Residential metal roof systems require underlayment, and it is critical for three reasons. One is to keep the structure dry during the roof system installation process. Another reason is to provide redundant protection. And third is to keep condensation on the back of the roof panels from saturating roof substrates.
Although for many years, 30 lb. asphalt saturated felt was the standard underlayment beneath metal roofing, many contractors have switched to newer synthetics underlayments that are lightweight and offer more coverage per roll. Self-adhering underlayments can also be used. However, it is critical these products always are smooth rather than mineralized so as not to damage the back of the metal panels.
During metal roofing installation, the individual roofing panels are fastened together with special screws that are color coordinated with the metal roof. The fasteners have a grounded washer that prevents water infiltration around the fastener.