2nd floor bath remodel. Concrete under tile. What do I have to remove?
Sounds like a standard old school &quot;float&quot; job: plywood, then (usually) tar paper, then expanded metal mesh (sometimes), then dry-pack concrete, then tile adhered with thin-set mortar. This was the standard way to get a strong flat substrate for tile before cement-board was common.
If you can get the tiles off with the cement still relatively flat and smooth (or if the cement can be made relatively flat and smooth) then it is perfectly fine to put new tile down on it... but it's a big if.