Residential rentals are depreciated over 27.5 years, but also with a mid-month convention. See this IRS table A-6, page 71. That means no matter when during a month, an asset is acquired or disposed of, it's assumed to have been placed in service or disposed of in the middle of that month.
I'm not sure why you are having to recalculate depreciation if you have already been using TurboTax in prior years. It's a one-time entry when you first place the property in service, then the 2nd year will be greater and remain the same until the year you dispose of it where it falls back down. If you have used TurboTax prior, you should have a depreciation report in your saved return.
If this is your first year to use TurboTax, then you will need to enter the prior date your property was placed in service and TurboTax will calculate the amount of depreciation that should have been taken, and calculate the amount going forward.
As for the roof, it's depreciated over 27.5 years in the same manner as the property itself. Add it to the asset section under 'Rental Real Estate Property,' then 'Residential Rental Real Estate.'
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