Why is my roof leaking around a vent?
Short answer. Most leaks around the base of a roof vent come from its flashing: a cracked rubber collar on a pipe boot, a vent flange laid on top of the shingles above instead of under them, nails that rusted or backed out, or sealant used in place of proper layering that has dried and cracked. Hail and age can also crack plastic vent bodies near the base.
The usual suspects at the base
- Cracked pipe boot collar: the rubber hugging a plumbing vent splits in the sun, often well before the shingles wear out, and water runs down the pipe into the attic.
- Flange installed wrong: the top edge of a box vent or exhaust cap flange must sit under the shingles above. If it sits on top, water running down the roof goes straight under it.
- Exposed or backed-out nails: nails through the lower flange or nearby shingles rust and rise, opening holes.
- Old sealant: roofing cement around the edges dries and cracks, and if it was the only thing keeping water out, the leak starts.
- Cracked vent body: hail or brittle age cracks plastic box vents and caps near the base.
- Decking rot: a leak that ran for months softens the deck so nails no longer hold.
How to confirm it
From the attic during or after rain, look at the wood around the vent opening. Stains that start at the pipe or the edge of the opening and run down the rafters point to the vent. Stains that start higher may mean water enters above the vent and travels down to it. From the yard, binoculars often show a cracked boot collar or a vent hood with its top edge exposed.
The fix
Replace failed boots, and reinstall vent flanges with the upper part under the shingles, sealing any exposed nail heads. Caulk is a temporary patch, not a repair. If the vent itself is damaged, replace it. See pipe boot leaks and common roof leak causes.