Direct Answer
Fishmouths in TPO flashing are raised, crescent-shaped openings or wrinkles that develop when the roofing membrane or flashing does not lie flat and creates a pocket or gap. They commonly occur around corners, angle changes, wall transitions, penetrations, and flashing overlaps. Fishmouths can result from wrinkles in the membrane, improper installation, bridging over changes in height, poor alignment, or movement within the roofing assembly.
Fishmouths are a concern because they can compromise the watertight integrity of a commercial roof. If the opening affects a welded seam or flashing detail, it can create a pathway for water intrusion and expose the roof assembly to moisture damage. In Florida, where commercial roofs experience intense rainfall, heat, and wind-driven rain, improperly repaired flashing defects can become recurring maintenance problems.
The appropriate response is to inspect the affected area, determine whether the fishmouth has compromised the seam or flashing, and repair it using the roofing manufacturer’s approved procedures. Simply pressing down the wrinkle or applying generic sealant over it may not provide a durable repair. GAF’s EverGuard single-ply roofing field guide specifically identifies fishmouths in TPO tape associated with wrinkles or changes in the height of the TPO deck sheet and describes a manufacturer-specified patching approach.
Who This Applies To
- Commercial property owners
- Property and facility managers
- Commercial roofing contractors
- General contractors and construction managers
- Roof consultants and Registered Roof Observers (RROs)
- Condominium and HOA boards responsible for roof maintenance
- Owners managing TPO roofing warranties and insurance requirements
Not for: This guidance is not a substitute for a manufacturer-specific repair specification, a roof warranty determination, or a professional assessment of a damaged roof assembly.
1. What Does a Fishmouth Look Like?
A fishmouth is a raised, pocket-like wrinkle that resembles the open mouth of a fish. It occurs when membrane material folds, overlaps unevenly, or fails to lie flat against the intended surface.
On a commercial TPO roof, fishmouths may appear:
- At TPO flashing tape overlaps
- At inside and outside corners
- Where a roof transitions from a horizontal surface to a vertical wall
- Around rooftop equipment curbs and penetrations
- At membrane transitions or changes in elevation
- Where flashing bridges an angle rather than conforming to it
- Along wrinkles that interfere with properly welded or adhered details
Not every visible wrinkle is automatically a leak. The important question is whether the fishmouth creates an opening, compromises a weld or bond, or prevents the flashing from forming a continuous watertight detail.
2. What Causes Fishmouths in TPO Flashing?
Improper Membrane Positioning
If the flashing material is not positioned correctly before it is welded or adhered, excess material can bunch up and create a raised pocket. Once the material is secured, the wrinkle may remain trapped in the finished detail.
Wrinkles in the Underlying Membrane
Existing wrinkles in the roof sheet can transfer into flashing tape or cover strips. The resulting uneven surface may prevent the flashing material from lying flat and making proper contact.
Changes in Height or Surface Geometry
Differences in elevation between adjacent roof surfaces can create difficult transitions. If the flashing bridges the transition rather than being properly formed around it, a fishmouth may develop.
GAF’s single-ply field guide identifies fishmouths in TPO tape associated with wrinkles and changes in the height of the TPO deck sheet. See the GAF EverGuard single-ply roofing field guide.
Improper Corner Detailing
Corners require careful fitting because the membrane must accommodate a change in direction without leaving gaps or creating excessive folds. Poorly fabricated or installed corner details can create fishmouths that interfere with the flashing’s watertight function.
Incorrect Welding or Bonding Procedures
For heat-welded TPO systems, the membrane surfaces must be clean, properly positioned, and welded using suitable equipment settings and installation procedures. Incorrect preparation or welding can leave defects in the finished seam. Adhesion-based details can also fail if the required surface preparation or application procedure is not followed.
Johns Manville’s TPO Membrane Application Guide explains the importance of proper membrane placement and seam-welding procedures in TPO roof installation.
Movement Within the Roofing Assembly
Building movement, thermal changes, and movement in roof components can contribute to stress around flashing details. These conditions should be considered when investigating recurring fishmouths, especially if the defect reappears after previous repairs.
3. Why Are Fishmouths a Problem?
They Can Compromise Watertight Details
Flashing protects vulnerable transitions and penetrations in the roof system. If a fishmouth leaves an opening or interrupts a properly sealed detail, water may enter the roof assembly.
They Can Lead to Recurring Leaks
A fishmouth that is covered without correcting the underlying defect may reopen or continue to permit water intrusion. Repeated repairs at the same location can indicate that the original cause has not been addressed.
They Can Affect Warranty Compliance
Roof warranties differ by manufacturer and system. A visible fishmouth does not automatically void a warranty, but an installation defect, inadequate repair, or failure to follow maintenance requirements may affect how a particular problem is evaluated.
Owners should review the applicable warranty and contact the manufacturer or authorized roofing contractor before undertaking repairs that could affect warranty coverage. Johns Manville’s Peak Advantage guarantee information identifies fishmouths among the conditions addressed under its published guarantee terms, subject to the applicable provisions and exclusions.
They Can Hide Additional Installation Deficiencies
A fishmouth may be a visible symptom of a broader problem, such as improperly fitted flashing, wrinkles in the underlying sheet, or deficient corner detailing. Inspecting only the raised area without examining the surrounding assembly may leave related defects undiscovered.
