Ice dams are caused by heat from the home melting the underside of roof snowpack. Meltwater flows down the roof until it reaches the cold roof overhang, where it re-freezes. As time passes this process creates an ice dam that increases the chances of roof leakage and can cause structure damage if it grows large enough. Shingles may be damaged by attempts to remove ice dams.
Drainage channels through ice dams can be created by filling nylon stockings with ice melt and placing them across the ice dam. Although this method alone won't remove an ice dam, it will prevent dams from growing by allowing snowmelt to drain from the roof.
The solution to preventing ice dams is to keep the roof cold by using effective roof structure ventilation. Inspector recommendations usually include addition of thermal insulation and/or blocking areas of heat leakage into the roof structure.