What is an ice dam?
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 roof. The water that backs up behind the dam can leak into your house and cause damage to walls, ceilings, insulation, and other areas.
What causes ice dams?
There is a interaction among the amount of heat loss from a house, snow cover, and outside temperatures that leads to ice dam formation. For ice dams to form there must be snow on the roof, and, at the same time, higher portions of the roof’s outside surface must be above Freezing while lower surfaces are below Freezing. For a portion of the roof to be below Freezing, outside temperatures must also be below Freezing for over sustained periods of time.
The snow on a roof surface will melt and snow above roof will remain frozen. As water flows down the roof it reaches the portion of the roof that is below Freezing and Freezes again forming an ice dam. The dam grows as it is fed by the melting snow above it, 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.(View Ice Dam Diagram Above)
· Removing 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.
· In an emergency situation where water is flowing into the house structure, making channels through the ice dam allows the water behind the dam to drain off the roof. Hosing with tap water on a warm day will do this job. Work upward from the lower edge of the dam. The channel will become ineffective within days and is only a temporary solution to ice dam damage.
Preventing ice dams in new homes
The proper new construction practices to prevent ice dams begin with following or exceeding the state code requirements for ceiling/roof insulation levels. The second absolutely necessary practice is to 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.