Suppose I want to cover a roof with corrugated steel sheets. Imagine a sheet laying on the roof. It will have "tops" - zones that are further from the roof surface and "bottoms" - zones that are closer to the roof surface (the line is the sheet cross-section, the roof is below the line, the outdoors is above the line):
I need to decide where I run the screws that will hold the sheet.
In case it was roofing slate I'd definitely run them through the "tops" ("B" on the drawing) because when it rains water will run along the "bottom" and into any hole it finds there.
However seems like the typical approach with corrugated steel is to run the screws through the "bottom" ("A" on the drawing) which puzzles me a lot.
Which do I choose - A or B - in case of corrugated steel and why?