shark fin

A shark fin is an upward curl or fold that forms along the side or end lap of roofing felt during installation. When overlapping sheets of felt are laid down, the edge of the upper sheet can lift and curl upward, creating a profile that resembles a shark's dorsal fin. This defect typically occurs at seams where felt sheets meet and are supposed to lie flat and sealed.

The problem arises most commonly during hot weather or when felt is installed over a substrate that has slight irregularities. As built-up roofing is laid in layers, each sheet overlaps the previous one by several inches. If the overlap is not properly adhered, weighted down, or torched in place during installation, the felt edge can peel back and stay curled. Wind pressure on a flat roof exacerbates the problem, catching under the curl and holding it open.

A raised shark fin is a serious defect because it breaks the watertight seal. Water, dirt, and debris collect in the gap between the curled felt and the substrate. The curl also creates a weak point where the felt is not bonded; traffic, thermal movement, or further wind can tear the material. Inspectors look for shark fins as a sign of poor installation workmanship.

Prevention and remedies

Prevention depends on technique and conditions. Installers must ensure felt is laid with adequate adhesive or bitumen at the overlap, weight it down with gravel or mineral surfacing immediately, or torch it with hot bitumen to set it flat. Curling happens less on cool days and when substrate is clean and dry. Once a shark fin has formed, it must be cut out and the area re-felt and sealed, adding cost and time to the project.

The name is literal and descriptive: the curled felt edge standing up from a flat roof looks like a shark's fin breaking the surface. The term appears in roofing and waterproofing trades where built-up membranes are the standard.