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What causes ice dams?

An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow from dripping off the roof. The water that backs up behind the dam can leak into a home and cause damage to walls, ceilings, insulation and other parts of the house.

An ice dam might form when

  • There is snow on the roof
  • Average outside temperature is below 32 degrees.
  • Roof surface temperature is above 32 degrees at its higher end and below 32 degrees at its lower end.

How it forms

  1. Indoor heating rises through the ceiling into the attic and warms the roof surface.

  2. Snow on the heated part of the roof melts and flows down until it reaches that part of the roof that is below 32 degrees. Water freezes into an ice dam.

  3. The ice dam grows as it is fed by melting snow above, but water held by the dam backs up and stays liquid.

  4. Eventually, the water finds cracks in the roof covering and flows into the attic, from where it could seep through the ceiling and interior walls.
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