Ice dams form at the roof eave as a result of the heat from the house escaping into the attic and melting the bottom layer of the snow piled up on the roof. The water then runs down the roof pitch until it reaches the point where it freezes forming an ice dam. Outdoor temperature cycles contribute to repetitive melting and freezing causing the ice dam to expand over the winter months.
Ice dams can damage roof shingles and lead to water leaking into the attic and into the house. Water-stained ceilings and walls, worn-out roof shingles, sagging gutters, icicles, peeling paint, and discolored roof sheathing are evidence of ice dam damage. Repeated water leaks can cause structural damage to roof sheathing and underlayment, thus reducing their wind resistance over time.