This old house was built in the 1940s and is located in NorthCentral PA. The subfloor beneath the floor tile and (I guess) leveling material appears to be poured concrete. A 4" test hole produced no wood shavings. Was that practiced in those days?
What are you trying to do?
There could be a subfloor deeper than 4". But there's no way to know, unless you open the floor.
Heck, we weren't even born then. Maybe A. Spruce was.
What you have is a classic 'mud' floor made from drypack, which is a 4:1 to 5:1 ratio of sqand to cement. (no lime)
When you go to remove the old cement tile base, you'll probably find the joists were hand chiseled into a pyramid shape. You'll also probably find the joists are undersized for tiling. Take a few pictures and measurements before heading over to the John Bridge Tile Forum where you can find an extensive library, get speedy answers and post pictures of your project.
Assuming you're over a raised foundation, I've seen tile mortar beds as deep as 3" over 2x6 T&G subfloor.
If you're not on a raised foundation, then you would indeed be digging up the slab upon which the house is built.
Its probably a raised mud bed "wet set" installation method. It was common practice to do that even in the mid to late 1970's. I was an apprentice carpenter during that period and seen it done many,many times. Back then all the tile setter's walked with a hunch back and had bad knees from knelling/bending all day.
GeeGuss the floor joist needed to be 10 to 12" on center with mud set tile. Sometimes even doubled up forcing the mechanical trades to hang the supplies from floor joist then box in with soffits.
I remember WonderBoard & Thinset came out around 1977 and the WET SET tile method dwindled away and so did floor deflection in time.
Does anyone know if Al Gore invented these products & method to... :rolleyes: