3/4" decking is good for 24" on center joist spacing for residential housing. it's what you'll find in most tract home subdivisions these days. while i wouldn't want my house on 24" spacing it's MORE than fine for a chicken coop. i built my coop on 24" centers with 1/2" decking. is it a soft floor? yep. do i care? nope.. i'm only in there once every few days. your roof decking is going to be 1/2" or LESS and it's more than suitable to shingle and occasionally walk on. if you ask me, you're just wasting money buying 3/4 and using 16" centers.