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 screws. Third, a long screw will be like a little lever because it is sticking up so high with its shank unsupported. As the metal roof expands and contracts, it is likely that, over time, the screw will snap off.

If for some reason you should decide to use nails, put them through the top of the main ribs. This is because a nail doesn’t seal as well, so should be kept out of the flat. Like the screws, the nails must have a neoprene washer under the head to provide a seal.

David Keener is an engineer with Fabral, a maker of metal roofing and siding in Lancaster, Pa.