We had an addition built in 1980. Just recently, we've found a few rather good size "soft spots" in the roof. On inspection, it appears the plywood has just disintegrated (one of the areas seems very dry, although we do have some moisture in another area). You can very easily push a screwdriver through the plywood, and nails don't hold very well.
My question is how is this typically repaired? Can we just put new sheating on top of the old roof shingles (one layer) and all? Or do we remove the shingles, and put the new sheathing over the old, or (least desirable), remove the old sheathing?
Any advice would be welcome!