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!
No, they're both supported by the floor joists, which might be sitting on a poured concrete wall that runs underneath the bathroom. It's a little hard to see exactly what's going on under the floor, visibility isn't great under the bathroom from the basement. Filling in the plywood section with mortar would require building out some sort of form to keep it from running into the basement.
The mortar seems pretty robust. The only way I'd be able to get it out would probably be the rotary hammer or a big sledge hammer or something. Ideally I'd like to just put SLC down as-is, I just want to make sure the cracks and divots won't cause any problems with that. The floor transitions to hardwood, and currently the bathroom subfloor sits about a 1/2" lower than the hardwood, so I'm hoping that's enough to fit a layer of SLC, ditra, and tile. I would like it to be flush, but I'm not super concerned if the bathroom floor ends up a tad taller.
I appreciate the feedback though, thank you!