How ice dams are caused and how to prevent ice build-up
December 17, 2010
The potentially damaging effects of ice dams are all too evident when watching the recent collapse of the roof of the Metrodome in Minneapolis.
“While the chance of your roof collapsing from an ice dam is very small, ice dams still can cause damage,” said Carl Pedersen, North Dakota State University Extension Service energy educator.
This is the time of year when ice dams will begin to develop on roofs and potentially cause damage. Homeowners have a few steps they can take to prevent ice dams.
“The key is to ensure your roof is constructed properly with sufficient attic insulation and ventilation to prevent the formation of ice dams,” Pedersen said.
An ice dam forms when snow melts and runs downs the surface of the roof. As the melted snow hits cooler parts of the roof near the eaves, it refreezes. This process continues and eventually creates a dam of ice on the roof.
As more snow melts, the dam stops the water, which freezes, making the dam bigger, or backs up the melt water, allowing it to leak under the shingles into the attic spaces or eaves. This can lead to water stains in the ceiling, structural damage and mold growth inside the home.
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Ice dams are clearly visible once they form, but to find out what causes them, a homeowner needs to look inside the attic.
Heat from inside the attic is the major cause of snowmelt on the roof forming ice dams. The greater the amount of heat in an attic, the greater the potential for snowmelt and ice dams.
The attic is warmed with heat leaking from a home’s living space.
Anyplace where plumbing, heating or electrical components protrude into the attic and the openings are not sealed properly can allow heat leaks to occur.
Recessed or “can” lights and attic access doors or ladders can be other areas where heat leaks into the attic.
Even if all the leaks into an attic from inside the home are sealed, heat from the home can be lost to the attic if the attic does not have proper insulation.
“The ability of insulation to stop heat loss depends on the type and thickness, but attics in northern climates need roughly 18 inches of insulation to be properly insulated,” Pedersen said.
Another cause of heat loss is improper ventilation of the attic space. Keeping all the heat out of the attic space is impossible, so allowing that heat to leave the attic through vents is important.
Depending on the construction of the home, this can be done through soffit vents, vents on the gable ends of the home or a ridge vent. Make sure the attic’s insulation does not block the air flow from the soffits or eaves. Use baffles to create a channel to hold insulation back from the soffit vents to allow air flow.
Removing snow from the lower portion of the roof so melting snow can run off is another way to prevent ice dams from forming.
“However, removing snow from the roof can be dangerous, so avoid getting on the roof by using a roof rake,” Pedersen advised. “You also need to take care to avoid overhead power lines.”