An ice dam is an occurrence where a ridge of ice forms along the edge of a roof such that melting water under the snow is blocked form leaving the roof by the ridge of ice.
Our roof has an area where ice dams form in a valley of the roof where water is channeled, which when it rains, is just fine. But in a longer, colder, snowier winter like we’re having this year, snow that is heated by heat from the house, or by the sun’s meager heat, melts and then refreezes as the cold gets to it. In most places on our roof (even other valleys), this isn’t a problem, and the water refreezes as icicles.
Small icicles are a result of small amounts of melting where the water flows under the snow, and freezes where it reaches the colder edge of the roof or gutters. But with an ice dam, the water sits behind the ridge of ice at the edge of the roof and freezes there, raising the height of the ridge, and producing a shelf of ice that goes back along the roof. If this goes on long and far enough, the melted water backing up behind the ridge/shelf of ice gets up in under the shingles, and the heat of the house can keep this liquid and allow it to flow in under the shingles, through the roofing materials and into your walls.
Ours is bad due t several factors. First, the valley is a place where you are more likely to get a larger amount of water. Second, our house is old (built in 1898), and wasn’t designed to be heat efficient, so heat gets to the roof more (though, if we had some serious coal heat, it would probably melt all the snow as it fell …). And third, the people who owned the house before we did had the bathroom just under the valley redone, and the contractors took out a bunch of the plaster, leaving an open space for hot air to go right up in under the roof and heat the snow just on the outside so the water flows down to the colder roof and freezes.
The major way to deal with an ice dam is to carve a channel through the ice to allow the backed up water to escape. The first step toward doing this is to clear the snow away from the dam and as far up the roof from it as you can, because snow not only can melt to make more water, but also provides a layer of insulation that allows your roof to be warmer which can also lead to more water.
The second step is to cut the channel. A relatively quick and easy way to do this is with a steam-cleaner. I used our Wagner 915, and it worked well, though with 8-10” of ice, it took several tankfulls of steam to cut channels and remove a large section of ice in the valley.
While this might seem slow, it is really safe for the roof, unlike trying to use a propane torch, heat gun/paint stripper, or even a hammer or hatchet.
Another option is to use calcium chloride (which melts ice at -25 degrees F), magnesium chloride (which melts ice at 0 degrees F), or rock salt (which melts ice at +25 degrees F) as a melting agent that is laid on the ice in a line to melt a channel. An easy way to insure that you have enough in the right place is to fill am old stocking or one leg of an old pair of pantyhose with the melting agent, and lay it so that it crosses the entire ice dam from the roof above the dam to where it can just hang over the edge. This will ensure that you melt a nice channel down through the dam, and if you have enough of a melting agent in the stocking, it can keep the channel open even after further snowfall and melting.
These channels, once opened up, allow the new melt-water to make it over the edge of the roof, perhaps giving you icicles, but keeping it from building up and causing damage inside the house.
There are some preventative options. Home Depot offers some calcium chloride bricks that you put up on your roof where you normally get ice dams before it snows and they will melt the snowfall and keep it liquid as it runs off the roof. They are supposedly good for 12″ of snow before needing to be replaced, but realize that that’s about an inch of rain, if you are in a location that might go back and forth between liquid and solid forms of precipitation.
Another option would be to get a ‘heat tape’ or ‘heat line’ to run along your roof and gutters.
Most all of these are ON as soon as you plug them in, so there are some temperature-based controllers that will turn them on when the temperature drops below a certain threshold, and turn them off when the temperature warms up enough, thus saving you on electricity costs, and from overheating your roofing.
These lines are clipped to your shingles and are usually run in a zig-zag along the eaves to provide channels over the cooler overhang parts of the roof.