What is a rain collar on a roof vent pipe, and does yours need one?
Why the gap exists
A metal flue for a gas furnace or water heater gets hot and grows. Its roof flashing is a cone or tapered jack that fits loosely around the pipe so the pipe can move and so there is clearance from the wood deck. That loose fit is an open path for water, which is why the storm collar sits on the pipe just above the top of the cone and works like an umbrella over it.
How it should be installed
- The collar slides down the pipe until it sits a little above the top of the flashing cone, overlapping it without binding, so the pipe can still move.
- It is tightened with its bolt or band so it cannot slip down.
- The joint between collar and pipe gets a bead of high temperature sealant rated for flues, because water running down the pipe will otherwise slip behind the collar.
- The flashing base is woven into the shingles: the upper part tucked under the courses above, the lower part lying on top of the courses below.
How they fail
The sealant at the top of the collar dries and cracks, the collar slides down onto the cone, or a cap was replaced during some earlier repair and the collar never went back on. The leak usually shows as rust streaks on the pipe in the attic, ceiling stains near the water heater closet, or rust on top of the appliance. It often drips only in wind-driven rain, which makes it look intermittent and hard to pin down.
Changes to the flue itself, such as its height, cap or clearances, belong to a licensed HVAC or plumbing pro. Plastic plumbing stacks use a different detail, covered in pipe boot leaks, and the way every flashing ties into shingles is in roof flashing explained.