An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow (water) from draining off. The water that backs up behind the dam can leak into a home causing damage to walls, ceilings, and insulation. Fortunately, there are ways you can prevent ice dams, but you have to know what causes an ice dam to know how to fix it.

What causes ice dams?

Nonuniform roof surface temperatures lead to ice dams. The snow on a roof surface that is above 32°F will melt. As water flows down it reaches the portion of the roof that is below 32°F and freezes.
The dam grows as it is fed by the melting snow above it, but it will limit itself to the portions of the roof that are on the average below 32°F. So the water above backs up behind the ice dam and remains a liquid. This water finds cracks and openings in the exterior roof covering and flows into the attic space. From the attic it could flow into exterior walls or through the ceiling insulation and stain the ceiling finish.

What causes different roof surface temperatures?

Since most ice dams form at the edge of the roof, there is obviously a heat source warming the roof elsewhere. This heat is primarily coming from the house. A blocked soffit or blocked soffit vent can also cut off ventilation to overhangs. The overhangs become colder than the rest of the roof which will lead to the freezing at the edges.

Preventing and dealing with ice dams

For emergencies, remove snow from the roof. This eliminates one of the ingredients necessary for the formation of an ice dam. A “roof rake” and push broom can be used to remove snow, but may damage the roofing materials.

For a long term solution, make the ceiling air tight so no warm, moist air can flow from the house into the attic space. After sealing air leakage paths between the house and attic space, consider increasing the ceiling/roof insulation to cut down on heat loss by conduction. Construct a continuous, 100% effective air barrier through the ceiling. There should not be any air leakage from the house into the attic space! Recessed lights, skylights, complicated roof designs, and heating ducts in the attic will all increase the risk of ice dam formation.