New Roof on Top of Old Shingles You should never install new shingles over the old shingle roof system. Your roof system will not be eligible for a warranty if you do not remove the existing shingle roof system before installing the new system. Installing over the old shingles can also cause unusual shapes to form in your new roof system and the roof system will not look smooth and level. Rot, Moisture, and Accelerated Roof Aging If the old…