Ice dams are a barrier of ice that is usually found at the eaves of the roof.  These obstructions are dangerous for many reasons, not only can they fall off the roof and severely injure someone, but they can cause serious and expensive damage to your home.  The dams prevent water from melting snow from flowing off the roof, and instead cause it to back up underneath the shingles.  This in turn can cause leaks in your home.  Of the many reasons for the cause of ice dams, the most common is a large amount of snow being heated during the day and freezing at night.

Another cause is a poorly insulated house.  Having warm air leak from the house into the attic results in the melting of the snow on the roof because of the warm air rising.  The best defence against this culprit is to seal all the places where the warm air could be leaking from the house with a caulking.  The most common places to find leaks, and in turn water, are around all light fixtures, ducts, plumbing stacks and attic hatches.  If all leakage problems have been addressed and the problem persists, consider soffit vents.  Soffit vent and gable vents expel the warm air from the attic space, preventing it from melting the snow on the roof.

Adding insulation results in a colder and less moist attic.  The insulation should be installed properly, with even distribution to ensure there all the nooks and crannies are covered.  Remember to avoid blocking vents with the insulation.  If you do find that there is a leak and it has affected the insulation, that piece must be replaced.

It is important to be aware of ice dams and the damage they can do.  Armed with this information, you can take the necessary steps in preventing the ice dam from forming in the first place.  As a plus, in correcting a leak of warm air into the attic, you will also be taking steps in reducing your energy bill.

Source: http://www.homeownercare.com/how-to-control-ice-and-snow-on-your-roof/2009/11/