Why put vents in a roof when it is supposed to keep water out?
Short answer. Vents go in a roof because an attic needs an exit for heat and moisture, and the top of the roof is where that air naturally collects. Roof vents are designed to shed water anyway: their flanges lap under the shingles above and over the shingles below, and their hoods, louvers, baffles and filters keep rain out while letting air through.
How a vent keeps water out
- Shingle lapping: the flange sits under the course above and over the course below, so water runs across it onto the next shingle.
- Hoods and louvers: box vents face their openings downslope or sideways, under a cover.
- External baffles: ridge vents deflect wind up and over, so it pulls air out instead of pushing rain in.
- Filters: fabric inside ridge vents blocks wind-driven rain and insects.
- Placement: the ridge is where water drains away in both directions.
Why not just use wall vents?
Gable wall vents help, but they work mainly with wind and ventilate the top of the attic unevenly. Roof exhaust near the ridge uses rising warm air every day, wind or not, and pairs with intake along every eave.
Where vent leaks really come from
Leaks usually come from installation, not the idea of venting: flanges laid on top of shingles and caulked, slots cut too wide, short nails that let a ridge lift, or hail cracking a vent hood. Well installed vents rarely leak in normal rain.
The tradeoff
A few carefully flashed openings are a far smaller risk than an unvented attic that rots the deck from the inside. Read roof flashing explained and roof ventilation basics.