An ice dam is formed when heat from the interior space of the structure vents upward through the ceiling and exits the roof causing the snow to melt. This melting snow then moves down the slope of the roof, coming in contact with the cold unheated overhang section of the roof, where it accumulates to form an ice dam. Once this ice is formed, water is then pushed back up the roof where it penetrates through seams, valleys and disturbed shingles. As water enters though the seams, it saturates the insulation and inner wall cavities. This damage can happen over several seasons before it is eventually observed or noticed through stained drywall and dripping. Ice dams also become a danger to persons and property below. As the weather warms, blocks of ice can suddenly release and fall to the ground. The weight of these ice blocks can be substantial and can cause severe damage. Homeowners typically need to be concerned where their vehicles are parked, as well as decks and other exterior amenities that can be severely damaged, destroyed-- or worse, someone can be seriously injured. These two photos are excellent examples of typical ice dams.