Icicles hanging along the eaves of your house may look beautiful, but they spell trouble.

That's because the same conditions that allow icicles to form—snow-covered roofs and freezing weather—also lead to ice dams: thick ridges of solid ice that build up along the eaves.

Dams can tear off gutters, loosen shingles, and cause water to back up and pour into your house. When that happens, the results aren't pretty: peeling paint, warped floors, stained and sagging ceilings. Not to mention soggy insulation in the attic, which loses R-value and becomes a magnet for mold and mildew.

Birth of an Ice Dam

1. Heat collects in the attic and warms the roof, except at the eaves
2. Snow melts on the warm roof and then freezes on the cold eaves
3. Ice accumulates along the eaves, forming a dam. Meltwater from the warm roof backs up behind it, flows under the shingles, and into the house.