An ice dam is formed when snow melts unevenly on a roof and refreezes into a dam at the edge of the roof, near the eaves. This dam prevents any further snow from melting and draining off of the roof. This standing water can back up under shingles, leak into a home, and cause significant water damage to ceilings, walls, and other areas. Ice dams can also tear off gutters and loosen shingles.