Just had home inspected and found that rafters and or plywood of roof sub-deck is sagging or delaminated??? Also it was either rafters or plywood only 1/4" (v. 5/8"?). If this all needs to be repaired/replaced, does that mean that roof and shingles also must be replaced? Assuming there are two layers of shingles (house built 1959, approx. 2500 sq ft), what should this whole thing roughly cost - just a range - PLEASE! We are doing last minute negotiations with the owner and only have a budget of about $15000 to spend on our own.