Ice dams are formed when heat from the inside of a home escapes into the attic
and warms the roof decking during the winter. This heat, combined with heat from
the sun, can melt snow on the roof. Melting snow then runs down toward the eaves
as water. When it reaches the cold eaves and gutters it refreezes. This
continual thaw and re-freeze process creates ice dams. The result is water
backing up under the roof shingles where it can soak through the roof decking or
wall sheathing, causing damage to attics, ceilings and walls.
There are three ways to defend against the damage ice dams cause: attic insulation, attic ventilation system, and water-proofing shingle underlayment. All three work together.