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I am redoing my second floor bathroom because of a leak under the tub which over many years has caused a leak downstairs. I thought it would be easy changing a rotten wooden subfloor… But when i started removing the old tiles, it was mortar or concrete? I cannot tell.. I attached a pic. Will this make the subfloor replacement harder or easier? Since its mortar or concrete can i just plug the hole with mortar and retile? Or does the whole subfloor need to be removed and filled with new mortar?

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  • When we renovated our 1939 upstairs bathroom the entire floor was laid on poured concrete a few inches thick. We had the same problem with a leaky tub ruining the ceiling below. Even some of the water pipes were embedded in it. The whole slab had to be removed. But we were doing a full renovation and expanded the bathroom about a foot.
    – tymish
    Mar 4 at 15:52
  • You've stumbled on the origin of the term "thin set" (mortar). It's the modern alternative to this conventional full mortar base.
    – isherwood
    Mar 4 at 15:59

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Presumably from the description it's a thick mortar bed method tile floor, typical for 1950s and earlier tile work.

You'll have to determine if the framing (likely wood, if the house is wood framed) holding it up (well below the floor level of the rest of the second floor) is rotten or not. That might be easiest from below, where you presumably need to fix the damaged ceiling anyway.

Despite being outdated, it's a good tile support - out of style these days since newer methods are faster and don't require the vastly different floor level at framing time. If what's holding it up is not rotten, patching with mortar should be all that's needed. Tile floors that have lasted 100+ years were typically put in this way.

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  • +1 to ‘check the framing for rot’. Open up a big section of the ceiling below to fully understand that scope of the problem. Mar 3 at 19:02

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