We've used metal roofs on all kinds of buildings, including 3 homes that we built, for nearly 30 years (the ribbed/ridged kind installed with screws - not standing seam). We've always used the roofing screws with rubber washers under the heads. The screws are placed on the "ridges" of the steel only and into 2x4 purlins. The 2x4's are laid flat, not on edge, about every 2 feet. There is no other roof decking under the steel, so when walking on this type of roof it's best to step where the purlins are, although not crucial (depends on how much you weigh). On insulated buildings, we do install Tyvek under the purlins in case there is any condensation from outside air on the underside of the roof steel. If there is ever any moisture, this would keep it from the insulation and would let it run down and out the vented soffits. The overlaps of the steel sheets are caulked as you go.
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