We recommend placing the screws in the flat area of the panel. The washer has a better opportunity to create a good seal. If done correctly, this will provide superior leak protection versus placing the screws in the rib itself. We all know that leaving a screw loose can cause leaking. However, over-tightening of a screw is probably a much more common cause of leaks and roof failures. Luckily, it is a problem that can easily be avoided.