Ice Dams occur when temperatures drift between 10 and 30 degrees farenhiet. Ice builds up on the edge of the roof, but snow will melt above the ice ridge and the water will collect and flow under the shingles and into the house, causing tea-colored stains on walls and ceilings and destroying wallboard, plaster and insulation.
Icicles, a key indicator ice dams may be present, are common on many homes in snowy climates. Icicles hang in crystal rows and cluster at the corners of the roof. They can be a beautiful winter sight but they are dangerous to a home.
To determine if your roof has ice dams or is merely sporting innocent winter decoration look for a ridge of ice at the roof's edge, or a bulging area under a layer of snow above the icicles. Those are ICE DAMS. You often have to remove the snow buildup to see the ice ridge.