I had an area of bathroom tile that had fallen lower than the rest of the floor. When I started to explore under the 67 year old tile, I found a masonry "sub floor" that is crumbling. Under the masonry is a wood sub floor. My question-- Should I remove all the old crumbling masonry and rebuild a sub floor to support my new tile floor? (The old floor has settled quite a bit- at least an inch in the corner.)picture of masonry filling

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    Since you have settling and the layer of cement or sand and mortar is crumbling you should remove the damaged base and start fresh. Your new tile job will last longer and any rot or just settling issues can be repaired. – Ed Beal Aug 22 '16 at 20:25
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    That's a perfectly standard mortar bed (thick bed) install, by the look of it. If the sub-floor is moving, I'd look further down than the mortar bed for the actual problem. It's a very good tile substrate even if many younger folks have never heard of it. – Ecnerwal Aug 22 '16 at 21:19
  • I agree you should remove, but can I also add that US home building baffles me sometimes. Is this upstairs or ground floor? – Tom Busby Feb 27 '17 at 11:15

You need to get rid of everything under the tile. The bed at one time was probably really firm but as it gets water it will start to break down and those sand bases really don't dry out well once they get wet.

If you need to add height just add plywood but top layer should be cement board.

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If you don't want to dig out all that concrete (or whatever it is), I'd consider putting in 3/4 plywood over it, then staple on wire mesh, and put the thinset on this and the tile. It will float the tile over the subfloor. I've seen this done several times successfully. If you want ot go high tech, get the Schsulter system plastic underlayment and use that instead. I've see that laid on to of sculped styrofoam in a contoured shower pan. I'd maybe put floor leveler concrete on your broken concrete to lock it all it before the Schuler, but not the 3/4 plywood.

Not sure why it broke like that - is it an unstable area, or was the concrete mix just poorly done?

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    Some of the masonry filling feels like sand, while other areas are much more solid like concrete. I think I'm going to remove all that old sand and concrete and rebuild. That way it will last several years. THANKS! – Wayne Aug 22 '16 at 21:12

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