Here in the northeast we experience a great deal of ice damming on the roof of our homes during the winter season. A leaking roof, especially during the winter time, is a complete hassle to deal with. Ice damming occurs when heat transfer through your attic begins to melt the snow on your roof. As melted snow makes it over the unheated soffit area it begins to refreeze causing a water back up under your shingles and into your home. The process usaully could take a few days or weeks to occur depending on outside temperature and the R-value of your attic insulation.
Ice damming can all be prevented with ice dam material that is applied to the roof deck before shingles are applied. It prevents water from penetrating your roof deck and seals around the nails used to fasten your shingles. Ice dam material is applied on the lower edge of your roof and three feet up(sometimes doubled to six feet). It should also be applied in valleys and around chimneys and skylights. In some areas building departments have made it code to install ice dam with your new roof. It is the cheapest and most effective way to eliminate the chances of ice damming. If you experience ice damming the best course of action is to leave it be and correct the problem when it warms. If you try to break the ice on the roof you will 99 percent of the time do more damage to the roof shingles and not to mention a dangerous practice.