A shark fin is a defect in a rolled roofing membrane where a side lap or end lap buckles and curls upward instead of adhering flat to the underlying layer. This usually happens when the felt or membrane roll is applied crooked, forcing the material to wrinkle. These raised fins are highly susceptible to being stepped on, cracking, and allowing water infiltration.
Heard on the job
You rolled that sheet out with too much tension and now we've a shark fin right in the middle of the seam.
Insider tip
Shark fins must be sliced open, laid flat, and patched over with a new piece of membrane to prevent leaks.
Do not mix it up with
- Fishmouth: A fishmouth is a half-cylindrical opening at a seam edge, while a shark fin is a sharp, upward-pointing crease or curl.