Glossary
Ice dam
A ridge of ice that forms at the cold eave when heat escaping into the attic melts snow higher up the roof and the meltwater refreezes at the edge.
In practice
An ice dam is a symptom of heat loss and poor ventilation, not of a bad roof covering. Water backs up behind the ice and runs uphill under the shingles, which is exactly what the ice barrier at the eave exists to stop.