Metal roofing that screws down requires on average 70 exposed screws per sq. of roofing. On an average sized home this can add up to something like 2,500-3,000 screws or 3,000 potential leaks down the road. Extreme temperature differences between summer heat waves and polar vortex cold snaps cause expansion and contraction of the metal roofing along with the deterioration of the rubber washer that seals the water out. with so many screws on the roof it can be very difficult to figure out which ones are leaking.  we often see metal roofing screws and nails going bad some where between year 9 and 13.