An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow from draining. As water backs up behind the dam, it can leak through the roof and cause damage to walls, ceilings, insulation and other areas. Contrary to popular belief, gutters do not cause ice dams to form. Gutters do, however, help concentrate ice from the dam in a vulnerable area, where parts of the house can peel away under the weight of the ice and come crashing to the ground. Metal roofs are less susceptible to ice damming than asphalt shingles. The slick surface of the metal roof provides little resistance where asphalt shingles tend to grip the ice. In most cases proper insulation and ventilation will keep ice dams at bay. In some cases heat tape may be needed to prevent damage.