Water is insidious and, given sufficient volume and time, it will defeat the newest, best designed roof and find ways into a home. Rather than the result of clogged gutters or faulty roofing, ice dams are most frequently caused by too much heat escaping outside and warming the roof. The warming occurs unevenly with the warmer area at the higher part of the roof melting the snow and then the cooler, lower area, particularly the roof edge, permitting the water to refreeze and then accumulate. The heated roof is usually caused by poor insulation or improper ventilation. Inadequate insulation lets too much heat escape into the attic and this creates a warmer roof.
Improper ventilation creates moisture and heat buildup due to the lack of air movement.
Compare the way the snow is melting from the living area of your home with how snow appears on the roof over an unheated area such as a garage or shed. Compare how your snow covered roof looks with your neighbors' homes. Look for icicles. These are pretty to look at but heavy icicle buildup means that interior heat is melting a lot of snow and may contribute to ice dams. How to prevent ice dams.