You will be fine nailing shingles straight to plywood if the roof has enough of a slope (so water will run off easily). If your roof is flat you should use metal or rubber roofing material instead of shingles.
(1) Are you new to chickens / when did you first get chickens?
A: No / probably when I was 3 or 4
(2) How many chickens do you have right now?
A: 21 and 16 in a brooder
(3) What breeds do you have?
A: Barred Rock, Golden Comet, Rhode Islan Red, Black Sex Link, Brown Leghorn, White...