How do ridge vents keep water out?
Short answer. Ridge vents keep water out with layers: an external baffle deflects wind and rain over the top, a weather filter or mesh inside catches droplets, the openings face sideways under overlapping cap shingles rather than up, and the deck slot stops short of the windy ends. When one layer is missing, such as an end plug or a properly cut slot, leaks follow.
Layer by layer
- Cap shingles: they cover the top of the vent and overlap each other like the rest of the roof, so rain falling straight down never reaches an opening.
- Side openings: air exits through louvers or mesh along the sides of the vent, down near the shingles, not on top.
- External baffle: a raised lip along the outer edge pushes wind up and over the vent. That creates low pressure at the openings, so air flows out and rain is carried past.
- Weather filter: a fabric or fiber mat inside the vent stops wind-driven rain and insects that get past the baffle.
- Slot setbacks: the deck slot stops several inches short of gable ends and further back from walls and chimneys, where wind hits hardest.
- End plugs: closures seal the open ends of the vent at each gable.
When water does get in
- Mixed exhaust: a power fan or gable vent can reverse airflow and suck rain in through the ridge.
- Slot too wide: the vent no longer covers it fully.
- Missing end plugs or lifted caps after wind.
- Short nails that let caps and vent lift in gusts.
- Old vents without baffles, especially early rolled products.
Telling a ridge leak from condensation
A ridge leak leaves stains at the peak after storms, often at the gable ends. Damp decking across the whole upper attic in cool weather, with no storm, points to moisture from the house condensing instead. See common roof leak causes and ridge vent vs box vents.