How do you fix a roof leak around a vent pipe when you are not sure the boot is the problem?
Short answer. Start by finding what is actually letting water in: a cracked rubber collar on the pipe boot is the most common cause, but exposed nails at the boot's flange, damaged shingles above it, squirrel chewed lead, a boot installed backward, or condensation on the pipe in the attic can all look the same from inside. Each one has a different fix, and caulk alone fixes none of them for long.
Diagnose by symptom
- Drip only during rain, stain right around the pipe: likely the boot collar or flange.
- Drip in cold weather with no rain: condensation on the pipe in the attic. Insulating the pipe in the attic fixes it.
- Stain a foot or two below the pipe: water may be entering above the boot through damaged shingles or nail holes and running down.
- Wet pipe inside the wall: a cracked pipe or failed joint, a plumber's call.
What to look for on the roof
- A rubber collar that is cracked, split or pulling away from the pipe.
- Exposed nail heads on the boot's lower flange, which rust and leak.
- A boot whose lower flange is tucked under the shingles below it instead of lapping over them.
- Lead boots with chewed edges.
- Cracked or lifted shingles just above the boot.
The fixes
- Cracked collar: replace the boot, which means loosening the shingles above it, pulling the nails and sliding a new boot in. A rubber repair sleeve slipped over the old collar is a stopgap.
- Exposed nails: pull and relocate them under the shingle above, or seal heads as a temporary measure.
- Backward boot: reinstall so the upper flange goes under and the lower flange over the shingles.
- Chewed lead: replace with a new boot and consider a pipe cap.
More detail is in pipe boot leaks and common roof leak causes.