You should never install new shingles over an old shingle roof system. Your shingles would not be eligible for warranties, and it can cause your new roof to look crooked or lumpy. Furthermore, if the old roof layer is not removed, we are unable to inspect the plywood decking for rot or deterioration. This can create a much larger problem in the future. Moisture from the previous roof layer can get trapped under the new shingles and accelerate roof aging.