I had to have my old three-handle tub faucet replaced by a plumber, because the valves in the handles were rotted into the handle bodies. When the tile came off, we found that the wall behind the tile was not backerboard, but the old-style cement laid on a wire mesh. So, the plumber had to punch a hole in the cement to get to the pipes. Coming in from behind the wall was, unfortunately, impractical because there's a double sink in another bathroom on the wall behind the shower, and moving that out of the way would have been even more trouble. So now my new faucet is located in about a 16" by 14" (four tiles wide by about three and one half tiles high) hole surrounded by cement, which I have to suspend tile over.

The plumber did a pretty good job of making a square hole in the cement, although it's not possible to make a perfect square when chiseling cement, and he installed backerboard into the hole for me. So now I've got backerboard cut and installed, but there are smallish (about 1/4") gaps where the cement isn't perfectly square with the backerboard. The backerboard comes out to almost the right depth, although it looks like the new tiling is going to be sticking out a little bit compared to the original tile.

I'm pretty comfortable trying to install the tile myself, I'm mostly just worried about the gaps between the backerboard and the cement, and how to deal with tiles that will stick out a little bit from the rest of the wall.

The faucet, and the hole, is located directly on top of the tub.

So, my questions:

Will thin-set mortar suitably fill in the gaps between the cement and backerboard?

Is there anything special I need to do to prevent leakage on the transition between the new tiles and old tiles, where the new tiles are going to stick out about 1/8" from the old tiles, or can I just apply grout liberally?

I've placed a vapor barrier (3 mil plastic sheeting) between the first and second layers of backerboard. I tucked the plastic sheeting around the rear piece and poked the sheeting into the gaps between the backerboard and cement around the sides and top of the backerboard. At the bottom where the hole meets the tub body, I've let the plastic sheeting lay over the curved "drip edge" of the tub, so any condensation that might form between the cement board and sheeting should drip back into the tub. Should I do something different with the sheeting before I put mortar into the gaps?

I'm really hoping that I don't have to take down the whole side of the wall where the faucet is installed just to make everything uniform. Honestly, I'm okay with a solution that would drop tiles in 5 years time (but not a solution that would leak into the wall, obviously), I just don't have the budget, time or experience right now to undertake a "big" project. Alternative solutions that fit in my constraints are also welcome, e.g. perhaps I could get some more wire mesh to put cement on to more closely match the original wall?

Absent any more answers, I'm going to proceed with plan "A" - use plain thin-set mortar to fill the gaps and let the new tiles be proud and just use extra grout on the raised edge.

  • How did your project turn out? I'm having the same problem. Thanks!
    – Anna
    Commented Jan 23, 2019 at 1:54
  • @Anna, Still holding up 8 years later. I used two thin layers of Hardibacker with a thin plastic vapor barrier placed between them, which "almost" lined up the new tiles with the old tiles, then used modified thinset to attach the tiles. The "new" 8 year old tiles do stick out a little bit from the surrounding tiles, but it is holding well. Good luck!
    – Sam Skuce
    Commented Jan 23, 2019 at 13:58

2 Answers 2


I would probably use unmodified thin set to cover the gaps and give a good base for applying the tile.

Could you do something a little decorative and use a border tile around the repaired area, sort of like a picture frame? Then use a different tile to fill in the area. The border would make the change in depth harder to see.

  • We are going to do our best to make it look good on the transition. The original tile was over 30 yrs old anyway, so we never would have got a perfect match, therefore we were always going to have deal with the aesthetic transition between old and new tiles, regardless of any difference in depth. My primary concern is just if having the new tiles proud of the old tiles makes it more prone to leakage, etc.
    – Sam Skuce
    Commented Dec 4, 2010 at 4:24
  • The backerboard I'm putting the tile on is Hardibacker cement board. The manufacturer specs[jameshardie.com/homeowner/pdf/backer-install-us.pdf] say that I should use modified thinset to apply the tile. Is there a reason you recommended unmodified thinset? From what I understand unmodified is generally used in 'airtight' spaces where the moisture won't escape while the mortar cures.
    – Sam Skuce
    Commented Dec 7, 2010 at 0:04
  • I read just the opposite that modified is used in 'airtight' locations, like under Ditra or some other impermeable barrier. However, I would always follow the manufacturer's recommendation.
    – ChrisP
    Commented Dec 7, 2010 at 0:28
  • Accepted with the caveat that I'm following the manufacturer's recommendations and using modified thinset.
    – Sam Skuce
    Commented Dec 10, 2010 at 18:11

For the gap, I would use tape it with a mesh (fiberglass tape) then fill the gap with mortar. Unmodified thinset is better since it is strickly a filler, but modified thinset would work as well. This is what we do to join 2 cement boards to prepare for tile installation.

For making the tiles flush, I would take a grinder with a masonry disk and remove a bit of the cement board or the back of your tiles.

  • This is a good suggestion. I never thought about grinding down the Hardibacker board to make it flush, nor using mesh tape to cover the gaps before applying mortar. Luckily, the mortar did fill in the gaps just fine even without the extra base, and the tiles are still securely attached 8 years later even though they do stick out a little bit from the rest of the tiles.
    – Sam Skuce
    Commented Jan 23, 2019 at 17:11
  • oh i didn't realize this was an old question. Not sure why it shows up in my feed 🤔
    – Quoc Vu
    Commented Jan 24, 2019 at 4:29

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