Ice dams develop as snow on the upper part of the roof melts. Water runs down the roof slope under the blanket of snow and refreezes into a sheet of ice at the roof’s edge creating a dam. Additional snow-melt pools against the dam and eventually leaks into the building through the roof.
The reason this ice forms along the roof’s lower edge, usually above the overhang, is because the upper roof surface is at a temperature that is above freezing and the lower part of the roof surface is below freezing. The upper roof surface is located directly above the warm living space. Heat lost from the living space warms this section of the roof, melting the snow in this area. During periods of freezing temperatures, the lower regions of the roof deck remains below freezing because roof overhangs are not warmed by the indoor heat loss.
Much of this water damage is apparent such as water stained ceilings, dislodged roof shingles, sagging gutters, peeling paint, and damaged plaster, but you may not notice the rotten roof decking or framing for quite some time.
How do I prevent them?
The most effective way is to try to maintain the entire roof surface as close as possible to the ambient outdoor temperature. Just look at the roof of an unheated shed or garage. Ice dams don’t form on these structures because there is no uneven melting and freezing.
Increase attic ventilation. A well ventilated attic will be mush colder and that means less snow melting on the roof. The object here is to get the temperature of the roof as close as possible to the temperature of the outside air. This will prevent the snow from melting, running down the roof and freezing again.
Add more insulation. The idea here is to stop the heat from escaping into the attic. Just make sure you don't close off the openings between the rafters which allow air to flow up from the soffit vents.
Self sticking sheets of rubber could be installed under the roof shingles wherever an ice dams are possible. If any water leaks through the shingles, these waterproof membranes will prevent the water from leaking into the attic.
Metal flashing called ice belts can be installed at the lower end of the roof. This flashing extends up and covers the area of the roof where the ice forms. They will provide a smooth surface which allows the snow and ice to slide off the roof and they also act as a shield and will not allow water to flow up under the shingles.
Think about installing a metal roof. They will eliminate the problem completely. The snow and ice will slide right off, and there no shingles that water can get up under.
Heat tape seems like a good idea in theory but in my opinion, it's a bad idea. In fact, the heat tape can actually create a dam just above it.
It may be necessary to remove the gutters since they can hold the snow and ice at the edge of the roof. Gutters do not cause ice dams but they do help concentrate ice and water at a very vulnerable roof areas. As gutters fill with ice, they often bend and rip away from the house.
Some people even resort to physically removing the snow and ice with hammers, shovels and ice picks. Even though carefully removing snow from roofs with roof snow rakes does help, more damage could be done to life, limb and roof in the process.
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