They Can Increase Maintenance Costs
Recurring flashing defects can lead to repeated service calls, interior repairs, operational disruption, and additional inspections. Identifying and correcting the cause early can reduce the likelihood of repeated maintenance at the same location.
4. How Should a Roofing Professional Inspect a Fishmouth?
The inspection should establish whether the defect is cosmetic, affects the seam or flashing, or creates an active water-entry risk.
A qualified commercial roofing professional should:
- Photograph the fishmouth and record its location.
- Identify the affected flashing detail and TPO roof system.
- Determine whether the opening affects a welded seam, tape, or bonded detail.
- Inspect adjacent flashing, membrane, corners, and transitions for related defects.
- Check for evidence of water intrusion or previous unsuccessful repairs.
- Review the applicable manufacturer’s installation and repair instructions.
- Document the proposed corrective action.
Where the roof is under warranty, the owner should establish whether the manufacturer or warranty provider must inspect or authorize the repair before work begins.
5. How Are Fishmouths in TPO Flashing Repaired?
The repair depends on the location, size, condition, and cause of the defect. A repair should restore the integrity of the flashing detail rather than simply conceal the raised area.
Correct the Underlying Wrinkle
If an underlying wrinkle or improperly formed transition caused the fishmouth, the repair may require correcting that condition before installing the appropriate patch or replacement flashing.
Use Manufacturer-Approved Repair Materials
Repairs should use compatible materials and follow the requirements for the specific TPO system. Generic sealants or incompatible products should not be substituted for the specified repair materials.
Install the Required Patch or Flashing Detail
Depending on the defect, the repair may require a properly fitted TPO patch, cover tape, or replacement flashing. The repair must provide the required overlap and sealing at the affected detail.
GAF’s single-ply field guide describes installing a TPO cover-tape patch over identified fishmouths in TPO tape and sealing the affected area in accordance with its specified procedure. The correct method should be confirmed against the current instructions for the installed roofing system.
Verify the Completed Repair
After the repair, inspect the finished detail to confirm that the material lies properly, the required seams are secure, and the repair follows the applicable manufacturer’s specifications. Document the completed work with photographs and a service report.
6. Can Fishmouths Be Ignored If the Roof Is Not Leaking?
No. The absence of an interior leak does not prove that a fishmouth is harmless.
Some defects may not currently allow water through the roof assembly, but an opening or compromised flashing detail can become a problem as conditions change. The appropriate response is to assess the actual condition and repair defects that compromise the roofing system in accordance with the manufacturer’s requirements.
At the same time, not every surface wrinkle requires the same repair. A qualified professional should distinguish harmless surface irregularities from defects that affect seam integrity or flashing performance.
7. How Can Property Owners Prevent Fishmouths?
- Use installers trained in the selected TPO roofing system.
- Prepare and position flashing carefully before welding or bonding.
- Ensure that corners and changes in elevation are properly detailed.
- Follow manufacturer requirements for cleaning, welding, and patching.
- Inspect flashing details during installation rather than waiting until project completion.
- Document installation deficiencies and verify corrections.
- Include flashing and penetration details in routine roof inspections.
- Coordinate rooftop equipment work to reduce damage from other trades.
For new commercial construction or roof replacement, inspection of flashing details during installation can help identify workmanship problems before final acceptance and closeout.
8. What Should Be Included in the Repair Record?
Maintain a record that establishes what was found and how the defect was corrected.
- Location and description of the fishmouth
- Before photographs
- Inspection findings
- Identified cause, where established
- Repair scope and materials used
- Contractor service report
- After photographs
- Warranty-related correspondence, if applicable
- Any follow-up inspection requirements
These records can support future maintenance planning, warranty discussions, property transactions, and insurance documentation when relevant.
Bottom Line
Fishmouths in TPO flashing develop when the membrane or flashing does not lie flat, often because of wrinkles, improper positioning, difficult transitions, or installation deficiencies. They matter because they can compromise a watertight flashing detail, contribute to recurring leaks, and create additional maintenance or warranty concerns.
The best approach is to inspect the affected detail, identify the underlying cause, and complete a manufacturer-approved repair. Do not assume every wrinkle is a leak, but do not ignore a fishmouth that compromises the flashing or welded seam.
Related Questions
- What should you do when an underwriting inspection flags open seams or membrane damage?
- What causes commercial roof leaks at flashing details?
- What voids a commercial roof warranty?
Sources
- GAF — EverGuard Single-Ply Roofing Field Guide
- Johns Manville — TPO Membrane Application Guide
- Johns Manville — Peak Advantage Guarantee Information
- GAF — Understanding the Layers of a Single-Ply Roofing Assembly
Last reviewed: October 2026
Related Resources
What Is a Commercial Roof Condition Report?
Need Commercial Roofing Help?
ShieldLine Roofing can help commercial property owners investigate TPO flashing defects, evaluate seam and membrane conditions, and determine the appropriate corrective work for their roofing system.
Disclaimer: This information is provided for general educational purposes and is not a site-specific roof diagnosis, engineering opinion, warranty determination, or insurance coverage decision. Repair methods should follow the requirements of the installed roofing system and applicable manufacturer instructions.
Commercial Roof Inspection Services
