Winter brings snow-packed roofs and damaging ice dams. If your attic is not properly insulated, heat from the attic will melt snow on the roof and the melted water can pool up and leak into the house. Also once the melted water hits the cold eves, it freezes and forms an ice dam.
To help prevent ice dams keep the roof cold. To maintain low temperatures make sure there is adequate insulation and seal gaps that let warm air into the attic. Also, ventilate the attic so that cold air pushes the heated air out. You may need a motorized vent at one end of the attic
if the gable and ridge vents do not generate enough air movement to dissipate the heat.
Increased insulation in the attic will help also. Before insulating, install polystyrene rafter air channels, then insulate the areas between the rafters. Keep an air space between the roof deck and the insulation to prevent a condensation build up. You could also use sheet-metal ice belts or heat tapes.
Whatever plan you decide, focus on the cause. Ice dams are created by the heat lost from the house.