Ice dams occur when the warm air in your attic melts the snow on your roof, causing it to run down the roof toward the ground. When it hits the eaves and breaks contact with the warm attic roof, the frigid air refreezes the water. As new melt hits the ice, it also freezes, building up the dam and can even be forced back up under your shingles and into your roof and attic, causing water damage, ruining your insulation and creating ideal conditions for mildew growth. Though salt may seem like the best melting solution, several salts can cause other damage to your roof or gutters.
Rock salt is common for de-icing driveways and paths. It should be no surprise then that it is the first thing many people think of to remove ice dams. However, rock salt is a form of sodium chloride, which corrodes metal, including the metal nails holding your shingles in place. Rock salt can also leave permanent stains on your roof and walls. It also does not work well below 20 degrees Fahrenheit. For these reasons, rock salt is not a good choice when it comes to getting rid of ice dams.
Some people use table salt for de-icing instead of rock salt. However, this is no better, as chemically they are the same substance. Table salt is just rock salt broken down into smaller crystals. It will corrode and stain just like rock salt does. Breaking the ice up with tools is also a bad idea, as that can easily damage your roof even worse than the rock salt could.
Magnesium and Potassium Chlorides
Magnesium and potassium chlorides also both melt ice. However, magnesium chloride is only effective if temperatures stay above 5 degrees Fahrenheit. Potassium chloride loses effectiveness below 12 degrees.
Calcium chloride is the best salt to use on ice dams, according to handyman Glenn Haege. It is not as likely to stain or cause corrosion as sodium chloride, but it can damage wooden gutters. It melts ice faster than sodium chloride does, works at lower temperatures than sodium chloride and also melts larger volumes of ice. It works at lower temperatures than either magnesium or potassium chloride.
Using Calcium Chloride
However, simply scattering calcium chloride across your entire ice dam, hoping to melt it, is not likely to work well. Plus, this would be an inefficient use of salt even if it does work. An ice dam can easily be made of hundreds of gallons of water, so melting it all requires a lot of salt. Instead, fill old pantyhose with the calcium chloride and lay it across the dam. This will melt a channel to allow water to safely run off your roof and break up the dam for manual removal.