Fishmouths in membrane roofing are caused by what condition?

• A. Improper alignment of felts
• B. Improper flashing
• C. Too much heat
• D. Nail misplacement

Answer: (A)

Fishmouths show up when the edges of the membrane laps don’t line up correctly as the sheets are laid and joined. If the felts or membranes along a seam aren’t aligned, the upper sheet can overhang or underhang relative to the one beneath, creating a small opening at the seam. When heat and pressure are applied during installation, or as the material settles, that misaligned edge folds or pulls away enough to form a triangular, gap-like opening that resembles a fish mouth. Proper alignment of the felts ensures the seam sits smoothly with no gaps, preventing the characteristic fishmouth from developing.

Imperfect flashing would tend to cause leaks around penetrations rather than a seam opening. Excessive heat can cause blistering or wrinkling, but not the specific fishmouth effect caused by misaligned laps. Nail placement issues typically result in punctures or leaks at fastener points, not a fishmouth along the seam.