When replacing a roof, you will need to decide if you want to remove the old shingles first. Most roofs are designed to hold one layer of shingles, but some people decide to add a second layer to save on the cost of removing the first layer. However, Ontario Building Code (Sec. 9.26) states that you may have no more than two layers of shingles, so you will end up spending more to remove both layers of shingles next time you replace the roof.