If the existing plywood is rotten or damaged it may need to be replaced. Also, if your home was built prior to 1960 it may have been sheeted with 1” boards. Often when roofs were sheeted in this fashion the builders left gap spacing between the boards to maximize building material usage. The “gap sheeting” should always be sheeted over with CDX plywood or OSB in a re-roofing project if the gaps are >1/2”.
