Something to remember is that keeping your roof shoveled off completely will prevent your ice dam issues. All to often you see the bottom four or five feet shoveled off and the rest of the roof is still covered. This will still cause ice dams to form they will just be four to five feet up the roof as well as keep all the weight higher on the roof and thus putting just as much strain on the framing as if you hadn't shoveled any snow off. It is virtually impossible to prevent ice dams in New England, warming from the sun and/or heat loss happen on every roof. The only way to help prevent the ice dam is to keep the snow from melting and that requires removing all the snow from the roof as often as possible. And think about how well you will sleep knowing that you do not have a ton of weight or more hanging over your head.
Ice melt is a good idea, but stay away from rock salt for all roofs, not only will it stain your house, but it will kill any vegitation under your eaves. If you have architectural shingles you will not want to use calcium either, like SIKSUKR said there are products that are gentler than others. Just remember you are adding more moisture to the issue, so make sure the water has a place to go first.
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