INFORMATION SERIES: Ice Damming Conditions

Many times homeowners experiencing water leakage and damage around exterior walls and ceilings during winter months think they have a faulty roof when actually the culprit is an ice dam. Ice dams are caused by melting snow. Attic heat rises to the highest point along the ridge line of the roof causing the snow cover to begin melting. As this melting snow runs down the roof surface below the snow, it reaches the cold areas of the roof along the edge or eave. As it arrives, the colder conditions at this low point of the roof cause the melted snow to freeze, thereby forming an ice dam. As further melted snow water arrives at the ice dam, it begins backing up. When it reaches a point of above freezing temperature, it remains a liquid and seeps in under the roofing materials and starts causing damage to the sheathing and the walls and ceilings below.

Ice dams can occur in any area where January temperatures average 35°F or where the mean average total snowfall is 6 to 8 inches. Ice dams are also more likely to occur in suburban areas with heavily treed tracts which minimize the roof clearing effect of prevailing winds.

The HouseMaster Technical Staff advises that there are many solutions offered to eliminate ice dams but most do work. Snow removal after a storm is the ultimate solution but unless the removal is total, the melting process will still begin near the ridge and ice dams will eventually form. Total snow cover removal is dangerous for inexperienced home owners and expensive if contracted after each snow fall.

Also to be considered is possible damage to the roof surface caused by the snow removal equipment. Steam and water removal of the snow cover presents further difficulty with the melted roof run-of and its refreezing around shrubs and walks.

Electric heating cables along the roof edge or in roof valleys have been employed as a solution but secondary ice dams form above the line of cables. Even the installation of two or three feet of metal flashings under the roof materials at the eaves has not solved the dilemma of ice dams. Secondary ice dams have occurred above the level of the extended eave flashing.

Most roofing experts now agree that the only possible solution is the development of a "cold roof." This solution involves a two step approach. First, the amount of heated air reaching the roof must be minimized and controlled. Properly installed insulation (no voids) by professional insulation contractors will reduce heated air contact with the roof surface. The second step and the most difficult and expensive for a previously built house involves the installation of increased attic ventilation. At the plates near the top of the exterior walls, continuous insulation must be installed as well as a source of air flow from the soffit vents to the roof ridge. Where there is insufficient clearance to install a natural airway, prefabricated air channels may be considered. Ridge vents or mechanical vent fans (set to operate automatically in cold weather) should draw air and moisture out of the house.

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