What Would Cause a Shingle Roof to Leak in Cold Weather?

Find the Source

• Just because that rusty spot forms in the ceiling of the kids' room doesn't mean that the problem is located in the roof above it. Rafters, trusses and piles of insulation all provide pathways for water to travel throughout your attic. The only way to find the source of that leak is to get up into the attic and look for wet or discolored wood and insulation. Provided wind hasn't torn shingles off, or ripped a satellite dish off the roof, the point of entry might be along the eaves or near vents, stacks or chimneys.

Ventilation and Humidity

• Leaky winter roofs typically are rooted in poor attic ventilation. Ideally, cold outdoor air rushes into the attic through the vents lining the soffits on the underside of the eaves and is drawn upward by roof or end vents until it exits the attic at the vents along the peak of the roof. When air intake at the eaves is slowed by insulation or framing -- or roof vents are too small -- heat builds up underneath the roof. Bathroom fans, or stacks that open into the attic rather than traveling through the roof to the exterior, add humidity that condenses on cold rafters and slides back downward. Gaps around stacks, chimneys, light fixtures -- even wallboard joints -- all offer pathways to water. Correct ventilation to keep the roof uniformly cool to slow melting and seal openings to pathways for leaks.

Ice Dams

• In places where winter means long stretches of subfreezing temperatures and frequent snows, sunlight and the warmth inside the attic causes snow to melt and run down from the peak to the gutters. The melted snow runs under accumulated snow, resulting in full gutters and thick drifts of slushy snow along the eaves of the roof. When the sun goes down the water along the eaves, which hang outside the walls of the house, freezes. Each day this happens, more snow melts, moves down toward the eaves and freezes. If snow is plentiful and the freeze-thaw cycle goes on long enough, icicles start forming and water begins backing up under shingles. As the water works backward under the shingles, it again encounters a warm surface where it begins to seep through the deck of the roof.

Flashing Failure

• Missing, warped or misapplied flashing all provide gateways for leaks. Snow melts and settles into any gaps, then expands as it freezes, widening the gaps. Freeze-thaw action can also pry open flashing along valleys -- the places where two different planes of roofing meet. Repairing or replacing faulty flashing seals these gaps, keeping water out of the warm attic.