@Carl Partial dispositions are absolutely okay to do and there are reasonable methodologies allowed by the IRS to come up with the amount to be disposed. There's nothing "screwed up" about the depreciation history. You basically split it into two assets---old roof and everything else---and allocate the depreciation between the two as if they were always separate. It would be completely appropriate AND strategic to dispose of the old roof which no longer exists. You get to take a loss now, and if you sell the property, there's less accumulated depreciation to recapture.
