Solid roof decks are either 1X8 ship lap, which is 3/4" thick standard boards (rare) or plywood, usually 1/2" thick. Occasionally you'll see 3/8" plywood (BAD IDEA!) or perhaps 5/8", which is much stronger than half-inch.
Thicker plywood is a benefit in high wind areas because the fasteners have more wood to bite into; thus more 'holding power'. The half-inch CDX is usually 4-ply, but 5-ply is available and is significantly stronger and should be used when spanning rafters. If putting plywood over 'skip' or 'spaced' sheathing boards (which is used on a wood shake roof), then the 4-ply is fine.
Plywood is graded as having an A, B, C, or D side. AC should be used at open visible overhangs with the A side down. An alternative to this is CCPTS, which means two 'C' sides with one side (PTS) plugged (with little football shaped patches) and touch sanded. Standard 'CDX' plywood translates to a 'C' side and a 'D' side, with the 'X' meaning exterior glue.
Avoid OSB (wafer board)! Unfortunately, many ‘roofers’ do use it. Why? BECAUSE IT IS CHEAP. IT IS RUBBISH. OSB can swell with moisture and is not nearly as strong as real plywood. If you are using plywood spanning just rafters with no spaced sheathing, then upgrade either to a 5-ply 1/2" CDX or 5/8” CDX. DO IT RIGHT!
