Ice Dam Cause
Anyone who has lived in a snowy climate has seen ice dams. Thick bands of ice form along the eaves of homes, causing millions of dollars of structural damage every year. Water-stained ceilings, dislodged roof shingles, sagging gutters, peeling paint, and damaged plaster are familiar results of ice dams.
Ice dams need three things to form: snow, heat to melt the snow, and cold to refreeze the melted snow into solid ice. As little as 1 or 2 inches of snow accumulation on a roof can cause ice dams to form. Snow on the upper part of the roof melts, runs down the roof under the blanket of snow to the roof's edge, and refreezes into a dam of ice. As more snowmelt runs down the roof, it pools against the ice dam. Eventually, water backs up under the shingles and leaks into the structure.
The reason ice dams form along the roof's edge, usually above the overhang, is straightforward. Heat and warm air leaking from living space below melts the snow, which trickles down to the colder edge of the roof (above the eaves) and refreezes. Every inch of snow that accumulates on the roof insulates the roof deck a little more (about R-1 per inch), keeping more heat from the living space in, which further heats the roof deck. Frigid outdoor temperatures ensure a fast and deep freeze at the eaves. The worst ice dams usually occur when a deep snow is followed by very cold weather.