If ice dams are a common occurrence in your area, there is a good chance that the building codes required your roofing contractor to place a water proof membrane on the lower portion of your roof. The steeper your roof the less chance that a large lake can physically form behind the dam. Ice dams are a big problem with low pitched roofs. Insulation is usually pinched where the roof rafters connect to the exterior walls. Many homes were built with the roof rafter cut to form a flat surface to attach to the wall. If this is the case, there is no room to place additional insulation or for proper ventilation. Ventilation is critical for wood shingle or shake roofs. In fact, most warranty claims are voided without proper ventilation.