Severe Underside Corrosion At Mid-Lap

| Case Study

Overview

This case study documents a severe instance of concealed corrosion within the lap joints of a profiled metal roof, resulting in the complete loss of the lower sheet across the purlin line.

The condition was identified during inspection despite the absence of internal water ingress, highlighting the risk of significant hidden deterioration within lap joints.

Roof Construction (Observed)

Profiled metal roof (material not confirmed on site)
Built-up construction with internal lining panels
Mid-lap overlaps with applied sealant
Roof supported on steel purlins

Note: The outer sheet material was not tested on site. Observations are based on visible condition only.

General Roof Condition (Topside)

From a top-side inspection:

The roof appears broadly serviceable
Isolated corrosion spots and coating degradation are visible
Mid-lap joints have been previously sealed
No widespread failure of the outer sheet is immediately apparent

Importantly: The roof was not actively leaking internally at the time of inspection.

image: general view of metal roof sheet with sealed mid-lap cut edge corrosion treatment

General Roof Condition: Appears broadly serviceable with no obvious indication of concealed failure.

Progression of Deterioration At Mid-Lap Joints

image: metal profiled roof sheet with corroded elements

Complete loss of the original protective finish throughout the roof

image: underside corrosion spots evident on a failing cut edge corrosion treatment

Historical cut edge corroison treatment with sealed mid-lap joint & spot-corrosion

Image: perforated metal roof sheets underside corrosion on failed cut edge corrosion treatment

Perforations evident to the uppermost sheets at the mid-lap joints

image: cut edge corrosion treatment with sealed mid-laps and underside corrosion resulting in sheet loss

Loss of structural integrity to the bottom sheets at the mid-lap joints

Key Finding

The lower sheet had corroded to the extent that no intact structural material remained, with the lap effectively held together by the applied sealant.

From above, the roof appeared serviceable. Within the lap, the sheet had already failed.

Condition Within the Lap

Inspection into the lap void confirmed:

Absence of continuous lower sheet material
Remnants adhered to the upper sheet
Material loss occurring within the joint

The primary deterioration has occurred out of sight within the lap.

Important Observation

The most severe deterioration is occurring where moisture is retained—not where exposure is greatest.

Where lap joints have been sealed or previously treated, consideration should be given to how moisture may behave within the system over time.

Reason for Refusal

Structural integrity compromised at the support line

Defect extends beyond surface corrosion

Coating would not restore structural capacity

The roof was assessed as unstable and unsuitable for a coating-based repair.

Summary

Severe concealed corrosion within lap joints
Complete loss of lower sheet at purlin line
No internal leakage despite structural failure
Driven primarily by moisture retention
Repair not viable using coating systems

Inspection Footage – Internal Condition at Mid-Lap

This footage shows the condition within the lap joint following localised failure.

The lower sheet has completely deteriorated within the joint, with no continuous material remaining across the purlin line.