Yes. You can apply new shingles over existing shingles, depending on the condition of the roof. If the roof has one layer of shingles that are lying flat and the deck is in good condition, the existing shingles typically do not have to be removed. Check with local officials to make sure that building codes are being followed. During re-roofing is a good opportunity to examine roof ventilation to ensure vents are sufficient in number, positioned properly and are unobstructed.