How can I prevent ice dams in a northern climate with very heavy snow? My cabin has a cathedral ceiling with R-48 insulation and a very good vapor barrier. The roofing is 50 year asphalt shingles with ice and water shield a few feet above the wall line. It is typical to have 3 to 4 feet of snow on the roof for several months. The ice dams were about 8 inches thick this winter.