Spray polyurethane foam can be installed over a commercial metal roof. However, most of the time, it’s installed over an existing roof.  The first step is to clean the existing roof with a power washer to remove debris and dirt. One weakness of metal roofs is rust, so your roofing contractor will use a rust inhibitor primer to stop the oxidation process. It is important not to skip this step because rust will continue to grow on your metal roof which can decrease the lifespan of your roofing system. Once the primer is installed, the roofing contractor can now begin installing spray foam.