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. Ice dams usually occur after a heavy snowfall and several days of freezing temperatures. Usually caused by poor attic insulation and ventilation, the ice dam is formed when the warm air inside your home leaks into the attic and warms the underside of the roof causing snow and ice on the roof to melt.The melted water drains along the roof, until it reaches the cold overhang. The overhang tends to be at the same temperature as the outdoors and the melted water refreezes and forms an ice dam which can cause icicles. As more snow melts the ice dam grows and runoff water is trapped behind the ice dam. Eventually, this dam can force the water to back up under the shingles and sometimes into the ceiling or walls of the home. Contrary to popular opinion, gutters do not cause ice dams. They can occur whether or not gutters are present.