As snow melts and refreezes it causes ice to build up in valleys, around vents and along the first 6 feet of the roof. Although from the ground you may simply see icicles forming along the edge of the roof, this is a sign of ice damming. Ice damming will prevent new water and snow from properly draining off the roof, leaving it to back up under the shingles behind the ice.

 

In order to prevent this from happening and causing damage to your shingles or worse - leaks, you need to remove the snow and ice. Depending on the weather this may need to be done once or twice a season.