You can install metal roofing over existing shingles, but it is not always recommended. It is possible if the shingle roof is in excellent condition and was installed correctly with the best materials, such as a high-grade synthetic underlayment. In this scenario, the metal is only being installed over the shingle because of style preference. In other cases where the asphalt shingle roof is past its useful life and has experienced some level of leakage or failure, installing a metal roof over it is highly discouraged. In this scenario, one would be installing a new product on top of a bad foundation. The roof structure has already been damaged, and it could also be trapping moisture underneath the new metal roof.