An ice dam is an accumulation of frozen water in the gutter system and at the roof edge that prevents subsequent drainage of melting snow from leaving the roof/gutter system. Ice dams are common in areas that receive heavy snow buildups.
In most cases, ice dams begin inside the house, when heated air leaks up into the unheated attic. In the winter, the roof above the unheated attic is cold. When warm air leaks into the attic, it creates warm areas on the roof, which cause the snow on the exterior of the roof to melt. The melting snow moves down the roof slope until it reaches the cold overhang, where it refreezes. The process continues, causing ice to build up along the eaves and form a dam. Eventually, this dam forces the water to back up under the shingles and sometimes into the ceiling or wall inside the home. This phenomenon may cause structural framing members to decay, metal fasteners to corrode, and mold to form in the attic and the wall surfaces. Few homeowners policies pay for ice dam removal. Interior or exterior damage, however, caused by an ice dam on the roof is typically covered under a special perils homeowners form.
There are measures that home owners in colder climates can take to reduce the chance of ice dams, including the following.
The services of a professional should be employed to remove heavy snow from your roof. This eliminates one of the ingredients necessary for the formation of an ice dam. Professionals are also able to address emergency situations in which water is flowing into the house structure. This is accomplished by making channels through the ice dam to allow the water behind the dam to drain off your roof. However, the channel becomes ineffective within days and is only a temporary solution to ice dam damage.
Your ceiling/roof insulation should be increased to reduce heat loss by conduction. Some state codes require an R-value of 38 above the ceiling for new homes. In narrow spaces, insulation products with high R-value (6–7) per inch are recommended. It is imperative that the ceiling be made airtight to prevent warm air within your home from flowing into the attic space.
Verify that there are sufficient soffit and gable end vents in your attic. These help to quickly vent any of the warm air that does get into the attic out into the atmosphere.
Source: Copyright 2012, International Risk Management Institute, Inc.