Ice dams are those sheets of ice and snow that form at the edge of your roof. They often look beautiful and result and in long, lovely icicles hanging from your eaves. I love looking at them and as a child I loved breaking icicles off our roof and sucking on them. They were nature’s popsicles. While these make a quintessential New England picture, they are actually quite bad for your roof and should be removed.
Ice dams are formed when there is freezing and melting on your roof. This is bad because as water freezes it expands. This results in the water getting up under your shingles, then when it freezes it expands, pushing the roofing apart and making bigger holes and degrading the quality of your roof. They can also cause the water to leak into your home when it pools up. They repeat the same actions in your gutters, resulting in the gutters getting warped and deformed over time as well.
Ice dams frequently form when buildings do not have adequate air sealing between the body of the house and the attic. Attics should have an air-sealing barrier and a layer of insulation between the living areas and the attic space, to prevent the heat from escaping into the attic. When warm air is in the attic, the portion of the roof over the warm attic is also warm, melting snow on the roof. However, the edge of the roof that hangs out over the wall is surrounded by cold air and is colder. When the melted snow runs to the edge of the roof it freezes creating a dam that holds more water above it. This water ponds and finds cracks in the roof that it would have otherwise run right over, and infiltrates into the attic and walls of the house causing water damage.
When attics are well sealed you don’t see ice dams as frequently because the air temperature is even and cold under the entire expanse of the roof, so the water more easily runs right off the edge of the roof. It is still possible to get ice dams with a well insulated attic, but they are usually not as frequent or severe.
The best thing to prevent ice dams is to get your house properly air sealed and insulated. However, if you have ice dams right now, there is a safe way to get rid of them.
Fill an old stocking (panty hose) with calcium chloride. If you can make it longer than the width of your ice damn, that is great, otherwise, a foot or two is good. Tie off the end. Toss the stocking up on the edge of your roof, on top of the ice dam, such that it lies vertically up your roof and hangs off the edge a little. The ice will begin to melt under the stocking, creating a channel that will let the melted water above the ice dam run down. How far apart you space them probably depends on what the ice dams look like and how the water is ponding above it, but I’ve seen every 2 feet as a suggestion. Here are our step by step instructions for making Sexy Ice Dam Breakers.
A stocking is better than a sock, because the material is thinner and you can make a longer tube. There are a variety of ice-melts out there, but calcium chloride is most often recommended for this purpose. Salt is not recommended because its corrosive properties might damage your roof. You can also see in our picture how calcium chloride worked quite well while rock salt and potassium chloride did not work well at all.
Update: We now know why Calcium Chloride worked so much better than the other ice-melts. Calcium Chloride has an effective melting temperature of -25°F while Potassium Chloride is only effective just below freezing at 25°F. Sodium Chloride/Rock Salt is good down to 20°F, but can cause damage to your roof. Details from Consumer Reports.