How do you fit a roof vent so it does not leak?
Short answer. A roof vent is fitted so its flange sheds water like a shingle: the top edge and upper sides slide under the course above, the bottom edge lies on top of the course below, and the nails sit where the overlapping shingles cover them. Sealant is only a backup at exposed nail heads and edges, never the main waterproofing.
The lapping rule
Water runs downhill, so every layer must overlap the one below it. If any part of a vent flange sits on top of a shingle above it, water will run under the flange and into the roof. That is why the flange is woven into the shingle courses rather than laid on top.
Fitting it step by step
- Position the hole: between rafters, and for exhaust vents, high on the slope. Cut the opening to the vent's template size, not larger.
- Free the shingles above: break the sealant strips of the courses above and to the sides, and remove nails in the way.
- Slide the flange in: tuck the top and side edges under the shingles. The bottom flange should overlap the shingles below.
- Nail: fasten the flange through the upper portion and sides, where the overlying shingles will cover the nail heads.
- Reset the shingles: renail any shingles you loosened, trimming them around the vent hood so they do not dam water.
- Seal: dab roofing sealant under the loosened shingle tabs and over any exposed nail heads.
Common fitting mistakes
- A flange laid entirely on top of the shingles and caulked around the edges.
- Nails driven through the exposed lower flange.
- An oversized hole that leaves the flange little to grip.
- Upper shingles cut too short, exposing the flange edge.
- A vent designed for shingles used on a metal or tile roof.
On metal roofs, vents need curb style or flexible bases made for the panel profile. See roof flashing explained and common roof leak causes.