(0:47) An Ice Dam is a hump of ice that forms at the edge of a roof under certain wintertime conditions. An ice dam can damage both your roof and the inside of your home.
An ice dam forms when the roof over the attic gets warm enough to melt the underside of the layer of snow on the roof. The water trickles down between the layer of snow and the shingles until it reaches the eave of the roof, which stays cold because it extends beyond the side of the house. There, the water freezes, gradually growing into a mound of ice.
The flatter the pitch of the roof, the easier it is for an ice dam to get a grip. Gutters at the eaves can also trap snow and ice. If snow and ice build up high enough in the gutter, it can provide a foundation for an ice dam.