Case Study
Moisture Retention at Sealed Mid-Lap Without Underside Corrosion
This case study documents a localised instance of moisture retention within a sealed mid-lap joint on a large industrial metal roof.
The roof had an existing cut edge corrosion treatment, with sealed mid-laps present across the roof. The treatment was generally in serviceable condition, although isolated areas of sealant splitting were identified during inspection.
Unlike more severe cases of concealed lap corrosion, inspection of the affected areas did not identify underside corrosion, sheet perforation, or material loss. This made the case useful in demonstrating the importance of assessment rather than assumption.
Roof Context
The roof inspection was conducted in anticipation of a wider refurbishment strategy to recoat the roof.
An existing cut edge corrosion treatment was present, including sealed mid-lap joints. In general, the previous treatment remained in reasonable condition, but isolated splits were observed in the sealant at some lap locations.
Because sealed mid-laps can create conditions where moisture may be retained within the joint, these areas were examined for evidence of underside corrosion, sheet perforation, and loss of structural integrity.
Inspection Observation
Several areas of split sealant were examined during the inspection. In most locations, no evidence of significant deterioration was identified beneath or around the lap joint.
However, at one localised area, moisture was released from within the sealed lap when the roof sheet was trafficked. This indicated that water had become retained within the joint, despite the absence of visible corrosion or perforation at that location.
This observation was significant because it confirmed moisture retention within the lap, but without the more serious deterioration that is sometimes associated with this type of condition.
Inspection Footage
The following footage shows moisture being released from the sealed mid-lap joint during inspection.
The video is provided for general illustration of the observed condition and should be considered alongside the written findings.
Interpretation
The presence of moisture within a sealed lap does not, in itself, confirm active corrosion or sheet failure. In this case, the surrounding inspection did not identify hidden cut edge corrosion, associated perforation, significant material loss, or widespread underside deterioration.
Based on the localised nature of the finding, the moisture source appeared more likely to be associated with a defective seal or localised entry point rather than widespread capillary-driven moisture movement through the lap.
This distinction is important. Moisture retained within a lap joint should always be investigated, but the appropriate remedial response depends on what is found during inspection, not simply on the presence of moisture alone.
Why This Matters
This case demonstrates that similar visual symptoms can have different implications depending on the underlying condition of the roof.
In some cases, split sealant at lap joints may be associated with concealed corrosion, perforation, or loss of sheet integrity. In this instance, the condition appeared to be localised, with no significant underside corrosion identified during inspection.
The finding reinforces the need to inspect suspect areas carefully before determining whether a roof remains suitable for coating-based refurbishment.
Specification Implication
The roof remained a candidate for coating-based refurbishment, subject to appropriate preparation and treatment of defective details.
Because the moisture appeared to be associated with isolated seal or fixing-related defects, the refurbishment strategy should address potential points of water entry before application of the coating system.
In this case, replacement of the fixings would provide the most robust approach for long-term performance. Where fixing replacement is not adopted, sealing of fixings with a purpose-made sealant may be considered as part of the coating process.
The key point is that the coating system should be applied only after the likely sources of localised moisture ingress have been addressed.
Recommended Approach
The preferred approach for long-term durability would be to replace the existing fixings as part of the roof refurbishment works. This would address a common point of potential water entry and reduce reliance on localised sealant repairs.
As an alternative, where fixing replacement is not undertaken, each fixing should be inspected and treated appropriately. This may include sealing with Liquasil Non-Sag Sealer before the application of the roof coating system.
Any split or defective lap sealant should also be reviewed and locally remediated where required before coating works proceed.
In this instance, for longevity of warranty, the existing cut edge corrosion treatment is to be removed and reinstated with a Liquasil equivalent as an integral part of the refurbishment process.
Key Technical Distinction
This case differs from more severe examples of concealed lap corrosion because moisture retention was observed without associated underside corrosion or loss of sheet integrity.
That distinction matters. The presence of water within a sealed lap requires investigation, but it does not automatically mean that the roof is unsuitable for coating.
Where the condition is localised, the substrate remains intact, and likely moisture entry points can be addressed, coating-based refurbishment may remain a viable approach.
Summary
This inspection identified localised moisture retention within a sealed mid-lap joint on a large industrial metal roof.
Although moisture was released when the sheet was trafficked, no associated underside corrosion, perforation, or sheet loss was identified in the inspected area.
The condition appeared to be localised and more likely associated with a defective seal or fixing-related entry point than widespread capillary-driven moisture movement.
The roof remained suitable for coating-based refurbishment, provided defective details were addressed as part of the preparation and coating process.
Related Guidance
👉 Severe Underside Corrosion at Mid-Lap – Case Study
👉 Progression of Underside Corrosion at Mid-Lap
👉 Cut Edge Corrosion Treatment
👉 Metal Roof Coating