I would focus on getting that househack first.You can take advantage of a lower downpayment, get a roof over your head, and an investment property all in one.

I sold a beachfront home last year that was ICF constructed in 2000 with a metal roof on concrete pilings and the insurance went up triple what my larger 2019 stick built with shingle roof 6 lots down is. 

If you are buying a house of any kind with mechanicals and roofs, you need reserves.

Is this a decision about one or the other - either write off the old roof OR depreciate the new roof? 

There's no roof or sewer for you to manage.

With your estimate, you might earn $800/month for 2 years and then incur a vacancy at the same time as you need a roof replacement and for that tax year your cash-flow might be -$1,200/month.

How to know when is time to replace the roof. 

Has a unique garage that is open air, but  roofed and on the same slab as the  rest of the house. 

I just closed last week on a duplex, new roof, windows, siding, furnaces, HW tanks, all cosmetic for ready,.....70k, Rented for 1800.

Savannah all I have is one of my houses has a damaged roof, that's it and feel fortunate.