01/08/13, 06:40 PM
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Join Date: Jun 2010
Location: North Central MN
Posts: 3,020
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Ice dams form when warm air gets into the attic because the attic insulation is insufficient or it gets around gaps where things like chiminys or vent stacks penetrate the floor of the attic, especially if the attic doesn't have enough ventilation. The the outside air temperature has to be just below freezing too.
The warmth in the attic melts the snow on the roof above the attic. The water runs down the roof until it hits the eave, which is below freezing because it's hanging out there in the below freezing air. The water freezes. Repeat and the ice builds up high enough to trap a lake of water behind it. This water is warmed by the warm attic and stays liquid. Eventually it finds a way into your house and damages the ceiling and walls.
The fix is to make your attic cooler by fixing the cause. You can also put up electric heater wires on the eaves to melt channels through the ice dams. Both of these need to be done in warm weather.
Right now you can fill a nylon stocking with sidewalk salt to make a "sausage" about 4 inches in diameter. Lay this across the ice dam, perpendicular to it, and it will melt a channel for the water to escape. For the rest of the winter, repeat as needed. Shoveling the snow off doesn't melt the ice dams but can help reduce the volume of melted water.
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