When I build something for myself or friends, I use 'Rainguard Galvalume'. It's stronger and faster since it covers 3' per sheet rather than 2'. You can't solder it, but there are some good glues these days like 'Great Stuff'. It will take purlin spacing of 30" instead of 24 which saves on materials and time. Don't put it on top of existing asphalt shingles or tarpaper. You need some spacers for a thermal break and keeping the bottom dry. My dealer will cut it to length and deliver although I use a trailer to pick it up. Don't screw it in the flat, only on top of the ridges. The screws will back out at the top and bottom due to thermal expansion if screwed in the flat. Don't overtighten the screws, you will crush the gasket. If you are collecting rainwater off of it, it's worth it to have industrial gutters made for it since you will get less spill over. When feeding you tanks, keep your feeder pipes in the air to avoid standing water in the pipe. Sunlight will penetrate PVC and that causes algae growth. Also paint the pipes since sunlight degrades PVC. Wow, I got into a little rant there didn't I. Hope this helps! 
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