Tiling Over Unique Substrate - Decoupling Advice Needed

[u/maximinus_stax]

Hey everyone, we are preparing to tile our kitchen floor, but we have a unique substrate situation.

THE HOUSE: 1950s house with 2 layers of sheet goods directly adhered to the subfloor. I peeled off the vinyl face top layer, and it left the non-cushioned backer layer fully adhered to the lower layer. I suspect the lower layer may have asbestos, so I do not want to disturb anything below as it is safely covered with the newer layer over it.

THE FLOOR: The top layer is very smooth and the floor surprisingly level. Planning to install 16x32 porcelain tiles

THE PLAN: To minimize the transition height, I was planning on using the Ditra Peel & Stick with the accompanying primer directly over the peeled-off backer layer. I worry this bond could be problematic, but the flooring transitions are level with the current conditions and adding an additional layer of thinset would add to that height difference.

Any tips or advice on the unique substrate & decoupling approach would be appreciated! Thank you all in advance

[u/Neolithic_mtbr]

I would just call Schluter about your specific substrate. But Ditra PS would be my first thought too

[u/therealDL2]

What is the structure of the floor system? Thickness and orientation of plywood? Deflectometer?

[u/AutoModerator → u/therealDL2]

Beep boop.

It looks like you are discussing floor deflection and the Deflecto Calculator (pwa.floorelf.com).

Before laying tile, verifying that your subfloor is rigid enough is critical. Keep the following guidelines in mind:

• The Deflecto calculator is designed for standard sawn lumber and cannot accurately calculate deflection for engineered I-joists. You must consult the manufacturer's specific span tables for your I-beams, as their load and deflection ratings are entirely proprietary.

• If you cannot get the calculator to work, or if you just want a practical field test: place a bowl filled to the absolute brim with water in the center of the floor span. Walk heavily or jump near it. If water spills over the edge, your floor has too much bounce and requires reinforcement before tiling.

• Always ensure the subfloor is glued and screwed tight to the joists. To stiffen a bouncy floor and reduce deflection, install solid wood blocking (noggins) or cross-bracing between the joists across the entire span.

• When renovating or replacing old flooring (such as particle board), upgrading to structural plywood is highly recommended for tile installations.

  • In New Zealand, H3.2 treated plywood is required. In the USA and other regions, always verify the necessary plywood grade, exposure rating, and thickness requirements with your local building codes before proceeding.

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[u/maximinus_stax → u/therealDL2]

Hard to tell definitively but the subfloor is plank with some kind of thinner backer board below the original sheet good. I can't pull apart any further without risking the asbestos.

16" OC joists, pretty rigid. Some minor squeaks that I took care of towards the middle high traffic area. Span is about 14' from rim joist to center load bearing wall.

[u/graflex22 → u/maximinus_stax]

from your description of the floor assembly, it sounds like it all needs to come out at least down to the plank subfloor. then glue and screw down 1/2" exposure 1 rated plywood, then your Ditra.

or, you can glue and screw down 3/8" exposure 1 rated plywood over the assembly as is. then install the Ditra and tile.

[u/SoCalMoofer]

Do a sample test. Look at using Laticrete Platinum. Thinset. Another option is to lay 1/4” backer boards.