A lead stack flashing is a pipe-shaped lead sheet with a flat, square base perpendicular to the main/vent stack pipe. It covers a building’s main or vent stack, and protects it from the elements.
A lead stack flashing slips over the main/vent stack, and then the top is folded over the top of the stack, draping over the inside of the pipe. The square base is then sealed to the roof in a manner consistent with the roof style.
There are other methods of stack sealing, or “flashing,” including aluminum or rubber boots. However, lead stack flashing is most common.