Ice dams, a familiar scourge of those who live in northern climates, are ridges of ice that from at the edge of a roof, preventing water from draining off. Little can be done once ice dams form on a roof, except to wait for the inevitable damage as the ice slowly melts and ruins walls and ceilings. So preventing them in the first place is your best strategy.
Easy Heat de-icing cable melts away the troublesome ice and snow, and uses only five watts per foot. As shown here, it comes with 240 feet of wire, and uses standard household current.