How do you get leaking if screwed in the rib?  I did a very, very small lean-to roof and I thought it made sense to screw into the rib thinking that water won't be flowing down the rib like it would in the valleys (flat)?????  I started  looking at other roofs and saw that everyone of them had the screws in the flat.  I can't believe I was right and everyone else was wrong, but, I never looked into.