The roof is fine until it leaks, decks are fine until they are not, things wear out, line blockages don't happen overnight they most often are buildups over time until the result IS seen overnight. 

They know the tenant base, median rents of different areas, how much to increase rent each year, ranges of repair costs (ex: tenant calls and says AC isn't working, no hot water, faucet is leaking, the HVAC system goes out).

Thank you Ned, that is exactly right, we actually check a condo/townhouse about 2 weeks ago, the thing was a condo with a basement, I cannot tell if it was a condo or townhome, anyway, the ceiling in the first floor was bowing, the 2nd floor, one of the bedroom was sinking, could not tell how this damage was done but the walls came apart from the floor, you could see the roof of the unit bowing inwards towards the structure a bit, and their "general contractor" estimate that for 5k they could straighten the property and level everything, even 1 window and 1 door looked so uneven that you would think  there was something wrong with your eyes...5k for structural damage, good luck with that one, we ran as far as we could from that....but we have seen they do inflate their ARV, since we learned the hard way , we go through these deals with a magnifying glass, I knew I was not so picky, numbers do not lie.Another thing that caught my attention, which I do not know if I am overly suspicious now that I am older, but they want to get the closing for us at the same time/day they are getting the property from whoever they buying it, which it seems to me they try to save on the closing by having a "dual" closing when in reality is just 1 closing and we are the one paying for it...I do not know there are few things that do not click right with this folks.

When purchasing, things that will amplify rates are roofs that have less than 10 years life, old water heaters, flood zones etc.

It is already leased and cashflowing, all new major systems except the roof which is 4 years old.

He earned quick money as a wholesaler, and I have a house to keep)Rehab cost: $20k (vinyl floor, interior paint, new dual pane windows, new toilets and bathroom vanities, lights, new landscape, new AC condenser, all done by handyman, spent some extra money for minor plumbing and electric jobs, hydro jet sewer lines, clean up attic (had mice), added some insulationTime spent: 3 weeks (two handymen for the first two weeks, one for the last week)Value after repair: probably around $440kJobs still need to be done later: some dryrots, roof work (it's tile roof, roof doctor quoted me $9k, handyman said probably $5k, home inspector said $15k you can get a new shingle roof.

They are the oldest properties, in lower class neighborhoods, sometimes without tile roofs or stucco siding. 

(Obviously in Vegas we have zero siding or roof expenses so your mileage may vary.)Good luck. 

It seems as if the options for owner-occupiers of three-unit properties are pretty limited, especially if the building has a flat roof. 

The hired leak detection company states there is no leak, the roof has been replaced as a result, and the crawl space has also been encapsulated.