Can I install Onyx mosaics in my Shower that don't have grout joints?


My shower stall has two sides each with 1/2″ USG ultralight foam board installed, which is waterproof. I plan on installing 2 rows of 12″ x 12″onyx mosaic tile in center of each wall with 12" x 24" porcelain on each side of the mosaic tiles. The mosaic tiles have a mesh backing and can be flexed open in either direction, and look like this:

I plan on installing 12" wide Schulter Ditra membrane on top of the foam board using Laticrete 254 Platinum thinset and installing the onyx on top of the Ditra also with the 254 Platinum. The reason for this is so the onyx will be flush with the porcelain on each side. The 254 is made for use in swimming pools.

Someone told me that because there is no room between the rows to put the grout that mold would form between/behind the onyx even if the onyx is sealed because sealers are not a miracle product, even with the 511 impregnator sealer I said I would be using. He also said onyx is sensitive and will be hard to clean, darken/etch/stain over time when exposed to water.

I am asking you to get a 2nd opinion - will the problems mentioned above happen to the onyx even if using a sealer such as 511 impregnator? If so, is there another sealer that would prevent those problems?


ANSWER - First of all what happen to the the 3rd wall in the shower?  I will assume it has a different substrate or you aren't tiling it.

Not sure why you are using Schluter Ditra on a wall.  It is a uncoupling membrane that is primarily used on floors.  Plus you already have a waterproof foam backer board if you install it to be waterproof.   If you are trying to adjust the level of the wall surface because the thickness of the mosaics is less than the porcelain tile by using it, it would be more practical and less prone to problems if you install the porcelain tile leaving out the mosaics, but leaving space for them.  Then skim coat that area with thin-set, which should not be more than 1/4" thick. Let it cure and the adhere the mosaics over it.

The stone and tile industry standards always say never butt tiles together due to potential expansion problems.  Although some imported mosaics come that way.  For an accent strip it might not be a problem.   Sealing the onyx will help make it easier to clean, but it won't keep debris or soap scum to get into the tight joints.  When you have organic material and moisture there is always the potential to get microbial growth if it isn't kept clean.   I don't see it as a problem as long as you keep it clean and maintain it.

4 thoughts on “Can I install Onyx mosaics in my Shower that don’t have grout joints?

  1. mike says:

    Yes, I’m using the Ditra on the wall so the mosaic tile will be even with the porcelain on each side.

    The mosaics tiles I’m using have fingers on each end that interlock so there will be no room for grout unless I separate them a little but not sure if much point in doing that because of the many other smaller pieces within each tile that cannot be grouted. However, I’ve considered flexing each row open and running a bead of matching colored silicone between each row/pressing together and wiping off excess.

    I am considering two options for installing the Ditra:

    1) Drill small holes in each square of the Ditra before applying to the wall, so if water gets in, there will be way for water to drain out at bottom (there will be 1/8 gap between last tile and ledge at bottom), and install orange side facing out. This should work if there are any voids in the thinset between the mosiac and the Ditra.

    2) Install the Ditra backwards since the white fiber backing is porous and if water gets behind the tiles it may be possible for any water can drain to bottom it there are any voids in the thinset.

    Which of two options is best?

    I was at tile store today and there is a new underlayment by Mapei called Mapeguard that’s same thickness as the Ditra. Have you heard of it and do you think it would be better to use than the Ditra behind the mosaic tiles?

    • Donato Pompo says:

      The way you are suggesting in not using the products as they were designed and intended to be used, then that makes what you are doing an experiment. With experiments they may or may not work well, and there are no guarantees.

      Our recommendations are always follow industry standards and the product manufacturers’ directions.

  2. Thomas E. Ward says:

    I’ve seen recommendations to use 5/8 backerboard/cement board when using onyx on floors. Does that mean even well designed, aged and mature concrete basement floors which should be stable after decades of aging (assuming not in earth quake zones).

    • Donato Pompo says:

      There is no practical reasons to install a backer board over a concrete substrate. Backer boards are meant for surfaces that don’t have the more substantial concrete or mortar substrate.

      If the concrete substrate is not flat or level or sloped the way you want it, by installing a backer board, then nothing has changed. Normally a concrete substrate not designed for a tile installation will be irregular. So to adjust it, all you need to do is scarify the concrete to remove any contaminates and open up the pores. Then apply a mortar bed or a patching mortar to adjust the surface to the intended flat finish.

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