Ice dams develop as snow on the upper part of the roof melts. Water runs down the roof slope under the blanket of snow and refreezes into a band of ice at the roof’s edge creating a “dam”. Additional snow-melt pools against the dam and eventually leaks into the building through the roof or roof trim. Taking precautionary measures before the snow flies can greatly reduce the potential damage and costs caused by ice dams.
This article explains the ways in which you can properly prepare your roof for snow and ice: