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Old 03-09-2018, 11:49 AM   #1
Analise
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Low-Quality Subfloor Prep Question

Hi everyone! I'm new here... I did a search to see if I could find an answer to my question without bothering anyone, but I couldn't find my specific issue. Apparently no one has as bad of a subfloor as we have.

I am helping friends install large-scale wood-look porcelain plank tile into their kitchen (8x48). I've personally worked with large plank porcelain before and I realize how important a very flat subfloor is. Here's the problem.

After we pulled up the old linoleum we discovered that the 30s-era subfloor is of a very poor quality. It is only 1/4" ply laid on top of 6" planks (not tongue and groove) on 16" O.C. joists.

The correct thing to do would be to pull it all up and put down a proper 3/4 ply, but we were kind of hoping that we could get away with just fortifying the existing instead. Would an uncoupling membrane like Ditra be enough to handle any movement? Or should we lay down backerboard beneath the Ditra? OR... should we just bite the bullet and pull the ply up and replace with a proper 3/4?

Basically I'm hoping someone can tell me that there is a solution to our issue without having to pull it up and replace Partly because the transition to the dining room is very thin wood flooring and we are hoping to keep the two areas as close to each other in elevation as possible... we are already looking at quite a steep transition issue as it stands with the tile all by itself, much less adding another 1/2" of ply + mortar + tile.

Thank you,
Analise
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Last edited by Analise; 03-09-2018 at 12:03 PM.
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Old 03-10-2018, 08:07 AM   #2
RichVT
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I would pull everything down to the joists, flatten the joists and install new plywood. I wouldn't want anything of "very poor quality" anywhere in my tile assembly.
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Old 03-10-2018, 09:05 AM   #3
cx
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Welcome, Analise.

The 1/4" plywood must be removed, regardless what else you elect to do.

The tile industry standards require that you install a minimum of nominal 1/2" plywood over your board subfloor. That presumes the subfloor boards to be T&G style and in good condition. It's not uncommon for folks to install the plywood over good board subflooring without the T&G feature and you can try that if you like.

Best option would be what Rich has recommended above.

My opinion; worth price charged.
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Old 03-10-2018, 09:00 PM   #4
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Are the boards perpendicular to joists, or on a 45 degree angle?
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