An ice dam is an ice build-up on the eaves of sloped roofs of heated buildings that results from melting snow under a snow pack reaching the eave and freezing there.
After several days of melting-freezing cycles, it's common for the melted water and ice to move under the shingles until water enters the attic and eventually does damage to the ceilings, wall and contents.
In cases where the ice dam goes unnoticed for an extended period of time, it can do significant damage to the building and its contents. There's no way to guarantee an ice dam won't damage your home, but you can take steps to decrease chances of ice dams forming with ice dam kits from Green Ice Melt!