What is roof vent flashing?
Short answer. Roof vent flashing is the piece that seals the joint between a roof penetration, such as a box vent, exhaust cap or plumbing pipe, and the shingles around it. It works by layering: the upper part tucks under the shingles above so water runs over it, and the lower part lies on top of the shingles below so water runs off it.
Types you will see
- Integral vent flanges: box vents, exhaust caps and power fans have a wide flange formed as part of the vent. The flange is the flashing.
- Pipe boots: a flat base with a rubber or neoprene collar that hugs a plumbing vent. The collar is the part that wears out first in the Georgia sun.
- Lead flashing: a soft lead sleeve and base folded down into the top of the pipe. Long lasting, though squirrels sometimes chew it.
- Flue cones with storm collars: for metal gas appliance flues, with a separate collar clamped above the cone.
- Step and counter flashing: for larger penetrations like chimneys and skylights, a separate system.
How it should be layered
Shingle courses above the vent overlap the top and sides of the flange, and the bottom edge of the flange lies on top of the course below. Nails go through the flange where the shingles above will cover them, or get sealed if they are exposed. Sealant is a backup, not the main waterproofing. A vent that depends on a bead of caulk to stay dry was installed wrong.
How it fails
- Cracked rubber collars on pipe boots.
- Flanges laid on top of the shingles all the way around, so water runs under the top edge.
- Exposed nails that rust and back out.
- Plastic vent hoods cracked by hail or gone brittle with age.
For the full family of flashing types, read roof flashing explained, and for the most common vent leak, pipe boot leaks.