Fishmouth: A Defect Shaped Like Its Name

To most of us, a fishmouth is simply the opening where a fish takes in water. The term is visual and literal: it describes the shape of the mouth, which is exactly how the word entered construction vocabulary.

In roofing and waterproofing, a fishmouth is a half-cylindrical or half-conical void that appears where two sheets of membrane, flashing or cladding material are lapped and joined. It occurs when one ply wrinkles, buckles or shifts sideways during installation, leaving the overlapping edge unsealed and raised above the substrate. The resulting opening resembles a fish's mouth, hence the name. Unlike a true seam, which bonds the plies together, a fishmouth creates a gap that can admit water, air and contaminants directly into the assembly below.

Fishmouths form most commonly in single-ply membrane roofing (TPO, EPDM, PVC), sheet metal cladding, and tar-and-gravel built-up roofing. They occur when a sheet is not laid flat and square before it is sealed, welded, glued or nailed. Tension, wrinkles and misalignment during the laying process are the root causes. Once present, a fishmouth cannot be sealed by the adhesive or weld bead that bonds the overlapping sheets, since the void sits outside the bond line.

The metaphor is so natural that it carries almost no risk of confusion. A fish's open mouth is a cavity; the construction defect is also a cavity shaped the same way. The term may have originated in sheet metal roofing, where the shape was easiest to spot, and later transferred to membrane and synthetic roofing as those products became standard. No other trade appears to use the term in a meaningfully different sense.

Detection requires visual inspection and sometimes probing: the fishmouth can be felt as a gap or hollow space where the seam should be continuous. Repair typically involves cutting out the defective section, re-laying the sheets without wrinkles, and re-sealing the joint. Prevention depends on careful sheet handling, flat substrate preparation, proper alignment before bonding, and trained installers who recognize the risk during the work.