An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow (water) from draining 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 areas.
Moisture entering the home from ice dams can lead to the growth of mold and mildew.
What causes ice dams?
Heat loss from a house, snow cover and outside temperatures interact to form ice dams. For ice dams to form there must be snow on the roof and, at the same time, higher portions of the roof’s outside surface must be above 32 degrees F (freezing) while lower surfaces are below 32F. These are average temperatures over sustained periods of time. For a portion of the roof to be below freezing, outside temperatures must also be below freezing.
The snow on a roof surface that is above freezing will melt. As water flows down the roof it reaches the portion of the roof that is below 32F and freezes. This causes the ice dam.
The dam grows as it is fed by the melting snow above it, but it will limit itself to the portions of the roof that average below 32F. So, the water above backs up behind the ice dam and remains a liquid. This water finds cracks and openings in the exterior roof covering and flows into the attic space. From the attic it could flow into exterior walls or through the ceiling insulation and stain the ceiling finish.
What to do to alleviate the ice dam