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I recently started renovating one of our bathrooms. After removing a few tiles there was a second layer of tile beneath. Below the second set was a concrete/mortar base that is ~1.5 inches thick and has broken in a few places to reveal sand inside.

According to my research this is an older style of base for laying tiles. The house is from the early 60s so that lines up. However I have not found much info beyond that. I am wondering whether I can put tile back down on top of it and how.

Additionally there are a few cracks where the top came off while removing tile and revealing the sand beneath. Most are small but one is larger. Would this require a complete demo of the mud base? Can I patch it with concrete or something similar? The big hole is by my kids step stool in bottom right, it is about 5-6 inches at it's longest.

Actual floor shot

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I'd just rip it all out, lay down some new floor sheathing then cement board on top of that. Slap your mortar down and start sticking tile. It might sound like a bit more work than just patching a few holes but you already have the floor ripped up may as well finish the modern approach.

From the pics, it's hard to see a 1.5" depth in that stuff. If the rest of the floor feels solid, I think you could get away with just patching it with your thinset to level it out if you really don't want to do the extra demo work. Given the thickness, maybe even smooth out the spots and let it sit a day or so before starting to lay the tile.

  • My first thought was to demo the mud floor but I am a bit concerned about the pipe work through it. The toilet return definitely is set in it and I'm not sure if anything else runs through it. My worry is damaging that and then having to rip the ceiling in the basement out as well to repair it. This is our long term house, maybe the effort is worth it? – James Apr 30 at 16:10
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it is probably old mud that was used back in the days, now we remove the old mud and build it back up with the concrete board to where you need it to be, screw it down and glue your tile down.

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The mud bed is usually a cement and sand mixture. if it is really Sandy and crumbling, I would recommend removing it. If it is mostly stable, you can fill it in with thin-set or grout. Although, if the hole is a half inch deep or more you will want to use a solid patch, cut to fit in the hole. Clean the hole, coat the bottom with thin-set, insert solid patch ( tile, cement board, etc.), Flash over with thinset, let dry before installing tile.

  • It does not seem crumbly and relatively good shape to what I have seen others repair online. Torn between taking it down and rebuilding and just fixing it. My other concern is that OSB on top of picture has a slant to it. It's just a thin strip that may not be that stable and crack the tile. – James Apr 30 at 16:36

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