I am planning to retile my fireplace hearth extension. As I was removing the old tile, the front edge of the concrete bed crumbled away. I presume this was because it was much thinner than the remaining concrete (~1"), and because there was no mesh lathe holding it together. The remaining concrete appears to be ~3 inches thick, based on what I can see from the basement. The remaining concrete is recessed from the wood floor by 3/4", and the tile I plan to use is 3/8" thick.

How should I patch this missing section of concrete so that it is a strong bed for the tile to sit? I need to add about 3/4" to bring it to the same height as the rest of the concrete.

My initial thought was to either use cement board, or float a bed of mortar up to the same level as the existing concrete, and then secure a 1/4" cement board with tapcon concrete screws and thinset to make a uniform surface. This will leave 1/2" left for the thinset and tile. Research has shown that that might not be adequate and that it isn't recommended to use concrete screws on cement board, but I can't tell if that is just because it is tedious to drill all the pilot holes (it's a small area, so I'm not concerned with that) or if there is another reason not to do that.

Any suggestions is greatly appreciated.enter image description here

  • I believe the concrete has deteriorated, suggest just replace with new cement board. – r13 Mar 16 at 21:40
  • Why not just use backer screws they drill right through the concrete and sink into the backer if you want to overlay. I would thin set them together old concrete and mortar is very brittle. Test a couple and see if the surface crumbles if it crumbles go nuts on it with a hammer put the pieces in the garbage and then use 1/2” or thicker backer with no worries. – Ed Beal Mar 16 at 22:23

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