends:
- removal of loose coating and corrosion
- preparation to a firm substrate
- application of primer and coating to affected areas
Importantly:
There is no requirement to seal overlapping sheets as part of standard cut edge corrosion treatment.
This supports a targeted treatment approach, focusing only on areas affected by corrosion.
A More Targeted Approach
A defect-led specification typically involves:
- treating only the affected cut edge
- applying a suitable corrosion protection system
- protecting the treatment with a durable topcoat
This approach:
- reduces material usage
- simplifies application
- avoids introducing unnecessary elements into the roof system
Economic Comparison (Conceptual)
| Approach | Material Use | Labour | Risk Profile |
|---|---|---|---|
| Full overlap treatment | High | High | Potential for trapped moisture |
| Targeted cut edge treatment | Lower | Lower | Lower risk of unintended consequences |
Practical Specification Considerations
When reviewing or writing a specification, consider:
- Is the defect confined to the top sheet?
- Is there evidence that overlaps require sealing?
- Does the treatment introduce elements that could create future issues?
- Is the scope proportionate to the defect observed?
When Sealing May Be Justified
Balance is important.
Sealing overlaps may be appropriate where:
- the original butyl seal has failed
- there is active water ingress at laps
- the roof design relies on lap sealing for performance
- an older cut edge corrosion treatment with sealed laps is already present
There may be cases where sealing is justified, such as where existing lap seals have failed or active ingress is present. However, this should be determined by inspection, not applied as a default approach.
Underside Corrosion at Mid-Lap Joints
Unlike cut edge corrosion, which is typically visible and progresses from exposed sheet edges, underside corrosion develops out of sight within the lap between roof sheets.
In many built-up metal roof systems, the underside of the sheets remains in good condition for extended periods, as it is protected from direct exposure to weathering and ultraviolet degradation. However, where moisture is able to enter the lap and becomes retained, the internal environment of the joint can change significantly.
Where sealing or additional materials are introduced at overlaps, particularly in areas subject to thermal movement or where bonding is not continuous, there is potential for moisture to be drawn into the joint through capillary action. If that moisture cannot readily evaporate or drain, it may remain in contact with the underside of the sheet.
Over time, this can lead to corrosion developing on the internal face of the lower sheet—an area that cannot be readily inspected or maintained.
In a recent inspection, corrosion of this nature had progressed across the purlin line to the extent that the underside of the lower sheet had effectively deteriorated completely. The remaining integrity of the lap was, in practical terms, being maintained by the previously applied sealant, with no intact steel remaining beneath.
This type of deterioration is particularly significant because, unlike surface corrosion at cut edges, it is often not possible to treat effectively once established. In many cases, the only viable outcome is localised or full sheet replacement.
For this reason, consideration should be given not only to how visible defects are treated, but also to whether the proposed approach may alter the moisture dynamics within the roof in a way that could contribute to concealed deterioration over time.
Summary
- Cut edge corrosion is common and usually straightforward to treat
- Traditional specifications can introduce unnecessary cost
- Treating non-affected areas is often not required
- Sealing overlaps may introduce risk if not justified
- A targeted approach is typically more economical and appropriate
Where treatment extends beyond the defect itself, additional cost is incurred not only in materials and labour, but also in introducing elements that may alter how the roof performs over time.
CPD Value
This guidance is suitable for informal CPD and reflective learning for building surveyors and property professionals.
Estimated reading time: 10–15 minutes