Susan,

Are you sure your roof is subroofed with plywood. In the time frame your home was build, some manufacters used chip board and particle board to cut costs. Once wet, these products were no longer structurely sound and may very likey harbor mold. In addition, once wet, shingle fasteners especially staples no longer held even if the subroof subsequently dried.

I am not trying to scare you but if chip or particle board was used and got wet, those products should be replaced. They are no longer safe to walk on or hold your roof trusses together. Current building codes no longer allow these products to be used in new roofs.

One option is to use a reflective roof material. The current tax code allows an energy tax deduction for qualifing reflective roofing materials.