Spongy decking can be caused by a lot of factors. Usually the reason is that the rafters may be too wide, around 24 inch wide rafter spacing and the deck boards might be too thin like less thin than 1/2 inch.
The fact that poor roof ventilation can cause boards to delaminate, or the glue in plywood or osb to separate and go bad will be a reason for these boards to be spongy.
This would mean a complete reroof of this project, new boards and new shingles.
