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This was what was under the tile in my bathroom. Probably from the 50s. I think it is thick mortar? It is crumbly in places. I don't want to demo the entire thing, I'd have to hire someone to do it. Can I patch it and tile over it?

I'd love it if I could confidently identify what this even is. I haven't found too many photos online of people sharing a similar situation.

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tub

Sarah
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If the cement floor is constructed the way I seen many original floors of homes your age the joists of the floor are beveled at the top 2" and there are ledgers on the sides of the joists, pieces of wood that hold other pieces of wood to fill in between the joists that create a surface to lay cement between the joists.

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Typically where the joists are is where the cracks occur. The only way to keep the cracks from coming back is to remove all the cement, and depending on a few factors like thickness and depth of the joists, the span the joist make, you may be able to flatten the tops of the joists by 3/4" to add plywood over the tops of the flattened joists, but if the joists can't be reduced in depth to flatten the tops, then the joists need to be "sistered" to accept the new subfloor.

Either way it is not a simple patch and go on from there.

Jack
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  • That's helpful context, thank you. I havent seen any wood yet but this is an apartment building. I am most concerned about going too deep and ruining anything structural. I'll see if I can get another photo to show depth of this top layer. – Sarah Dec 07 '23 at 18:03
  • The beveled tops should be somewhere close to the cracks, usually about 1/2" below the surface. You will not hit anything structural while chipping away at the mud bed. It should be about 2" thick, except at the joists. Now if for some reason, the joists are lowered at the mud bed, it will be only the concrete everywhere, and the crack may be formed by a drain line that is in the bed, seen that too. It will be a very thick metal, hard to damage, unless it has issues, then it needs to be replaced anyway. Better to find that out now rather than later.... the hard way.... – Jack Dec 08 '23 at 02:55