Am I calculating the trowel size correctly here?

Question

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.

Answer 1

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

Answer 2

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.