Some of the worst winter roof leaks we see start with ice, not a storm. An ice dam forms when heat escaping into the attic melts snow on the upper roof, the water runs down to the cold eave, and it refreezes into a ridge of ice. Water pools behind that dam and works its way under the shingles and into the house. The fix is mostly about what happens inside the attic.
Why ice dams form
The root cause is a warm roof deck. Heat leaking from the living space, through gaps and poor insulation, warms the upper roof enough to melt snow even on a freezing day. The eave, which hangs past the warm interior, stays cold, so the meltwater refreezes there. A consistently cold, well-ventilated roof deck does not form dams.
Attic ventilation and insulation
The two defenses are insulation and ventilation. Good attic insulation keeps household heat out of the attic; balanced intake and exhaust ventilation keeps the attic, and the roof deck, near outdoor temperature. We balance ventilation as part of every roof replacement, and a roof inspection is a good time to check intake and exhaust before winter.
Ice-and-water shield at the eaves
Even a well-built roof can see an occasional dam in a brutal winter, which is why we install ice-and-water shield along the eaves and in the valleys on every replacement. It is a self-sealing membrane that blocks water that backs up under the shingles, the last line of defense against a dam.
What to do if you have a dam now
Do not climb up and chip at it; that damages shingles. Carefully clearing snow from the lower roof with a roof rake reduces the melt feeding the dam. If water is getting inside, that is a leak, and our guide to common roof leaks and a free roof inspection will get to the source.