Ice weighs about 50 lbs. per cubic foot. Snow thathas been compacted by daytime warming and nightime freezing weighs about 20 lbs. per cubic foot. Structurally it is difficult to design for this tremendous ice or snow load at the edge of a roof. This is a complicated by ice extending beyond the roof edge and pulling on the roofing material, fascia and siding. A combination of two factors causes the formation of ice. First,heat inside the building rises through the roof insulation. Depending on the depth of snow, amount of insulation and outside temperature, this will melt the snow. If the exterior temp is cold enough, the melting snow will turn to ice at the eaves.
Secondly, changing outside temperatures effect ice dams. If the outside temperature rises, the snow starts melting. As the temperature drops, the snow begins to re-freeze from the outside. The snow closest to the roof re-freezes more slowly. This already melted snow water runs down the roof and freezes at the cold eaves.
