Roof repair life span

[Trey Moore]

Some background: I have been looking at a duplex in north Florida that would be my first venture into REI, I have never owned real estate before so my experience has been reading, more reading, the podcast, and talking to non-investors that are long-time home owners.

The property claims to have a "10-yr old roof" - but after looking at the property, it appears they just put an additional layer of (asphalt) shingles on top of the ones that were already on there.  As far as I know, there are no leaks or issues with the roof currently.  I have a few questions about this and roof repairs in general:

1. Calling the roof "10 yrs old" feels dishonest to me, is simply layering over existing roofing enough to call the roof "new"?  I was under the impression you really needed to strip it down to the wood, replace boards as needed, and re-roof the whole thing.
2. I totally understand it's cheaper/easier to just layer over, but is this normal/standard/acceptable?
3. How do others typically plan for these big capex expenses? More specifically:
   1. How do you estimate the time remaining on the roof? 
   2. Should I seek out that roofer that replaced the roof last and ask them what the lifetime is? 
   3. How can I come up with an accurate estimate of the remaining lifetime and how do I know when it's time to replace it? 
   4. Do you deduct the cost (e.g. $15k for a roof in 8 years) from the cash flow over the next eight years?
   5. Can a roof be replaced with tenants in the multifamily?