Thermoplastic Polyolefin (TPO) roofing systems are widely used on commercial buildings because they are energy-efficient, durable, and resistant to UV exposure. However, like any roofing system, proper installation is critical to long-term performance. One installation defect that can significantly reduce the effectiveness of a TPO roof is the formation of fishmouths in TPO flashing.
Fishmouths may appear minor at first glance, but they can become entry points for water, compromise the roof’s weatherproofing, and shorten the lifespan of the roofing system. Understanding what causes fishmouths and why they should be repaired promptly can help building owners avoid costly leaks and premature roof failures.
What Is a Fishmouth in TPO Flashing?
A fishmouth is a wrinkle, fold, or raised opening that develops along the edge of a TPO flashing membrane. Instead of lying completely flat against the roof surface or vertical penetration, a portion of the flashing lifts and creates a small gap that resembles an open fish’s mouth.
Fishmouths most commonly occur around:
- Roof penetrations such as pipes and vents
- HVAC curbs
- Parapet walls
- Corners and transitions
- Equipment supports
- Drain flashings
Because flashing is designed to create a watertight seal around these vulnerable areas, even a small opening can reduce the roof’s ability to keep moisture out.
What Causes Fishmouths?
Several installation and material-related issues can lead to fishmouth formation.
Poor Installation Technique
The most common cause is improper installation. If the flashing membrane is not aligned correctly before welding or adhesive application, excess material can bunch together and create wrinkles.
Experienced installers carefully smooth the membrane before permanently attaching it.
Excessive Material Tension
Stretching the TPO membrane too tightly during installation creates internal stress. As temperatures fluctuate, the material naturally expands and contracts, causing stressed sections to lift and form fishmouths.
Proper membrane relaxation before fastening helps prevent this issue.
Improper Heat Welding
TPO seams rely on heat welding to create a permanent bond. If insufficient heat, pressure, or welding speed is used, portions of the flashing may fail to bond completely.
Weak welds often separate over time, creating openings that resemble fishmouths.
Complex Roof Details
Inside corners, outside corners, curved surfaces, and irregular penetrations require carefully fabricated flashing pieces.
Attempting to force flat membrane into complicated shapes often results in folds and raised edges.
Membrane Contamination
Dust, dirt, moisture, oils, or debris trapped beneath the flashing can prevent proper adhesion or welding.
Clean, dry surfaces are essential for durable flashing installation.
Thermal Movement
Commercial roofs experience continuous expansion and contraction from changing temperatures.
If flashing is installed without allowing for normal movement, stress may eventually create wrinkles or lifted edges.
Why Are Fishmouths a Serious Problem?
Although fishmouths may appear cosmetic initially, they often become functional roofing failures.
Water Intrusion
The primary concern is water infiltration.
Rainwater can enter through even a small opening and migrate beneath the membrane, causing hidden moisture damage before leaks appear inside the building.
Seam Failure
Fishmouths frequently indicate incomplete or weakened welds.
Over time, wind, moisture, and thermal movement enlarge these openings, allowing seam failures to spread.
Wind Damage
Raised flashing edges are vulnerable to wind uplift.
Strong winds can catch lifted sections, pulling them farther away from the substrate and increasing the risk of membrane damage during storms.
Reduced Roof Lifespan
Repeated moisture intrusion can damage insulation, deteriorate roof decking, and accelerate aging of the roofing assembly.
Ignoring small flashing defects often results in significantly larger repair projects later.
Warranty Concerns
Many roofing manufacturers require flashings to be installed according to strict specifications.
Improper flashing details or unrepaired fishmouths may affect warranty coverage if they are determined to result from installation deficiencies rather than material defects.
Can Fishmouths Be Repaired?
Yes. In many cases, fishmouths can be repaired successfully if discovered early.
Professional repair typically includes:
- Inspecting the surrounding flashing and seams
- Determining whether the membrane has been contaminated or damaged
- Trimming or relieving the fishmouth if necessary
- Re-welding the membrane using manufacturer-approved procedures
- Installing a properly welded TPO cover patch when required
- Testing the repaired seam to verify a watertight bond
Large or widespread flashing defects may require replacement of the affected flashing assembly instead of a localized repair.
How Can Fishmouths Be Prevented?
Prevention begins with quality installation and routine roof inspections.
Best practices include:
- Hiring experienced commercial roofing contractors familiar with TPO systems
- Following manufacturer-approved flashing and welding procedures
- Allowing membrane relaxation before fastening
- Maintaining clean welding surfaces
- Performing seam probing and quality control after installation
- Scheduling regular roof inspections to identify minor defects before they worsen
Shieldline Roofing’s Expert Opinion
Fishmouths in TPO flashing are usually a sign of poor detailing, inadequate membrane preparation, or improper installation. We treat them as defects that should be corrected promptly because they can create weak points where water can enter and compromise the flashing system.
Our Key Insights
A fishmouth is a wrinkle or raised fold that prevents the flashing from lying flat and bonding properly. It can create an unsealed area or stress point, increasing the risk of water intrusion and premature failure—particularly around corners, curbs, and other roof penetrations.
Trust Shieldline Roofing for Expert TPO Roof Installation and Repairs
Fishmouths in TPO flashing are more than cosmetic imperfections—they are warning signs that the roof’s waterproofing system may be compromised. Early detection and professional repairs help prevent leaks, preserve insulation, maintain manufacturer warranties, and extend the service life of your commercial roof.
At Shieldline Roofing, our experienced commercial roofing specialists install, inspect, repair, and maintain TPO roofing systems according to industry best practices and manufacturer specifications. Whether you need flashing repairs, preventive maintenance, or a complete roof evaluation, we provide reliable solutions that protect your building and maximize the performance of your roofing investment. Learn More
