The roof on my 30+ year old house is shot and I'm going to replace it. The roof structure is rafters that run up to a ridge board. The wood surface of the roof is sltaggered slats. Two of the rafters are sagging and the ridge board is broken. Some of the slats are soft and will not hold nails very well. I want to cut the slats on rafter centers to access the bad rafters, replace the sagging rafters, install plywood to level repaired area with original roof surface, then sheet the entire roof with 1/4 inch plywood to make it all nice and flat. Is this type of overlayment acceptable or do I need to remove the entire slat surface and replace all with plywood?