fishmouth
A half-cylindrical or half-conical shaped opening or void in a lapped edge or seam, usually caused by wrinkling or shifting of ply sheets during installation.
fishmouth: wrinkled gap where roof membranes meet
A fishmouth is a void or gap that forms at the overlap between two sheets of roofing membrane, typically single-ply (TPO, EPDM, or PVC) or built-up roofing felts. The opening has a curved, half-cylindrical or half-conical shape that resembles an open fish mouth, hence the name. It occurs at the seam where one sheet should lie flat against another, but instead creates an air pocket that exposes the substrate or lower membrane below.
The defect forms almost always during installation, particularly on sloped roofs or in warm weather when membrane material becomes more pliable. As workers pull and position the top ply across the overlap, small wrinkles or buckles can form in the material. If these are not smoothed out before heat welding or adhesive application, they lock in place. When the wrinkle is along the edge of the overlap rather than in the field, it creates the characteristic mouth-shaped opening. On mechanically fastened systems, fishmouths sometimes appear where membrane shifts slightly under foot traffic before fasteners are properly seated.
Even a small fishmouth is a water intrusion pathway. Wind and rain pressure can push water into the gap, which then travels beneath the membrane. Moisture trapped in the void accelerates degradation of the substrate and underlying insulation. On low-slope roofs with pooling water, fishmouths become collection points that extend saturation. The defect also creates a stress concentration: the curved void is a weak point where the seam can split under thermal movement or membrane tension.
Inspection protocols specifically flag fishmouths as installation defects that must be remedied before final sign-off. The fix typically involves cutting out the wrinkled section, removing the damaged material, and re-lapping and re-welding or re-adhering the overlap. This is labor-intensive and expensive if found after the membrane is fully installed. Prevention requires trained crews, adequate surface preparation, proper temperature control of materials, and patience during positioning before sealing.