An ice dam is a build up of ice at the edge of the roof. It is called a dam, because the build up is large enough to dam up some water behind it.It is caused by the melting of snow on the roof, which flows down the roof to the edge, where it freezes and builds up. What causes this? Sometimes even the best roof designs can get them. In the winter it seems that a large number of roofs have them. Generally it is caused by a warm attic and cold eaves. That allows the snow to melt on the roof over the attic, and then re-freeze when the water reaches the eaves. The eaves are cold since they are not over the house, but they are hanging out there in the cold. If the ice doesn't fall off the roof, and hangs on, then the water melting above, will build up behind the ice.
What harm can they do?Water can build up behind the ice dam and can get high enough to get under the shingles and leak into the attic. It often shows up as water leaking from the window trim or wet spots in the ceiling close to the walls. If it happens once it can stain your woodwork and make those ugly water marks (stains) on your drywall. If it happens often it will rot the roof decking, and other framing members.
How do I eliminate them?
Insulate and ventilate the attic. (Click here to read about the importance of attic ventilation)
You want a cold roof that will keep the snow from melting on the roof. At least when it does melt on the roof, it will not freeze again when it reaches the eaves. Ideally what you want is a roof that is a constant temperature from top to bottom, from eaves to the peak.
Adding insulation to your attic floor will help. You should have enough insulation in your attic floor to give you R-49 (or you don't live in an area where ice-dams are a problem). That is about 20 inches of fiberglass. It means giving up the attic floor as a place to store things, since to get that much insulation you will have to go deeper than the joists, and preferrably lay it perpindicular to the joists. You should NOT use insulation with a vapor barrier (kraft faced) since you should not add a vapor barrier part way through your insulation. Before laying the insulation down, or before blowing it in, seal up any openings in the floor. Electrical wiring comes through holes, caulk them closed. The plumbing vent-lines run through the attic... caulk around those pipe openings. If you have light fixtures which require the insulation to be left away from them to dissipate the heat, then make sure you do that... you don't need to create a fire hazard here.
At the eaves, make sure the insulation does NOT close off the opening between the rafters that allow the air to flow up from the soffit-vents. If you are blowing in insulation or packing the batts in tight there, buy rafter vents. These are styrofoam panels that fit between the rafters to keep a channel open.
(Click here to read about the importance of attic ventilation)
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