Case Study
Progression of Underside Corrosion at Mid-Lap (Top Sheet Failure Sequence)
This case study documents the progressive deterioration of the upper sheet at a mid-lap joint on a large built-up metal roof, captured across multiple stages from initial coating breakdown through to full material loss.
Unlike more severe forms of concealed lap corrosion, the deterioration in this case was confined to the upper sheet. The lower sheet remained intact, although rust staining was visible where corrosion products had washed onto the sheet below during rainfall.
This made the case particularly useful from an inspection and specification perspective, as it demonstrated how the condition can progress over time before reaching complete failure.
Roof Context
The roof inspected was a large built-up metal roof of approximately 14,000m². Corrosion was present at mid-lap joints, together with more general coating loss across the wider roof area.
The deterioration was observed to affect the underside of the upper sheet only. A butyl seal was present above the highest point of corrosion, and this appeared to be preventing direct water ingress into the overlap from above.
This distinction is important, as it suggests a different corrosion environment from cases where moisture is continuously entering and cycling freely within the lap.
Progression of Corrosion
The images below illustrate four distinct phases in the progression of underside corrosion affecting the upper sheet at the mid-lap.
Stage 1 – Pre-Perforation
Corrosion has advanced to the point where coating loss and section thinning are evident, but perforation has not yet occurred.
Stage 2 – Initial Perforation
Small perforations begin to appear where corrosion has progressed through the remaining thickness of the upper sheet.
Stage 3 – Advanced Perforation
Perforations enlarge as material loss continues, reducing continuity of the upper sheet across the lap area.
Stage 4 – Complete Loss of Upper Sheet
The upper sheet has deteriorated completely at the affected location, exposing the lower sheet beneath.
Stage 1: Coating breakdown and corrosion immediately prior to perforation.
Stage 2: Initial perforation of the upper sheet at the mid-lap.
Stage 3: Enlargement of perforations as material loss progresses.
Stage 4: Complete loss of the upper sheet at the affected area.
Corrosion Mechanism (Assessment)
The condition observed is consistent with progressive underside corrosion of the upper sheet at the mid-lap.
The presence of staining to the lower sheet indicates that water is moving through the lap and carrying corrosion products from the upper sheet as deterioration progresses. However, the existence of a butyl seal above the highest point of corrosion appears to be limiting direct water ingress into the overlap.
This suggests that, in this case, the defect is not being driven by unrestricted moisture cycling throughout the lap void, but by a more contained deterioration mechanism affecting the underside of the upper sheet.
The suitability of any remedial approach therefore depends not only on the extent of corrosion, but on the overall condition of the original roof sheet finish and the broader condition of the roof system.
Why This Matters
This case is significant because it demonstrates visible progression of a defect that can move from localised corrosion to full material loss on the same sheet.
It also shows that:
- underside corrosion at laps does not always affect both sheets
- staining of the lower sheet does not necessarily indicate loss of the lower sheet itself
- the decision between localised treatment, full coating, or replacement depends on more than the extent of corrosion alone
In this case, the wider condition of the original plastisol finish across the roof was a major factor in considering the appropriate remedial strategy.
Inspection Overview
The video footage shown here was recorded during the inspection and provides a general overview of the roof condition, including the wider loss of finish across the roof surface and the progression of corrosion at mid-lap joints.
The footage is provided for general illustration of observed conditions and should be considered alongside the written findings.
Specification Options Considered
Based on the observed condition, two broad approaches were considered. The following outlines the technical basis of each approach and the associated project implications.
Option 1 – Replacement of Upper Sheets
Replacement of the affected upper sheets across the lap lines would remove deteriorated material and reinstate continuity of the outer weathering layer. This represents the most direct method of addressing material loss at source.
However, the extent of deterioration observed across the roof would require replacement of a substantial proportion of the roof sheets. On a roof of this scale, this introduces significant cost and programme implications.
In addition, where works involve replacement of a large proportion of the roof covering on a built-up system, consideration may need to be given to the potential application of current Building Regulations, including thermal performance requirements under Part L. This may introduce further design, cost, and programme considerations depending on the extent of intervention and project-specific circumstances.
Such works may also result in disruption to the operation of the building during the construction period.
Option 2 – Localised Remediation with Coating System
An alternative approach involves targeted remediation of the most severely affected areas, combined with a full coating-based refurbishment of the roof.
This would typically include:
- localised removal of severely deteriorated sections of the upper sheet
- mechanical preparation of exposed surfaces to ST3 standard
- application of corrosion protection to prepared areas
- installation of a reinforced lap detail across the joint
- application of a full protective coating system across the roof
This approach does not reinstate original sheet thickness or structural capacity, but is intended to reduce exposure to further deterioration and provide a continuous protective barrier across the roof surface.
In this case, the presence of an existing seal above the lap joint appears to limit direct water ingress into the overlap, which may influence how corrosion has developed and may be a relevant factor when considering the suitability of this type of intervention.
As this approach does not involve replacement of the roof build-up, it may avoid the need for more extensive regulatory or structural intervention, subject to project-specific assessment.
Key Technical Distinction
This case differs from more severe examples of concealed lap corrosion in that the lower sheet remained intact and active moisture ingress into the lap appeared to be limited by an existing seal above the corroded zone.
As a result, the issue in this instance was not simply the extent of corrosion, but the relationship between:
- the progressive loss of the upper sheet at the lap
- the wider condition of the original roof finish
- the scale and cost of alternative interventions
- the potential implications of extensive sheet replacement on a built-up roof
Summary
- progressive underside corrosion of the upper sheet was observed across four distinct stages
- the lower sheet remained intact, although rust staining was present
- the wider roof also exhibited general loss of original finish
- replacement of affected upper sheets would represent the most direct intervention, but with major cost and programme implications
- localised remediation combined with a full coating strategy may be considered where the defect is contained and the lower sheet remains intact
- the appropriate approach depends on the overall condition of the roof and the extent of intervention required