How do pipe boots work?

Part one: the flange

The hole cut in the decking for a plumbing vent is a bit larger than the pipe. The boot's flange, metal or plastic, covers that hole and extends well beyond it. Its upper half slides under the shingles above, and its lower half lies on top of the shingles below, the same lapping that makes shingles shed water. Rain flowing down the roof runs off the shingles above, onto the flange, around the pipe and back onto the shingles below. Nails go in the upper corners, where the next course covers them.

Part two: the collar

The raised collar is sized for the pipe, commonly 1.5, 2, 3 or 4 inches, and grips it like a snug sleeve. Water running down the pipe or splashing against it stays outside the collar and drains onto the flange. Some boots have a stepped cone you trim to fit; lead boots instead fold over into the top of the pipe.

How they fail

• The collar cracks. Sun, heat and ozone break down rubber, opening a ring-shaped crack that lets water run straight down the pipe into the attic.
• The flange sits on top of the shingles all the way around, so water from above runs under it.
• Exposed nails back out and leak.
• Animals chew soft lead or rubber.

Typical signs are a stain on the ceiling near a bathroom or kitchen wall, or water marks on the pipe in the attic. See pipe boot leaks and common roof leak causes.