The roof decking would be the layer of plywood or O.S.B. nailed towards the trusses or rafters. Roof decking is often seven/16” and is available in four’x8’ sheets. If you’re tearing off aged roofing, the roofer can seek out damage that needs to be repaired after the deck is bare. https://digitalbizgenius.com/top-5-roofing-contractors-for-joliet-il-expert-services-for-your-home/