Shell BuseyBlog - Ice DammingFebruary 14th 2018
Ice dams are the large mass of ice that collects on the lower edge of the roof or in the gutters. As more melting snow (or rain) runs down the roof, it meets this mass of ice and backs up, sometimes under the shingles and into the attic or the house.
Ice damming usually occurs with a significant depth of snow on the roof. If the attic temperature is above freezing, it warms the roof sheathing which melts the snow lying on the shingles. This water runs down the roof until it meets the roof overhang, which is not warmed by the attic and will be at the temperature of the surrounding air. If the air and the overhang are below freezing, then the water will freeze on the roof surface and start the ice dam.
Here is a video of Shell explaining ice damming...