The Roof Vent in the Bathroom Is Leaking When It Snows

How It Works

• Bathroom exhaust fans pull the hot, humid air out of the bathroom into a vent stack that runs from the port cut in the bathroom ceiling to a vent cap mounted over a hole in the roof. If the exhaust system works correctly, the fan pushes the moist air to the outside of the house before water in the air condenses inside the home. Extremely long runs of vent pipe or vent pipes with too many turns can prevent fans from completely exhausting the moisture. Sloppy installation of the fixtures at either end of the vent stack can also cause leaks and condensation problems.

Cap Installation

• The exhaust pipe extends a few inches through the roof and terminates inside a metal fixture. An overlapping cap suspended over the opening of the vent stops rain or snow from entering the vent. Rectangular metal flashing at the base of the cap fits the vent cap to the roof. If the installer nailed the flashing to the shingles and caulked the edges, weathering pulls the flashing away from the shingles, and water runs underneath. A correct installation slips the flashing beneath the shingles. Shingles trimmed to fit the shape of the vent carry water past the flashing.

Insulation

• To carry all the moisture to the outside of the house, the vent pipe must stay warm while the system operates. Uninsulated vent pipes running through cold attics can condense water inside the stack. Condensation runs back to the ceiling fitting in the bathroom, causing water damage that appears to come from the rooftop. Warm air rising through a poorly sealed ceiling fixture can condense moisture on the outside of the pipe as well. Covering all exposed metal vent surfaces in the attic with an approved insulation wrap keeps the pipe warm and prevents condensation problems.

Ice Dams

• Locating a vent stack too close to the edge of the roof could place the roof opening in the pool of water that collects behind an ice dam. If warm air from inside the house leaks into the attic, the heat can warm the roof enough to melt the bottom of the snow layer. Any part of the roof extending past the attic stays cold, so the water freezes at the roof's edge. As the ice builds up, a pool of water backs up behind it and seeps under shingles and vent fittings. Well-insulated, ventilated attics keep the entire roof at the same temperature and prevent ice dam formation.