Note also that for depreciation, you are required to use the *lower* value of:
1. What you paid for the property when you originally acquired it, plus what you paid for any property improvements after you acquired it, or;
2. The FMV of the property on the date you converted it from personal use, to a rental.
Since "you" did not pay for a new roof separate from the contracted purchase price, whatever the roof costs does not get figured in or added to the basis. Since the seller paid for it, the seller (not you) would get to add that cost to their cost basis to reduce their taxable gain on the sale.
While not impossible, I seriously doubt that the FMV of the property on the date you converted it to a rental, was lower than your acquisition costs. So you would use your acquisition cost, plus the cost of any property improvements that *you* paid for.
