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Mid-Lap Corrosion

Industrial Metal Roof 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

Metal Roof Defects

Metal Roof Defects

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.

Image showing how cut edge corrosion advances on a steel profiled roof. Taken during an inspection by Liquasil Ltd
image: cut edge corrosion treatment with sealed mid-laps and underside corrosion resulting in sheet loss
severe cut edge corrosion beneath an existing cut edge corrosion treatment
Image: perforated metal roof sheets underside corrosion on failed cut edge corrosion treatment
Image showing how cut edge corrosion advances on a steel profiled roof. Taken during an inspection by Liquasil Ltd
severe cut edge corrosion beneath an existing cut edge corrosion treatment
image: cut edge corrosion treatment with sealed mid-laps and underside corrosion resulting in sheet loss
Image: perforated metal roof sheets underside corrosion on failed cut edge corrosion treatment

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.

Best Practice Guides

 

Best Practice Guides

Frequently Asked Questions

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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:

  1. HP Butyl Tape – a purpose-made, geo-textile faced waterproofing tape designed to be overcoated or
  2. 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)