Roof Sheathing and its Deck
Making sure the appropriate roof sheathing/deck is under the shingles you have to install is critical to ensure optimal shingle performance.
The roof deck should be in good condition: dry, clean, flat, smooth, and securely nailed. Plywood sheathing at least 1/2″ thick are best.
If shingles are installed over an unacceptable roof deck, it may void the manufacturer’s warranty. Any part of the deck or sheathing that is rotted or warped must be replaced.