However, if you decide to roof directly over shingles, these tips should help you get the most out of your project. In this method, the purlins are screwed down to the roof deck, depending on how the metal roofing manufacturer has designed the panels. Ideas, metal roof purlins over shingles metal roof purlins Installing a metal roof over shingles. How to install metal roofing over shingles video . The second way is to install 1×4 purlins over the shingles. Sheffield metals always recommends removing the existing shingles so you can properly inspect, repair, and level the roof deck before installing a brand new roof. This is the fastest way to install but it doesn’t provide the flattest surface for metal roofing installation. Metal roofing, however, can last upwards of 50 years or more, making it a worthwhile investment. Bend the metal edging to your needs if necessary and fasten it to the roof with nails at every 4 to 6 inches. Both metal and shingle roofs are great option...