Mid-Lap Corrosion

Name: HP Butyl Tape
Brand: Liquasil
Availability: InStock

Technical Defect Analysis

What This Defect Is

Mid-lap corrosion is a term associated with cut edge corrosion, occurring at the horizontal overlap between two profiled steel roof sheets. There is no exposed cut edge on the lower sheet as this is protected by the overlap distance.

The cut edge is present on the upper sheet where it overlaps a bottom sheet, meaning that this exposed cut edge of the uppermost sheet is where coating breakdown and corrosion occurs.

This is essentially the same underlying defect as end-lap corrosion: loss of coating protection at a cut edge leading to progressive steel corrosion.

However, mid-lap corrosion can become more serious where historic lap-sealing cut edge corrosion systems have been applied, as these can split over time, trapping moisture and causing hidden underside corrosion.

Causes

Exposure of unprotected steel at the factory-cut edge of the upper sheet
UV degradation of Plastisol and polyester coatings
Moisture retention within the horizontal overlap
Capillary action drawing moisture into the joint
Historic mid-lap sealing treatments that fail and trap moisture

How to Identify

Peeling or curling of the original coating along the mid-lap line
Rust staining or “gingering” along the exposed cut edge
Cracked or delaminated coatings on the top sheet
Visible rust spots on silicone bands from older cut edge corrosion treatments

Risks if Untreated

Progressive corrosion into the sheet material
Hidden underside corrosion caused by failed mid-lap sealing treatments
Occasional corrosion to the lower sheet if long-term degradation has occurred
Water ingress along the mid-lap joint
Potential sheet perforation and increased repair costs
Potential sheet loss and structural integrity issues

Condition Ratings

Early

Minor coating lift with no active rust.

Moderate

Visible surface rust with early loss of coating protection.

Poor

Cracked coatings, heavier corrosion, and potential water tracking.

Severe

Advanced corrosion with coating loss, perforation, or underside corrosion resulting from failed historic cut edge corrosion treatments.

Recommended Treatment

Flexlap is Liquail’s BBA Approved, stand-alone cut edge corrosion treatment and can be used when a full roof recoat is not required.

As standard, our full metal roof coating includes treating cut edges as standard, using Metalseal rather than Flexlap as the top coat.

Clean the mid-lap area to remove debris, loose material and/or historically applied treatments and coatings.
Mechanically prepare the upper sheet to ST3 standard (bare, non-shiny steel).
Prepare in a straight line following the natural roof profile.
Prime prepared areas with Liquasil SWT Corrosion Primer.
Once dry, apply Liquasil Flexlap (or Metalseal 20/SF if coating the whole roof).
Apply at a wet film weight of approximately 350–400 microns.

Specification Wording For Treating Top Sheet Only

“Thoroughly clean the mid-lap area of the roof sheets to approximately 500mm above the highest point of corrosion. 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 Flexlap (or Metalseal 20/SF for full-roof treatment) to form a uniform, fully bonded protective coating at a wet film thickness of approximately 350–400 microns. Metalseal 20 may require two coats to achieve this film weight.”

Additional Specification Options

Apply Liquasil HP Butyl Tape over primed areas and overcoat with Flexlap or Metalseal if sealing mid-lap joints is specified.
Re-seal with silicone where an existing sealed cut edge corrosion treatment is being replaced. Note that underside corrosion will be present (slows progression but does not resolve the defect).
Replace affected sheets where perforation or underside corrosion is extensive.
Avoid sealing the entire mid-lap unless required, as this can trap moisture.

Suitable Liquasil Products

Liquasil SWT Primer
Flexlap (stand-alone CEC treatment)
Metalseal 20
Metalseal SF
HP Butyl Tape
Liquasil Lapsil Sealant

FAQs

Is mid-lap corrosion different from end-lap corrosion?

The mechanism for treating mid-lap corrosion is the same as for end-lap corrosion, but because of the way industrial roofs are constructed, mid-laps are more likely to suffer from holding moisture, most commonly hidden beneath historic cut edge corrosion treatments.

Can mid-lap joints be sealed?

We recommend that mid-lap joints are only sealed when there is good reason to seal them, for example, if the butyl weather proofing strip between the sheets has perished or is missing, or when an old cut edge corrosion treatment is being replaced.

Resealing the joint can trap moisture and accelerate underside corrosion if not done correctly. Liquasil has 2 options for sealing or resealing overlapping roof sheets:

HP Butyl Tape – a purpose-made, geo-textile faced waterproofing tape designed to be overcoated or
Lapsil Sealant – recommended only if replacing an historically sealed cut edge corrosion treatment

Does the bottom sheet usually corrode?

The bottom sheet does not usually corrode unless either another defect is present or the roof has been neglected over a long period, or where moisture has been held against the steel by older sealing systems.

How is Underside Corrosion Treated?

Please refer to our dedicated technical article for identifying and treating underside corrosion. This is a specialist resource for condition, dilapidations and acquisition surveyors.

In some cases, corrosion may develop progressively at lap joints, leading to partial or complete loss of the upper sheet.
See: Progression of Underside Corrosion at Mid-Lap (Case Study)

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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