Why do some houses have ice dams on the roof and others do not?
Ice dams develop when snow is on the roof, outdoor temperatures are only slightly below freezing, and there is spot or generalized heat loss from the house into the attic/roof section of your house. The heat melts the bottom of the snow pack, the melted snow runs down the roof until it gets to the overhang of the roof. Without the heat from the house, the water freezes, creating an ice dam. The solution lies in finding a way to keep the roof cold whenever it is cold outdoors. The snow will either melt and run off the roof, or stay put. Ventilation is the key. Anywhere insulation touches the roof, the ventilation cannot keep the roof frozen and heat will flow through the insulation to melt the snow. Complicated roofs have more problems because the structure tends to block the cold ventilation airflow. Provide continuous airflow to the underside of the roof and you will have no more ice.