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.