Repairing Severe Mid-Lap Corrosion Before Roof Coating
| Case Study
Localised Sheet-End Repair
This case study documents the repair of severe corrosion at the mid-lap joints of a profiled metal roof before the application of a protective coating system.
The deterioration had progressed beyond surface corrosion. Sections of the upper sheets had perforated and broken away during pressure washing, leaving insufficient sound metal for conventional preparation and coating.
Rather than concealing the damaged areas beneath sealant or coating, the deteriorated sheet ends were removed and new profiled metal repair sections were installed.
Roof Condition
The roof was formed from overlapping profiled metal sheets with horizontal mid-lap joints.
Corrosion was visible along the sheet ends before preparation began. However, pressure washing exposed a greater degree of deterioration than was initially apparent.
The corrosion had affected the underside and edges of the upper sheets. In several areas, the remaining steel was sufficiently thin that it broke away during cleaning.
Severe corrosion exposed after pressure washing. Sections of the upper sheet had perforated and separated from the remaining sound material.
Extent of Material Loss
Once the deteriorated sections were removed, the scale of the corrosion became clearer.
Large areas of the original sheet ends had suffered substantial section loss. The removed material showed that much of the steel surrounding the lap was no longer suitable for preparation and coating alone.
Removed sections of the original sheet ends showing extensive corrosion, perforation and loss of steel.
Removed sections of the original sheet ends showing extensive corrosion, perforation and loss of steel.
Extent of Material Loss
Once the deteriorated sections were removed, the scale of the corrosion became clearer.
Large areas of the original sheet ends had suffered substantial section loss. The removed material showed that much of the steel surrounding the lap was no longer suitable for preparation and coating alone.
Existing Mid-Lap Detail
​A butyl weathering strip was present between the upper and lower roof sheets.
The strip was positioned within the lap rather than directly at its exposed edge. This created a weather seal between the sheets while leaving the outer section of the lap capable of releasing incidental moisture.
The presence and position of the original butyl strip were relevant when considering how the repair sections would be installed.
The existing butyl strip remained visible between the upper and lower roof sheets after the deteriorated material had been removed.
Repair Approach
Complete replacement of the affected roof sheets may have provided a more comprehensive repair.
On this project, a localised repair was selected. The deteriorated sheet ends were cut back and new profiled metal sections were installed over the remaining upper sheets.
The repair sections were formed to match the existing roof profile and were bedded onto butyl tape before being mechanically fixed.
This approach reinstated metal across the areas where the original sheet ends had been lost, allowing the wider coating project to continue.
New profiled repair sections installed over the remaining upper sheets and fixed through butyl tape beneath.
New profiled repair sections installed over the remaining upper sheets and fixed through butyl tape beneath.
Repair Approach
Complete replacement of the affected roof sheets may have provided a more comprehensive repair.
On this project, a localised repair was selected. The deteriorated sheet ends were cut back and new profiled metal sections were installed over the remaining upper sheets.
The repair sections were formed to match the existing roof profile and were bedded onto butyl tape before being mechanically fixed.
This approach reinstated metal across the areas where the original sheet ends had been lost, allowing the wider coating project to continue.
Preparation Around the Repair
Existing fixings and sealant were removed where necessary to allow the underlying surfaces to be prepared.
Residual sealant was mechanically removed and exposed areas were cleaned and primed before new fixings were installed.
This preparation was necessary to avoid relying on deteriorated sealant, corrosion or poorly bonded material beneath the new repair detail.
Sealing and Initial Coating
Once fixed, the new profiled sections were sealed using Lapsil Sealant.
The repaired areas were then incorporated into the first stage of the Metalseal 20 coating work. This included coating the new metal, prepared surrounding surfaces, fixings and associated repair details.
Retaining the Lap Detail
The exposed end of the repaired lap was not sealed across its face.
The butyl strip beneath the repair section formed the weathering seal. Leaving the outer edge open avoided creating a sealed pocket at the front of the lap and allowed incidental moisture within that outer section to drain back out.
This is an important distinction. The repair reinstated the missing metal and weathering detail without attempting to encapsulate the entire mid-lap joint.
Completed repair detail. The exposed edge of the lap remains open, with the butyl weathering strip positioned beneath the new repair section.
Completed repair detail. The exposed edge of the lap remains open, with the butyl weathering strip positioned beneath the new repair section.
Retaining the Lap Detail
The exposed end of the repaired lap was not sealed across its face.
The butyl strip beneath the repair section formed the weathering seal. Leaving the outer edge open avoided creating a sealed pocket at the front of the lap and allowed incidental moisture within that outer section to drain back out.
This is an important distinction. The repair reinstated the missing metal and weathering detail without attempting to encapsulate the entire mid-lap joint.
Completed Roof Coating
Following completion of the localised repairs, the wider roof was coated using the Metalseal 20 system.
The finished coating provided a continuous protective finish across the original roof sheets and the newly installed repair sections.
The completed roof following localised sheet-end repairs and application of the Metalseal 20 coating system.
Why This Case Matters
This case demonstrates the difference between treating surface corrosion and responding to actual loss of steel.
Where sheet ends have perforated or broken away, coating alone cannot replace the missing material. Some form of metal repair or sheet replacement is likely to be required before a coating system can be considered.
It also demonstrates that roof coating projects do not always begin with a clean or visually attractive substrate. Preparation can uncover defects that require additional decisions before work can continue.
The appropriate response may depend on several factors, including:
- the extent and distribution of corrosion;
- the condition of the remaining roof sheets;
- the suitability of the existing roof structure;
- the intended service period;
- access and operational constraints;
- and the available project budget.
Project-Specific Limitations
This case study records the repair adopted on one particular roof. It should not be interpreted as a standard detail for every instance of severe mid-lap corrosion.
Complete sheet replacement may be more appropriate where deterioration is extensive, where insufficient sound steel remains, or where the condition of the roof covering makes localised repair unsuitable.
Localised repair may nevertheless be considered where the affected areas can be removed, suitable remaining material is available for fixing and the wider roof remains capable of refurbishment.
Any proposed repair should therefore be assessed in the context of the individual roof and the intended scope of the project.
Summary
- Pressure washing exposed severe perforation and material loss at the mid-lap joints.
- The unsound ends of the upper sheets were removed.
- An existing butyl weathering strip was identified within the original lap.
- New profiled metal repair sections were installed over the remaining upper sheets.
- Existing sealant and affected surfaces were prepared before refixing.
- The repairs were sealed and incorporated into the Metalseal 20 coating system.
- The outer edge of the lap remained open rather than being face-sealed.
- The repair represented a project-specific alternative to complete sheet replacement.