In the northern portions of the United States, winter weather causes a particular type of problem in the form of frozen water in the gutters of a home or building. Although it is an unfortunate problem, it is also quite common and there is an easy solution. To rephrase that statement, there is an easy resolution if the problem is addressed immediately before it causes further damage.
How an Ice Dam Forms
Ice dams form when the water freezes so fast it turns to ice in the gutter before draining through the downspout as intended. Once it has turned to ice, it prevents any more water from its proper flow, which in turn freezes. Eventually the gutter is completely filled with ice, which leaves no room for further snow melt or rain other than on your roof, which is the last place you want standing water to be. Your shingles were designed for water to drain off them, not sit on top of them. It might would be less of a problem if such water also froze, but the heat rising from your house keeps it just above freezing temperatures.
Damage Caused by an Ice Dam
The damage comes because sitting water soaks under your shingles rather than draining away. Water is always going to find some way to go downhill, even if the only path it has is through your attic. Once the water is inside, it's “game over,” as it immediately begins to rot the wood structure and cause mold to grow.
How to Repair an Ice Dam
Fortunately, the problem is common enough for engineers to have designed a very specific and simple repair for when an ice dam forms. There is a specialty tool, an insulated heating core, designed specifically to fit on or in your gutter and melt the ice. In layman's terms, think of it as your electric heating blanket, except sized for your gutter and properly grounded as appropriate for the outdoor weather. After it thaws and drains, according to the particulars of your situation and the weather forecast, there may be a preventative salt application to assure the gutter doesn't freeze again with the next storm.