“One never can know enough about snow”
     - George Leigh Mallory, 1923

Causes

Ice dams form when the roof's snow pack melts and seeps down to the surface of the roof where the water is prevented from freezing by the snow's insulation. The meltwater runs down the roof slope, only to refreeze when it hits the cold air or the cold eaves on the downslope edge. The ice causes a blockage and results in the formation of more ice dams and icicles (Figure 8). The ice dam builds as the flow of meltwater collects behind it, floods over the top or around the sides, and refreezes when it contacts freezing air. It is not uncommon to find ice dams several feet high at roof edges holding back large quantities of meltwater. This produces great water pressure on the roof surface below.

Four conditions are required for ice dams to form: 1) a sloping roof, 2) a snow pack resting on the roof's surface, 3) below-freezing air that chills the eaves, and 4) interior heat leaking through the roof and/or solar heat penetrating the snow pack and warming the roof surface (Figure 9). Ice damming can also occur on improperly drained flat roofs, decks, or patios—any place where melted snow refreezes before it can properly drain.

Mitigation

Installation of a cold roof or cold attic, which prevents escaping building heat from melting the snow on the roof, is the most common way to substantially mitigate ice dam formation. (Cold roofs are further described in the "Roof Type and Function" section below).

A steep, slippery roof can also mitigate ice dam formation to some extent. Generally, the steeper the roof and the more slippery the roofing material, the lower the probability of ice damming. However, steep, slippery roofs do not prevent ice dams if ice adheres to the roof edge. For example, with standing-seam metal roofs, it is not uncommon for snow and ice to freeze to the seams. In this case, the snow pack forms a bridge between standing seams, with its underside melted clear of the flat roof pan. When snow and ice avalanche off the roof, the snow runs on "rails" like a train, and the movement can be sudden and deadly (Figure 10). The designer must consider a "dump zone" for snow and ice below such roofs.

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Figure 8
Figure 10
Figure 9