According to Bob Schultheis, natural resource engineering specialist, University of Missouri Extension, non-uniform roof surface temperatures cause ice dams.
“In the winter, when warm air inside the house leaks into the unheated attic, it creates warm areas on the roof, which causes snow on the exterior of the roof to melt,” said Schultheis.
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," said Schultheis.
Besides dislodged roof shingles, sagging gutters, damaged insulation, and adding water stains on interior ceilings and walls, water from ice dams may cause structural framing members to decay, metal fasteners to corrode, and mold and mildew to form in attics and on walls.
Schultheis says the best way to prevent ice dams is to control heat loss from your home.
"In the short-term, remove snow from the roof using a roof rake or push broom, but take care not to damage the roofing materials. Doing this work on or below the roof can be very dangerous, and it’s a job best left to the professionals, said Schultheis.
Another short-term solution is to stop water from flowing into the structure. Schultheis recommends making channels through the ice dam by using a hose with warm tap water. Work up from the lower edge of the dam. The channel will become ineffective within days.
For the long-term, Schultheis says it is a good idea to increase the ceiling and roof insulation in your home to R-38 to cut down on heat loss. Make sure the ceiling is airtight so no warm air can flow from the house into the attic space. Do this by plugging gaps around plumbing vents, wiring, recessed lights, and chimneys.
"Keep the attic cold by providing at least two square feet of attic vent for each 150 square feet of attic area. Where the roof rafters meet the walls at the eaves, provide a 1-inch clearance between the roof sheathing and the ceiling insulation to allow ventilation," said Schultheis.