How to adjust cost basis of a rental property when selling it under 'Sale of Property/Depreciation' section?

I installed a roof-$4k when I bought the rental property a few years ago, and this roof has been a depreciated asset over the years. I sold this rental in 2018 and now I want to add the original price, $4k back to the cost of rental property to reduce the capital gain, of course, the past depreciations will be recaptured.

But I don't know two things:

1. where can I adjust cost basis in 'Sale of Property/Depreciation' section of this rental property?

2. How much sales proceeds should I allocate to this depreciated roof asset? I must enter some value there, otherwise turbo tax will not do the depreciation capture for this roof asset on Form 4797.

I understand I can do cost basis in the 'Sale of Business Property' section, but if so, I cannot allocate the sales proceeds to other depreciated assets in the rental.

Please help!