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There’s wooden subfloor in some areas and 3-4 in thick concrete with penny tile. I’m at a loss as these gaps are quite large and I don’t have much experience yet.


We plan on pouring self leveler over the subfloor but obviously need to fill these gaps/holes before doing so.

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    Please use the edit link underneath your post if you want to update it with additional information. Thanks and welcome to the site!
    – Niall C.
    Commented Oct 11, 2023 at 15:34
  • What type and size of tile do you plan on putting down over this? That is an important question and will change the answers Commented Oct 11, 2023 at 15:44
  • We’re planning on using penny tile
    – David
    Commented Oct 11, 2023 at 15:56

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You can use anything that will seal the gaps: foam, 1/8" ply, or even cardboard glued with silicone. Whatever holds the self-leveler from seeping through. This is assuming that you will add 1/4" cement backer on top of the self-leveler.

If you plan on installing small tiles, you need a surface that will not give in. Rip the wood floor and build a strong structure for the subfloor. This will include rim joists on which the subfloor edges will rest.

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  • Ok so foam will work? Everything I’ve seen online says to add more plywood. We’ll do that if that’s the best option.
    – David
    Commented Oct 11, 2023 at 15:56
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    @David The "best" option is to remove the wood subfloor, attach a rim joist to the cement, then add a new subfloor, and perhaps a thin layer of 1/8" ply. The cheapest/quickest option is to use foam or anything to seal it, then make sure to add cement boards, and to use large tiles on top.
    – Cheery
    Commented Oct 11, 2023 at 16:01
  • So it’s not possible to use penny tile?
    – David
    Commented Oct 11, 2023 at 16:38
  • @David if you plan on installing small tiles, you need a surface that will not give in. Rip the wood floor and build a strong structure for the subfloor
    – Cheery
    Commented Oct 11, 2023 at 16:49
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The space under that yellow stool's leg is too big to paper over. Cut a chunk of your plywood big enough to fasten to three sides of the hole and partially install a fastener sticking out of it as a handle. You're going to sneak this plywood chunk through the hole, lifting it by the handle to bear against the underside of the floor. As you hold it in place from above, screw around the edges every 3" to 4". You'll want to cut a filler piece to fasten on top of this lower piece (after removing the handle, of course).

The other gaps can be bridged with 1/8" plywood bearing on top of the joints. This 1/8" plywood should bear no more than 1/2" on the concrete side of the joint if you don't fasten it to the concrete. Be sure to fasten it to the plywood on the wood floor side.

Note that the right way to do that transition would be to physically attach the wood framing to the concrete. Assuming a jagged concrete break, I would have used hydraulic cement to make flat and plumb 4" tall by 8" wide bearing areas spaced every 16 inches along the side of the slab. I would have used wedge anchors (and a rotary hammer) to install 16" lengths of pressure treated 2x4 between each of these landing pads. The wood floor framing should be attached to a solid ledge like this.

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