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!!

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.

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 August 10, 2020

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

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