On metal roofing panels with ribs, why would one drive screws through the flat part of the panel instead of through the top of the rib?
The directions for installing metal roofing panels that are attached by exposed fasteners give different methods for nails vs screws. These panels have raised ribs. My natural inclination would be to insert fasteners through the top of the rib so that the hole in the panel would be as far above running water as possible. This is indeed the method that is recommended when nails are used. But when screws are used, the recommended method is to drive them through the flat area of the panel. Why would this be the superior way to use screws?
Both the nails and screws used in this work have flexible washers to seal the heads. The nails are not to be driven so hard that they would dent the panel. I think screws could driven with equal delicacy.
Example directions ( from http://www.fabral.com/installation/i...ation-5-09.pdf) :
The correct way to fasten steel panels with nails is to drive the nail
through the top of the rib so the washer is compressed securely
against the metal. Nail placement must be in the ribs for roofing
applications to minimize the potential for roof leaks. Over-driving the
nail can split the washer and dimple the metal, causing leaks.
Wood screws with combination metal and neoprene washers should be
installed in the flat area of the panel adjacent to the ribs, and
tightened such that the washer is compressed as illustrated above.
This will ensure a lasting, leak-proof seal. See pages 4 to 5 for the
correct fastener locations.