Interesting issue with the nails. I designed, marketed, sold and supervised metal roofing installations for more than six years. Including the World’s Largest single project at Walt Disney World. (Caribbean Beach Resort 1.5 million square feet)
I wonder how roof panels test for wind when nailed vs. screwed on. I’d almost bet that they don’t meet the Florida Wind Codes when nailed. Our Company never used nails, always used self tapping metal or wood screws based on the substrate we were installing the roof on.
The older the metal roof, the more likely the exposed screws will serve as possible areas of water penetration. The expansion and contraction of the roof panel will elongate the original screw hole. What was once a round hole has become an oblong hole. Usually a large diameter screw with a larger neoprene washer solves the problem.