Image of floor

I am trying to extend this penny tile floor. The existing penny tile floor is concrete + tile + penny tile on top. The new addition I have added is osb+plywood+cement board (as pictured on top).

I would like the bridge the gap between the cement board and existing addition.

My current plan is to fill the gap with tile thinset and cover with cement board tape. Not sure if this is the right way to go.

As far as the edge cracks go in the cement board picture, I am going to replace the cement board and drill pilot holes for the screws. I'm a beginner and luckily have more cement board! Oops.

  • 2
    Stay a bit further from the edges with the screws on the replacement cementboard. Also, bed the replacement in thinset - osb+plywood+thinset+cement board. And go read this answer.. diy.stackexchange.com/a/52675/18078
    – Ecnerwal
    Dec 5, 2022 at 3:52
  • Thank you. It's a tight fit, I'm not sure there is room for thinset unless i use a thinner piece of plywood or cement board. In my reading on this site, it seemed like the thinset under the cement board is not a hard requirement and only used for leveling purposes. Am I making a mistake?
    – byte
    Dec 5, 2022 at 3:55
  • 1
    If there's no gap, you're good either way. It's a small area, so it may be flat enough.
    – Ecnerwal
    Dec 5, 2022 at 3:59
  • "thinset under the cement board is not a hard requirement" - it is if you want it to never crack. But this will crack because of the shear plane under your toes, before that is ever the issue, so w/e. And you used isolation membrane (eventually? What's with the red stuff, but there's none on the floor?) so it's about as good as it's going to get, +1.
    – Mazura
    Dec 6, 2022 at 17:15
  • I ended up using thinset under the cement board when I redid it! It was helpful to get a feel for laying down thinset.
    – byte
    Dec 6, 2022 at 17:31

1 Answer 1


I ended up filling the gap with tile thinset and cement board tape.

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