Warming and freezing cycles can cause ice dams on the outside of the house. Dams can also form due to heat from the inside of the house. You’ll see ice buildup along the roof’s edge or in the gutter system. Where water is overflowing the gutter system, there may be icicles or a frozen fall area. Ice dams can form in a variety of places, including around dormers, roof valleys, vents, and roof overhangs. When water backs up onto the roof or travels beneath the shingle, ice dams impact interior insulation and moisture levels.