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Industrial Metal Roof Inspection Checklist for Building Surveyors

Technical Defect Analysis

Introduction

Industrial metal roofs can remain serviceable for many decades, but only if defects are identified early and appropriate maintenance or refurbishment measures are implemented.

During condition surveys, acquisition surveys and dilapidations inspections, building surveyors are often required to identify defects that may lead to water ingress or accelerated deterioration of the roof system.

This industrial metal roof inspection checklist is intended as a practical reference to assist surveyors when inspecting industrial metal roofs. It highlights the most common areas where defects occur and the indicators that may suggest further investigation or remedial works are required.

If initial findings suggest wider issues, refer to when a roof survey is required before specifying a coating system for guidance on next steps.

More detailed explanations of specific defects can be found within the Liquasil roof defect guidance pages.

Typical Areas of Inspection on Industrial Metal Roofs

1. Roof Sheet Condition

Inspect the overall condition of the metal roof sheets.

Surveyors should look for:

  • corrosion at exposed sheet edges

  • breakdown of the factory coating system

  • visible perforations or metal loss

  • impact damage or distortion of sheets

  • signs of previous repairs or patching

Where corrosion is present, the extent and progression should be assessed in order to determine whether the deterioration is cosmetic or likely to develop into a structural defect.

2. Roof Fixings

Roof fixings are a common source of water ingress on industrial metal roofs.

Inspect for:

Fixings that have deteriorated or were incorrectly installed during construction may allow water to enter around the penetration.

3. Roof Sheet Joints and Laps

Industrial metal roofs typically include both side lap joints and end lap joints where roof sheets overlap.

Surveyors should inspect for:

Defects within overlapping joints may only become apparent during periods of wind-driven rain.

4. Flashings and Penetrations

Penetrations through the roof covering are frequent sources of water ingress.

Inspect areas such as:

  • flues and ventilation penetrations

  • Dektite or rubber pipe flashings

  • ridge flashings

  • verge flashings

  • abutment details

Particular attention should be given to deteriorated seals, loose fixings or flashing details that do not adequately cover the junction they are intended to protect.

5. Rooflights

GRP rooflights are widely used within industrial buildings and can deteriorate significantly with age.

Surveyors should check for:

  • cracks or holes in the rooflight surface

  • degradation of the protective gel coating

  • failed fixings or sealing washers

  • evidence of water tracking beneath rooflight frames

Degraded rooflights are frequently mistaken for general roof leaks when water enters around the perimeter fixings.

6. Ridge Details and Fillers

The ridge of a metal roof often contains foam profile fillers designed to prevent wind-driven rain entering beneath the ridge flashing.

Inspect for:

  • missing or displaced profile fillers

  • degraded foam fillers

  • failed sealant or fixing points

  • obvious black soot-like staining beneath the ridge line, indicating failure of the profile fillers

Dislodged fillers can allow water or debris to enter the roof construction.

7. Industrial Gutters

Industrial gutters are exposed to constant moisture and debris accumulation and should always be inspected carefully.

Surveyors should assess:

  • corrosion of gutter bases and joints

  • perforation of the gutter lining

  • blocked outlets or debris accumulation

  • insufficient gutter falls causing standing water

  • evidence of previous lining systems

Gutter failure can result in significant internal water damage during heavy rainfall events.

Indicators of Potential Water Ingress

During inspection, surveyors should also look for indirect indicators that may suggest roof defects are present.

These may include:

  • rust staining on roof sheets

  • damp patches or staining internally

  • water marks beneath roof penetrations

  • debris accumulation in gutters

  • evidence of temporary repair materials

In some cases, water observed internally may be caused by condensation rather than roof leaks, particularly where warm moist air within the building condenses on colder roof surfaces.

Recording Inspection Findings

When inspecting an industrial roof, surveyors should typically record:

  • photographs of each identified defect

  • the approximate location of defects on a roof plan

  • the severity and extent of deterioration

  • any areas where further investigation may be required

Accurate documentation assists when preparing inspection reports and recommending appropriate remedial works.

When Further Investigation May Be Required

A more detailed inspection or specialist survey may be required where:

  • corrosion is widespread across the roof sheets

  • water ingress cannot be clearly traced to a specific defect

  • previous coating systems are present and their condition is uncertain

  • gutters show signs of significant deterioration

Where multiple defects are present, a comprehensive refurbishment strategy may be more appropriate than isolated repairs.

Related Technical Guidance

For further information, surveyors may wish to consult the following guidance pages:

These resources provide additional technical detail on inspection techniques and refurbishment strategies.

Frequently Asked Questions

What are the most common causes of leaks on industrial metal roofs?

The most common causes include deteriorated fixings, failed sealant within roof sheet laps, defective flashings, degraded rooflights and corrosion of industrial gutters. Regular inspection is important to identify these issues before significant water ingress occurs.

How often should industrial metal roofs be inspected?

Industrial metal roofs should typically be inspected at least annually, and more frequently where buildings are located in exposed environments or where previous defects have been identified.

Can corrosion on a metal roof be repaired without replacing the roof?

In many cases corrosion can be stabilised and treated using appropriate cut edge corrosion treatment, provided the underlying metal roof sheets remain structurally sound. Early identification of corrosion is important to maximise the effectiveness of refurbishment works.

Are roof coatings designed to waterproof metal roofs?

Protective roof coatings are generally designed to protect the metal substrate from further deterioration and extend the service life of the roof system. The underlying metal roof sheets remain the primary waterproofing element.

As you can see, a metal roof comprises many different components and each one can be source of water ingress. Attempting to seal everything on the roof can cause condensation and other defects, so the diligent surveyor should always check manufacturer’s claims.

Refer to this article about metal roof waterproofing.

Technical Reference

This guidance may be referenced by building surveyors when assessing industrial metal roof defects during condition surveys, acquisition surveys or dilapidations inspections.

All Roof Defects

Roof Defects Index

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.

Image showing mid-lap corrosion on a metal roof with loss of the roof sheet due to lack of maintenance

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.”

Applicable Liquasil Systems

Metalseal BBA Approved Full Metal Roof Coating

Flexlap Cut Edge Corrosion Treatment

Liquasil Lite Budget Roof Coating

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