I'm currently working on renovating the bathroom in our 1958 home. I just finished tearing out the shoddy job some flippers did a few years back, and found some issues with the floor that have got me scratching my head. I've learned that it was common practice back in the 50's to have a mortar slab subfloor, and it looks like that's what I have in our bathroom (except where the old bathtub was, where there was just 2 layers of 5/8" plywood). The tile was adhered directly to the slab with no decoupling membrane, and the only way I could get it and the thinset off was with a rotary hammer. I'm afraid that might have been too aggressive though, because now there's a bunch of cracks in the subfloor. I'm not certain if the cracks go all the way through because the bottom/sides of the slab are hidden. But I'm also not certain the actual original mortar slab is cracked, or if the flippers put a super thick layer or two of leveling compound down, and that's what's cracked, because in one area where I accidentally made a deeper gouge, it looks like there's about an inch of smooth grey mortar-like substance at the surface, but beneath that is a lighter-colored concrete-like substance with an aggregate mixed into it.
I'd like to get some more opinions on what's actually going on here, and what my next steps should be. Can I put leveling compound down as-is, do I need to patch/fill the cracks, do I need to fully chip away this smooth, grey top layer, or do I need to tear out the slab completely and lay a new plywood subfloor?
Imgur link for reference: https://imgur.com/a/xF7go4z
I can add more photos if needed. Thanks in advance!
Honestly, that doesn't look too bad. I'd say the plywood is filling in an area of concrete that was removed before. The smooth mortar stuff is self-leveling compound, the gray stuff is concrete. Is there any movement in the floor, and if you're planning on putting down tile, then movement is the enemy. In which case, I'd say tear the concrete out and rebuild the rest of the floor with plywood, screwed to the joists that are presumably inspected and reinforced if needed.
If you don't have time or money to do that, this could be used as is with some repair of the holes. Those cracks aren't really a show stopper, and depending on the flooring you plan to use, you might not need to do anything about them. The concrete debris in the walls annoys the shit out of me, though. And I'd fix whatever you've got going on over by the toilet, that looks grim.