Shark fin
Shark fin turns up in 2 fields: waterproofing, shop slang. Here is what it means to the people in each one.
Shark fin in waterproofing
An upward-curled edge or lap in a roofing membrane that fails to lay flat.
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.
"You rolled that sheet out with too much tension and now we've a shark fin right in the middle of the seam."
Shark fin in shop slang
A sharp, triangular sliver of metal left behind after a poor cut.
This dangerous burr occurs when a cutoff wheel or shear fails to sever a piece of material cleanly, leaving a jagged point sticking up. They are notorious for slicing through heavy leather work gloves and causing severe lacerations. Fabricators must immediately grind them flush to make the material safe to handle.
"Watch your hands on that angle iron, there's a nasty shark fin on the back edge."