I am trying to calculate what trowel size (fast set thickness) I need in order to obtain a flush level between the tiles and the green marked area in the diagram below. Here is some context first:

I am not planning to tile to the ceiling as I have uneven ceiling and I think it will look too oppressive to have tiles all the way up the to ceiling

For that reason I decided to go with 3 horizontal rows of tiles, starting at ~2" above the floor level. The first two inches between the tiles and the floor will be covered by a plinth (same tile as the wall tiles or like the floor tiles) and the shower base

The specs of the tiles I am using are here
https://www.anatoliatile.com/content/uploads/Prima_Porcelain_Tile_screen.pdf White Matte Rectified 24"x24"

So I am trying to determine if using a 1/4" trowel will get me flush with a layer of drywall over cement board (or maybe two layers of cement board) above the tiles (marked with green on the picture) -two layers of cement board = 1" -one layer of cement board + 3/8" the tile =7/8"

this means I have 1/8" thickness for the thinset and two times that, 1/4" for the trowel size Is my calculation correct ?

I am not sure if 1/8" thinset is enough for that size of the tiles (24"x24")

The glass panels will be 72" high (not sure if it will look nice to have the glass panels higher than the top line of the tiles)

I still have to determine the positions of the verticals (where to cut the tiles on the vertical) and how I place the tiles there, to look esthetically pleasing.

enter image description here

  • Do you have a plan for finishing the edges of the drywall against the tiles? I raised an eyebrow at that suggestion on your other post. It's not a standard approach, and unless you're going to install a metal or plastic bead over the drywall and against the tile I'm not sure how this will work. I wouldn't do it unless some other aspect of your design depends on it. Flush joints, whether in drywall or woodwork, are a royal pain. In the real world things just don't come out flat, and you need a solid plan for the joint.
    – isherwood
    Aug 31, 2021 at 2:46
  • BTW, a 1/4" trowel doesn't really leave you with 1/4" tall ridges of mortar. You apply with the trowel at an angle, so it's something less than 1/4". Of course you then need to be precise and consistent with your tile set to arrive at whatever you calculate. It's all sounding pretty sketchy to me. (24" tile is more often set using a 3/8"x3/8" or 1/2"x1/2" trowel anyway, I think. It's tough to get the mortar to squish with smaller notches. Larger tile, larger notch.)
    – isherwood
    Aug 31, 2021 at 2:49
  • yes a nice shiny contrasting metal bead is what my brother is law (architect) told that I could use. This new design is his idea but I am not sure how realistic it is as the tiles end at 70.08 above the shower base which is a little bit low in my opinion
    – MiniMe
    Aug 31, 2021 at 2:52
  • How will you tape or skim coat to a shiny bead? I don't follow. Has this architect done any field work?
    – isherwood
    Aug 31, 2021 at 2:53
  • yes I think I am going to go with your advice, double the cement board to get level with the tiles, skim coated as you said and then do what the other guy said prime it with an oil based primer
    – MiniMe
    Aug 31, 2021 at 2:53

2 Answers 2


The most important thing I can share from experience is that with large-format tile (which 24"x24" certainly is), you will almost always need to use a large notch trowel (1/2 U-notch or square-notch, or larger). If you don't use a large notch trowel, you may end up with voids behind the tile and lack the proper coverage to avoid a future failure of the installation.

Some things that will help guide your choice:

  • this is in a shower, where near total mortar contact/embedment is critical

  • the less flat your substrate is, the more mortar you will need to accommodate the imperfections

  • in most cases, design requirements should not affect trowel size choice - it should be a choice based on flatness of the substrate and size of the tile

  • yes I was aware of that and I see that that size can't get me what I need
    – MiniMe
    Aug 31, 2021 at 14:19

First you will for sure use a large notched trowel. But most importantly you will back butter each piece 100%.

Also given your layout and diagram I would trust 2 out of the 7 guys who do tile work for me to make this look 100% professional. This is a really really hard install to get the fine details right. You have a ton of exposed edging and are doing that piece above the tub? My suggestion is to take the tile all the way up. If you don't like that look then put accent tile in. I have ran glass mosaics in areas like that and very easy to get these to fit "snuggly" to the ceiling. I do not suggest stopping and putting in j-trim unless you you are ready to do some serious calculations on depth.

This may be the way to go over the toilet and right wall but in my opinion this is not what I would do above the shower/tub.

  • Thank you for the answers and comments. I do appreciate all these details and I am sure it is diffict to get this right. I have all the time to get this right so I will take my time to make the right decision. I do listen and take all the answers I get here and I am reading a lot these days about tiling. One way or the other I will finish this even if it wont look perfect or very professional. Point taken with that bead but what else can I use. I am reco sidering tiling up to the ceili in the shower enclosure but not above the sink
    – MiniMe
    Aug 31, 2021 at 23:40
  • You may want to look into a tile leveling system, watch some you tube videos. Large format tiles can be difficult to get level. Sep 1, 2021 at 2:43

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