I think it's a cement board. I smashed a corner tile and scraped off grout. You can see a screw from where it seems attached to the subfloor.

Hallett & Co,

I believe that what you have labeled ”Vinyl? Tarpaper?” is actually the air conditioning return ductwork that is covered with duct mastic (or a whole lotta dust.) The OP stated that he took the pic from an air return. The duct boot should extend all the way to the surface of the finished floor, but at least they used mastic to seal the seams!

A,

Thats correct. Any post about this house should come with a disclaimer that A LOT of little things are just not right 😅

I would only take it down to the osb and if in good shape work from there but you will not know till you get there which btw is a PITA. Rent a pneumatic tile scraper-worth every cent and even with that it is a dirty, tiring, long job.

Hardiebacker is NOT appearing to be bonded via thinset. Must come out.

If they screwed it down with proper fasteners and schedule which in your "Corner pic" it doesnt appear so. all needs to come out.

Caveat' being its going to tear up the osb unless you can YNSCREW each fastener. Last i checked there is only 2 OSB brands rated for a tile floor assembly and the TCNA doesnt spec it period yet the MFG of advantech and durastrand does.

If i were to do the job and wrrty it. likely id be removing it all down to the Joist, TJI, or trusses and replacing with PROPER TCNA approved EXP1 B/C plywood at 3/4" thick and then adding a membrane dependant on tile selection and deflection rating. Youll also need either T&G or every seam crossblocked.

Thats how its doe when done to industry approved spec.which is set by ANSI/TCNA not osb MFG.

Do it right do it once IMHO.