If during the process of replacing your roof shingles we discover you have rotten decking, we will replace as needed. It is extremely hard to determine if you have rotten wood and to what extent until we actually replace your shingles. We cannot shingle over rotten wood because the nails will not hold. We will set a pre-determine price for a sheet of plywood in our contract should that occur. We make every effort to avoid any surprises to our customers.