.”• I pay all the costs – things like Real Estate Commission - Title Insurance - Escrow Fees - Transfer Tax -Termite Clearance - Roof Certification – the list goes on and on..• And I buy them in an “as is” condition.

BUT, I had to put on a new roof which I didn't think I needed.

You can have two properties with similarly sized roofs in two completely different neighborhoods at different cost points. 10% "might" be a starting point, but it depends on a lot.

Lots of things come immediately to mind:  Septic problems, a new roof, HVAC failure, lower-than-projected rents. 

and most of the time is  bad management, bought the wrong price, Interest and Insurance to the roof and, I cant get out the Bridge loan to do a DSCR loan etc ...Simple hands on, work do research and minimize risk. 

Outside of real estate I am in the construction industry, in Commercial / Residential Roofing.

Rent paid for all of the expenses, so it cost me nothing and over the years (I kept it for ~20 years) price almost tripled by the time I sold it.Not sure what grad program is 3 years (I'm used to 2 for masters, 4 for PhD), but if you do need to sell one, I'd look at each property and see which one has the most equity, highest interest rates, lowest return, major expenses in the near future (eg new roof) and you mentioned you'd lived in two for 2 of the last 5 years, so have capital gains exemptions. 

5) Why was new roof NEEDED?

In these instances, you'd have to check things like flooring, plumbing for leaks, roof for leaks, hot water heater, and heating and air conditioning units. 

It is regional, but I find some of the newer homes where I am have more open living areas, but the bedrooms are smaller as are some of the lot sizes.To get a new mortgage, your rates are likely going to go up, but if the new home was well built, you shouldn't have to worry about things like new windows or a new roof for a while.