Cleveland, OH - The Metal Building Manufacturers Association (MBMA) released the white paper, Comparison of Retrofit Systems Over Existing Metal Roofs, by Brian Gardiner FRCI, RRC, CCS. This paper provides a detailed comparison of two options for putting a new roof layer over an existing metal roof: a new metal roof and a single-ply layer, most often consisting of Thermoplastic Polyolefin (TPO).