Uninsulated attics usually do not have ice dams because the heat coming through the attic tends to melt snow as it lands and prevents much accumulation. A well-sealed and insulated attic results in a cool roof and generally will also not have ice dams. Ice dams are more frequent if many valleys and dormers complicate the roof or there is a large overhang. More complicated types of roofs hold more snow because they are more prone to have more thermal bridging and air leakage areas.