The roof decking is definitely the layer of plywood or O.S.B. nailed on the trusses or rafters. Roof decking is usually seven/sixteen” and is available in four’x8’ sheets. If you’re tearing off outdated roofing, the roofer can look for hurt that should be fixed when the deck is bare. Rafters: https://elliotpuafi.empirewiki.com/8201277/getting_my_roofers_to_work