When I re-roofed the entire roof, not just the first 3 feet, was laid with bitchathane (AKA Grace Ice & Water Shield) before the shingles went on. This is pretty standard practice up this way but does add a pretty good labor expense.. Think of it as a barrier coat for your roof.... Never had a single leak and we don't shovel it unless there is a weight issue......... Right now multiple neighbors are dealing with ice dam leaks one is a good friend and whom I suggested go with bitchathane, when they did the roof three years ago, and did not..... Penny wise pound foolish.... D'oh.....