Safe to put tiles on top of two 3/4" fir board layers & Ditra-Heat?

Question

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!!

Vincent · Answer

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.

Answered by Vincent on December 8, 2021

Quoc Vu · Answer

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.

Safe to put tiles on top of two 3/4" fir board layers & Ditra-Heat?

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