Ice dam related leaks occur when warm, heated air from the home begins to migrate through the insulation and into the attic area of the house. Without proper ventilation, this warm air collects in the attic area and may warm the underside of the roof decking.
With outside temperatures keeping the top outside layer of moisture frozen, the warm decking melts the bottom layer of ice returning it to a liquid state. This underlayer of water will run down the shingles until it meets the “dam” at the gutter line. With no escape route, the water pools, then backs up underneath the shingles and enters the attic area through nail holes and seams in the roof decking.