Underside Corrosion To Metal Roof Sheets

Industrial Roof Technical Defect Analysis

What Is Underside Corrosion?

Underside corrosion is a form of metal degradation that occurs on the unexposed, internal face of profiled steel roof sheets, typically beneath the sheet laps or within areas where moisture becomes trapped. Unlike cut-edge corrosion or coating breakdown on the weathered surface, underside corrosion develops out of sight, often progressing unnoticed until the defect becomes advanced.

This form of corrosion can be present on:

Untreated metal roofs
Previously coated roofs
Roofs where cut edges or laps have been sealed as part of earlier refurbishment works

Because it is not visible from above, underside corrosion is frequently underestimated or even missed during routine inspections.

Side laps are often cited as corrosion-prone areas, but in practice, corrosion at side laps is uncommon on industrial metal roofs. Where deterioration is observed, it is more frequently associated with underside corrosion or end-lap detailing rather than the side lap itself.

Why Does Underside Corrosion Occur?

Underside corrosion is primarily driven by moisture entrapment combined with limited drying potential.

Common contributing factors include:

Overlapping sheet geometry that restricts air movement
Condensation formation on the colder internal face of roof sheets
Capillary action at laps, drawing moisture into confined interfaces
Reduced UV exposure, preventing natural drying
Sealed overlaps, which trap moisture that would otherwise evaporate

Where roof sheets are sealed at the mid-lap, either intentionally or as a by-product of cut-edge treatments, any moisture entering the gap can remain present for extended periods. This creates ideal conditions for corrosion to initiate and accelerate on the underside of the sheet.

Why the Defect Is Often Missed During Inspections

Underside corrosion is one of the most commonly overlooked metal roof defects, particularly during landlord-led or tenant-led inspections.

Key reasons include:

The defect is not visible from the external roof surface
Drone surveys and visual walkovers provide no indication of underside condition
Internal inspections will miss the defect due to the presence of internal roof liners
Greater focus is typically placed on visible defects such as cut-edge corrosion, as this is the easier and most obvious win for surveyors

As a result, roofs may be described as “treated” or “maintained” while significant corrosion is progressing unseen beneath the roof sheets.

How Does Underside Corrosion Differ From Cut Edge Corrosion?

Cut edge corrosion refers to “top-down” corrosion, which starts as edge-peel of the original finish.

As the edge of the finish recedes, the steel substrate is exposed and rust begins to form on the surface.

Cut edge corrosion can be treated relatively easily. Underside corrosion can be hidden, but it cannot usually be treated.

The image above shows a roof inspection where underside corrosion is present.

It is evidenced by the obvious staining to the bottom sheet on the mid-lap and also by the perforation to the uppermost roof sheet.

In this case, the sheets are tightly compressed together which can cause moisture to become trapped, which is how underside corrosion develops.

The rules are used to determine the presence of a butyl weatherproofing strip to protect against water ingress.

Risks Associated With Leaving Underside Corrosion Untreated

If left unaddressed, underside corrosion can lead to:

Progressive loss of sheet thickness
Localised perforation of roof sheets
Increased risk of water ingress during wind-driven rain
Structural weakening at laps and fixings
Escalating repair costs once the defect becomes visible

From a dilapidations perspective, underside corrosion represents a latent defect that may still fall within the tenant’s repairing obligations, even where superficial treatments have previously been applied.

It is vitally important for incoming tenants to have a thorough roof inspection conducted before signing a lease, because missing this defect could cost them dearly at the end of the lease.

How to Identify Underside Corrosion

The defect is usually present on metal roofs where a existing cut edge corrosion treatment is already present that seals the mid-lap joint with a sealant.

Surveyors should look for visual indicators such as rust staining streaks on the surface of the sheets, rust spots on the surface of existing cut edge corrosion treatments, splits to sealant and areas where existing treatments appear to have failed.

Underside Corrosion and Dilapidations Risk

Underside corrosion is particularly significant for dilapidations surveyors acting for landlords at lease end.

It is frequently encountered where:

A roof has been previously “refurbished” using surface-level treatments
Cut edges have been sealed without consideration of moisture movement
The roof appears visually acceptable from above

In these scenarios, the defect may be:

Present at lease commencement and expiry
Worsening due to earlier remedial works
Not immediately apparent to the tenant

This creates a strong evidential basis for further investigation and, where appropriate, remedial liability.

Common but Ineffective Repair Approaches

Several commonly specified interventions can inadvertently worsen underside corrosion:

Blanket Sealing of Overlaps

Sealing mid-laps or end-laps without addressing moisture pathways can trap water within the sheet interface, accelerating underside corrosion rather than preventing it.

Overcoating Existing Treatments

Applying new coatings over unknown or incompatible systems can conceal ongoing corrosion beneath, without arresting its progression.

Treating Visible Defects Only

Addressing cut-edge corrosion or surface coating breakdown in isolation often leaves underside corrosion untouched.

These approaches may improve appearance in the short term but do little to manage long-term risk.

Considerations for Long-Term Remediation

Effective management of underside corrosion requires:

Proper inspection and assessment, including internal viewing where practicable
Removal of existing treatments where compatibility or moisture entrapment is suspected
Avoidance of unnecessary lap sealing unless specifically justified
Use of systems that respect vapour movement and drying potential

Remedial strategies should be driven by defect pathology, not by convenience, appearance or length of a product warranty.

When a Coating-Based Solution May Be Appropriate

A coating-based refurbishment may be appropriate where:

The roof sheets remain structurally sound
Corrosion is at an early or moderate stage
Surface preparation allows proper assessment of existing conditions
The system design does not rely on indiscriminate lap sealing

In such cases, coating systems should be selected and detailed to address cut edges and exposed steel, while avoiding interventions that could increase the risk of concealed underside corrosion.

Where underside corrosion exists beneath an old cut edge corrosion treatment, it will not be possible to fully reinstate the condition of the roof without isolated or even wholesale sheet replacement. This should be factored into any specification, even if only as a contingency.

Summary for Surveyors

Underside corrosion is a high-risk, low-visibility defect that is frequently underestimated in metal roof assessments. Its presence can significantly alter the technical and commercial outcome of dilapidations negotiations, particularly where earlier refurbishment works have concealed or exacerbated the issue. It is important because it cannot be effectively treated, so the decision is whether to replace affected sheets or to hide and slow-down the defect with a robust sealing method such as a coating-compatible waterproofing tape.

Early identification and informed remediation are essential to avoid progressive deterioration and unexpected liability.

Liquasil Specification Wording Where Underside Corrosion Is Present

Liquasil System Considerations (Manufacturer Guidance)

Remove existing cut edge corrosion treatment at mid-laps, ensuring that any sealant is removed as far as reasonably practicable. Prepare mid-lap areas to ST3 Standard (bare, non-shiny metal) following the roof sheet profile across troughs, webs and crowns. Apply Liquasil SWT Corrosion Primer to all prepared areas and allow to dry. Apply Liquasil HP Butyl Tape directly to the primed areas ensuring that the mid-lap joint is fully encapsulated. Use penny rollers to remove any ballooning areas. Fully encapsulate the HP Butyl Tape with Liquasil Flexlap or Metalseal, paying particular attention the to the edges of the tape. Metalseal 20 may require two coats to achieve the recommended film weight of 350-500 microns.