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106 year old house - on top of the joists (16" on center) are wide 3/4" thick boards at a 45 degree angle, then 3/5 tongue and groove on top of that. Putting 3" marble hex down and plan to use a 1/2" thick Ditra-Heat uncoupling membrane.

Is that enough support under the tile or do I need to add hardibacker or plywood too? I am trying to minimize height, it's already above the wood floor in the hallway.

Thanks!!

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It does not sound like your tongue and groove flooring is a recommended subtrate to attach the DITRA-HEAT membrane based on the data sheet and detailed installation instructions. Hardiebacker, per instructions, needs to be attached to 5/8" exterior plywood. DITRA-HEAT can be attached directly to plywood or OSB so removing the T&G flooring and adding new 1/2" plywood sounds like the way to go. Schluter can ultimately answer detailed questions about your application.

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if you install over wood, you will eventually have cracked tiles and/or grout. it is not a stable substrate.

If height is an issue, I would take out the tongue and groove flooring, and install the cement board on top on the 3/4" subfloor.

For floor, i use 1/2" cement board.

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    I think this answer was downvoted because it doesn't appear to take the membrane into consideration. That's its purpose. Without that, I'd agree.
    – isherwood
    Feb 26, 2019 at 20:46
  • sorry i meant 1/2 cement board for floor vs the thiner 1/4 which i use for was.
    – Quoc Vu
    Mar 1, 2019 at 6:45
  • @isherwood the membrane would helps with expansion/contration but my concern is the floor flexing due to heavy foot traffic. I think it is fine on vertical application. i have seen several instances of the membrane disconnecting from the substrate. when it comes to tiling half of the job is to prepare the substate.
    – Quoc Vu
    Mar 1, 2019 at 7:02
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    OP was not specific on how the T&G was installed respectively to the boards below it, thus you could be right and that would be the best case scenario. But I feel in any case appling a heating membrane over wood is asking for problems. Wood is not dimensionally stable and having a heated floor amplifies the temperature swing leading to more acute dimension changes. Also, it is unclear what is the T&G material, but I suppose a house of that age, that would be solid wood and that Ditra-heat will quickly dry out the wood.
    – Quoc Vu
    Mar 1, 2019 at 17:34
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    Layers of wood were almost never installed in alignment. In the case of hardwood flooring, the t&g or shiplap sufloor was often installed at a 45 degree bias.
    – isherwood
    Mar 1, 2019 at 17:36

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