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I have a 4x4' hole (or recess) in my hardwood floor.

The bottom of the recess is 1/2" plywood instead of the recommended 3/4", because that's what would allow for the final tile installation to be flush with the hardwood.

To make up for the missing thickness of the plywood, I installed far more 2x4s across the 2x8 joists (it's a cabin) than I would have thought necessary. There's also 2x4s supporting any seams in the plywood. The 2x4s are hung using joist hangers. The floor joists are 16" apart on center. The supporting 2x4s are more like 8" apart. There's also five deck blocks and beams supporting the whole thing. The 1/2" plywood is screwed down to excess.

There is no perceptible give when walking anywhere on the surface. However, if I fill a large bowl as close to spilling as possible (only surface tension preventing it from spilling over), and I walk around it, I see a small amount of water flowing down the side of the bowl.

I had a fairly heavy dude jump on a piece of tile, straight on the plywood. It took a few tries before it cracked in the middle. It was placed on the suspected weakest spot of the whole thing, where the water overflow occurred in the bowl.

However, I'm a little worried about the fact that we'll be placing a very heavy wood stove in the center of it. There's a joist in the center, but still, two grown men will have to lift it into place, standing on the tiled area.

The tile is 1cm thick granite.

The end goal is to have the tile and hardwood be flush. The plywood subfloor is neither level nor flat. The hardwood floor is also not level, but there's differences in depth throughout the recess in the floor.

My current plan is to use concrete patch directly on the 1/2" plywood to make it flat. The deep areas will have in excess of 1/4" concrete, and the high parts will have zero concrete, as they're at the exact right depth to accommodate all the layers of thinset, concrete board, more thinset and tile.

The tile area is surrounded by four sides of hardwood. One of the sides partly consists of a chimney. Picture:

square recess in hardwood floor

My question is multi-faceted:

  1. Do I need any sort of reinforcing lath in the deeper parts of the floor patch?
  2. Can I put thinset right on top of the smooth concrete floor patch I just placed in order to flatten the area?
  3. Is this even a good idea? Should I use self-levelling concrete first? Do I need to screw in plywood pieces like a puzzle piece (due to the 2x4s going across the joists to support it) from underneath to minimize deflection?

Notice the use of the word "flatten" instead of "level": The floor is not level, so the 4x4' patch of tile should also not be level, but flush with the surrounding hardwood floor (which is not level).

Edit: A rough diagram (in Paint) of the plan (notice how the concrete patch is thicker on the sides than in the center):

Diagram of floor plan

2 Answers 2

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Assuming that the floor joists run perpendicular to the hardwood, you spanned your plywood in the wrong direction. Plywood has a "strength axis" that runs parallel to the grain in the outermost plies. This strength axis should have run parallel to the joists to exploit your 8" 2×4 spacing.

VersaBond LFT thinset, for instance, allows thinset thickness up to 3/4". Find an LFT thinset with a sufficient max thickness spec to eliminate the floor patch under your cement board. If your plywood isn't exterior grade, then prime the floor first to minimize water exposure. Install the cement board over a minimal thickness bedding layer of thinset with screws on 8" centers (be sure to use alkali resistant screws). The minimal thickness bedding layer will further minimize your plywood's exposure to water.

For on top of the cement board, as long as the LFT thinset's overall thickness doesn't exceed the maximum thickness spec, you're okay. You can therefore apply an initial shim layer and let it cure before applying a final layer of thinset along with your tiles. I expect that this "shim layer" step should make the final tile installation much easier. This step also has the added benefit of not allowing any thinset shrinkage of the shim layer to alter the even-ness of the tile and surrounding hardwood. LFT thinset is formulated to minimize shrinkage as long as you get the water ratio correct, so I would forgo the initial shim layer to save time.

Natural stone tile like your granite calls for back buttering anyway, so back butter and use a square notch trowel sized appropriately for your tile to apply notched thinset to the backs of the tiles. This way there's no need for notched thinset to complicate your thick layer of LFT thinset on top of the cement board.

Use a straight edge spanning from hardwood to hardwood to get your tile surface level to the adjacent floor surfaces.

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    @Audiopolis, my most recent edits address your questions. The shim layer is intended to simplify installation. It goes on top of the cement board, allowing you to trowel it into parallel with the hardwood surface. This way, uniformly applied thinset in the next layer will naturally make the tile surface parallel with the hardwood surface. Further, if the thinset's water ratio is off, then its shrinking will cause merely a sunken tile surface instead of an out-of-plane and sunken tile surface.
    – popham
    Dec 22, 2023 at 19:47
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    @Audiopolis, the TCNA Handbook recommends back buttering stone tile anyway, so back butter your tiles and apply the notched thinset on the back of the tiles. This way you don't need to worry about notching the thinset on the floor. Trowel the floor's thinset flat and at the correct depth.
    – popham
    Dec 22, 2023 at 19:53
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    @Audiopolis, the straight edge is for pressing tile down into the thinset with the top surface aligned to the hardwood surface. The straight edge should work better than other techniques like measuring tape plus laser. Using a straight edge also to plane the thinset isn't a terrible idea. My instinct is to lay the straight edge across the hardwood and use a combination square set to the correct offset to gauge down to the thinset surface. I would repeatedly mark depths with the combination square followed by trowelling until the combination square just barely skates across the thinset surface.
    – popham
    Dec 22, 2023 at 20:01
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    @Audipolis, and you bed the cement board in the thinset while it is still wet. The bedding layer should be thick enough that the 8" on center screws pull the cement board down for something like 80% coverage between it and the plywood.
    – popham
    Dec 22, 2023 at 20:05
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    @Audiopolis, depending on your plywood's span rating, I might not tear it up. 8" is a teeny tiny span, even for incorrectly oriented plywood. The weak axis stiffness is about 1/5 the strong axis stiffness. For uniformly distributed loads, I compute that your 8" spans are deflection equivalent to 13.7" spans of correctly oriented plywood. 8"×5^(1/3) after a bit of algebra.
    – popham
    Dec 22, 2023 at 20:46
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You can install concrete over plywood if you use a fortified Portland cement like Ardex or Webcrete. You must use a primer on raw plywood.

Also make sure the plywood is sound and secure, as excessive movement will cause new concrete to crack or break loose.

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  • "Also make ssd's..." That looks like a very "helpful" phone autocowreck. Care to edit to clarify that?
    – FreeMan
    Dec 20, 2023 at 18:14

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