Remodeling a bathroom and about to put in the floor. I have a plywood subfloor, and I'm about to install 1/2-in cement board on top, bedded in modified thinset spread with a quarter inch notched trowel. I plan to leave a 1/8-in in gap between all cement board sheets on the floor, with a 1/4-in gap at the walls. I'll be using a mosaic floor tile, and a subway tile on the walls with a cove base transition.

My tub is a porcelain enameled steel tub. It is solidly installed with the flange anchored on top of a ledge at the back of the wall. It is set into a bed of mortar which supports the entire base.

I have looked everywhere, but I can't seem to find any information as to how to properly transition the cement board on the floor with the skirt/apron/decorative panel of the tub. I have small kids that will definitely create splashes. Ideally I'll wipe them up quickly, but there will certainly be situations where the chaos of life leaves small puddles on the bathroom floor every now and then. I know of several people that have had issues with leaks from standing water at this transition point.

My thought is to leave an 1/8-in gap, and then fill that with thinset right up against the edge of the tub. Because I will be using Mapei Aqua Defense under the tile inside of the shower/tub enclosure, I could also apply a coat of this up against the edge of the tub at least, if not the entire bathroom floor (very small, 4x6 ft). I would then install tile as normal, leaving a 1/8-in gap, and caulking with 100% silicone or a siliconized grout matching caulk.

Does this make sense? Should I be doing anything different here? Another idea was leaving the 1/8-in gap open (cement board and tile), and filling the entire gap with silicone after installing tile.

  • sorry, having trouble envisioning what the tubs skirt/apron looks like. Photo? Is it a decorative detail that brings it away from the tubs face (side)?
    – ojait
    Dec 28, 2020 at 19:36
  • It's a very standard/simple tub side. Just white enameled steel like the rest of the tub. No detail. Dec 30, 2020 at 11:26

1 Answer 1


I think you have a good plan except do not fill the gap with thinset, leave a small gap between the cement board and the tub and fill with caulk.

You could apply some of the Mapei on the first few inches of the cement board.

  • Thanks. I actually just added that option in has an edit, the same time you were providing this answer. Good call on the expansion and contraction. Would you lay a bead of silicone directly following the cement board install, or should I wait until after tiling and fill the entire gap at once? Seems like the latter is is a better option, but the former would allow me to use 100% silicone below, and a a siliconized caulk color-matched to my grout on top. Dec 28, 2020 at 18:31
  • 1
    Caulk the first gap before tile. Caulk the tile gap after grouting.
    – Alaska Man
    Dec 28, 2020 at 18:51
  • There is colored caulk that matches the grout used, no need to work with exposed silicone
    – Jack
    Dec 29, 2020 at 4:37

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