Re: Screw holes in used metal galvanized roofing
You can reuse them, but expect leaks no matter what you do to seal them. It would be best if these were used as wall panels and buy new for the roof.
For the roof, you can seal the holes with a beutyl caulk, just scrub both sided down really well to clean most of the dirt off first, then force the caulk through each hole and leave a button on either side.
You will need to install 1x4 skip sheathing to lay the roof panels over. If the panels are the flat type with periodic ridges, you can install them directly onto the skip sheathing. If it is the corrugated type roofing you will need to purchase the appropriate "waffle board" to install between the panel and the 1x4. A waffle board is cut in the same corrugated pattern as the panels, this will support the panels fully and allow you to walk on the roof as you are installing and for maintenance purposes. Always nail/screw through the ridges, not the valleys. Additionally, all metal roofing is "one-way", meaning it has a definite front and back. Make sure the same side is up on all panels and that they are mating properly as you lay them out.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!