They should not overhang more than 2.5 cm (1 in.). Any distance more and the shingles will break off with the weight of snow and ice in the winter. What you do want on the edge of the roof is something called a drip-edge. When rain water or melted snow runs off the last shingle, most of it will drop to the ground because of gravity. Some will, however, work its way under the last shingle. If there is enough water there it will soak into the roof deck and may cause some damage. A drip-edge is a long strip of aluminum that sits underneath the last shingle. When excess water works its way up the inside of the last shingle, it will be blocked by the drip-edge. A simple trick that will prolong the life of the house!