New Flat Roof. Remove or cover current roof?

[Joe Gee]

Hello BP, 

I purchased a 4 plex recently and looking to replace the current flat roof.  The current roof is leaking in one area that the last owner patched up unsuccessfully.  The rest of the roof is holding up, but its pretty old and probably better to just put a new one on.  

My question is, do I rip off the old tar roof or just go over it with the new TPO roof.  My roofer said its better to just go over it, and if I rip it off it will cost a hell of a lot more money, not to mention open up a giant can of worms.  But if there is rotten wood underneath, I would want to deal with it now rather than cause more problems down the line.  

Has anyone dealt with a flat roof replacement?  And what do you recommend?  

I plan to keep this 4 plex in my portfolio as a long term buy and hold. 

Thanks