Ice Dams

The cause is heat escaping through the attic, which melts a layer of snow on the roof immediately above the shingles. The melt water is insulated from the cold air above by the thick blanket of snow. The water trickles down to the eave and freezes there because the eave is colder than the roof above. The cycle repeats itself until the ice at the eave forms a dam that literally blocks melt water from draining off of the roof. It's only a matter of time before the water backs up under the shingles and drips down into the attic where it soaks insulation and drips into living spaces below, potentially ruining wallboard, paint, and just about anything else that's susceptible to water damage. 

How do you keep it from happening again next year? The key is adequate insulation and ventilation, but it's sometimes a tough fix. Start by placing vents in the soffits and along the ridge to ensure a steady flow of cold air under the roof deck. Added insulation on the attic floor will keep the heat inside the house (where it belongs), but make sure that the insulation does not block the soffit vents. Finished attics must provide unobstructed cold-air channels between the soffits and ridge to ensure that the roof deck is not heated by thermal energy leaking out of the attic living space. 

An excellent low-cost look at ice dams and how to avoid them is provided by the booklet Roof Snow Behavior and Ice-Dam Prevention in Residential Housing ($3.50 postpaid). To order, call the Extension Service Distribution Center at the University of Minnesota: (800) 876-8636 -- include the booklet item number when ordering: NR-BU-0507-PM1. 
-- Roy Berendsohn