How does a roof pipe flashing boot work?
The parts
- Flange or base: galvanized steel, aluminum, plastic or lead, wide enough to reach well under the shingles above.
- Collar or cone: rubber, EPDM, silicone or lead, sized to the pipe.
- The deck opening: cut slightly larger than the pipe so the house can move without cracking the pipe.
Why the layering matters
Water coming down the roof has to land on top of the flange, so the flange's upper edge must sit under the shingles above it. If the upper edge sits on top of the shingles, water runs underneath it and straight into the hole. The lower edge lies on top of the shingles below so water leaving the flange continues down the roof surface. Nails go only in the upper corners, where shingles cover them. A boot laid on top of all the shingles and caulked around the edges is a common shortcut and a common leak.
Sizes and adjustable boots
Plumbing vents are usually 1.5, 2 or 3 inch PVC or cast iron, and boots are labeled by pipe size. Adjustable boots have stepped collars you cut to fit. A collar that is too large leaves a gap; one that is too small tears.
Not for hot pipes
Furnace and water heater flues are metal and run hot. They take a metal flashing with a storm collar and the clearances the vent maker requires, not a rubber boot.
See pipe boot leaks and roof flashing explained.