I am installing a new roofon a house in Toronto Canada where snow and ice can be an issue. My roofer has told me to use a cross over method in my valleys where the shingles are crossed over about 1 1/2 feet over one side and then crossed over down to the valley where they are cut. This is all done over a tar pape barrier. The old method was just using the metal valleys. He claims the new way, will prevent the ice from traveling under the shingles and is the more popular way these days. Any help would be appreciated.