The ice acts as a dam that prevents water from flowing off the roof. While ice dams can result from frozen or clogged gutters, dark shingles that absorb heat, or low roof pitch, they are most often caused by heat loss from the house. The roof above an unheated attic is cold during the winter months. Heat that escapes from interior rooms enters the attic and warms the outside surface of the roof, causing the snow to melt. Because the ice dam blocks the water from flowing off the roof, water eventually builds up and seeps under the shingles, where it can cause both interior and exterior damage to your home.
Ice dams can cause a variety of problems.
Outside, water buildup from ice dams can damage exterior walls, gutters, and roof shingles. When water gets inside, it can stain ceilings, peel paint, and damage insulation. In addition, water from ice dams can lead to mold and mildew growth in attics and on wall surfaces. The most effective way to prevent ice dams is to stop warm air from leaking from the interior rooms to the unheated attic. Increasing the ceiling or roof insulation will reduce the amount of warm air that flows from the house to the roof and will help prevent ice dams.