I didn't use screws, I didn't want more roof holes. I went with Renogy panels and used their Z brackets. I made mounting plates for the Z brackets to make it easier to remove the panels if needed. I used 4952 VHB tape to secure the mounting brackets to the roof and covered them with Eternabond tape. The VHB tape probably has all the strength you need to secure the panels to the roof but I wanted redundancy just to be sure the panels don't move. So I know used the VHB tape and then covered the mounts with Dicor. Dicor is very strong but make sure you wait about 5 days for it to fully cure and set. I attached a pdf with more details. You might think about using the Renogy tilt mounts,
https://www.amazon.com/Renogy-Adjust...-garden&sr=1-1. You wouldn't need to make any mounting plates, just put VHB on the rails and stick in place. I would still cover with 2" Eternabond tape just to seal it in place and add a redundant layer of adhesion. The tilt mounts let's you get easy access under the panels for when you are hooking up the system and them after everything is installed, they let you raise the panels to clean the roof or do any maintenance. I also like that you can tilt them to better a better sun angle, like in the winter when the sun is much lower on the horizon. They didn't have them when I did my install, to bad I would have liked to use them.
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