A.David Keener responds: Our company definitely recommends screws over nails for attaching metal roofing to the purlins. Screws are much stronger than nails in terms of pull-out strength, they don’t back out as readily as nails, and it’s possible to get a better seal with screws than nails. Screws should always be installed in the flat of the panels. This may appear to be incorrect because it places the screw in the waterline. This isn’t a problem, however, because the screws used for installing metal roofing have neoprene washers that compress to form a good seal.

You shouldn’t put the screws through the ribs for several reasons. First, one of two things will probably happen when you try to compress the neoprene washer. Either the crest of the rib will dent or the washer won’t compress properly. In either case, you will get a poor seal. Second, you will need long screws, which will cost more than short...

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