What is a roof pipe boot, and which pipes need one?
Short answer. A roof pipe boot is a flashing collar that seals around a round pipe where it passes through the roof deck, and on most homes it is used on plumbing vent pipes. Hot metal flues from furnaces and water heaters need a different metal flashing with a storm collar, and bath fans and kitchen hoods exit through vent caps rather than pipe boots.
Pipes that get a boot
- Plumbing vents: PVC or cast iron pipes, usually 1.5 to 3 inch, venting the drain system. This is the classic pipe boot.
- Radon mitigation pipes: PVC exhaust pipes that run up through the roof.
- Some high-efficiency furnace and water heater pipes: PVC exhaust and intake pipes, when they exit through the roof instead of a wall.
Pipes that need something else
- Gas furnace and water heater flues: metal vent pipe that runs hot and needs the flashing and clearances its manufacturer specifies, plus a storm collar.
- Wood stove chimneys: listed chimney flashing kits made for that system.
- Electrical service masts: a mast flashing made for the purpose.
- Bath fans and kitchen exhausts: roof vent caps with their own flanges, not boots.
How to tell which you have
Plumbing vents are open-topped plastic or cast iron pipes. Gas flues are metal with a rain cap on top. Exhaust caps are low, louvered or hooded boxes.
Why it matters
A rubber boot on a hot flue can soften and fail, and it ignores the clearance the flue requires. A boot that is the wrong size for a plumbing vent leaves a gap. When roof penetrations are replaced, ask what each one is and what flashing it will get. See pipe boot leaks and roof anatomy explained.