Cover holes in shower floor with cement board

beauty_brainsAugust 29, 2014

We are remodeling our bathroom The plumbers cut out a section of plywood in order to move the drain Then came the tile guy and covers those holes with cement board Is it ok that he did that Should he have used plywood to replace the plywood that was cut out I need help I don't want them to continue if that is not the proper way

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Much depends on exactly WHAT kind of shower you are building. Typically, you will cover the floor with a waterproof membrane, then attach expanded lathe with staples or nails,

At this point, you put a "mud bed" over that.

NOW, if it is a "traditional" shower floor, a rubber membrane is put in, attached to the drain and over the curb and 10" up the walls. The curb than has the same wire lathe "overbended to the curb and attached ONLY on the outside by fasteners. It is then "mudded" with mason mix with a slight slope inward. At this point, the final mud bed is installed and can be tiled over.

The other process is, after the initial mud bed is installed, a "SAM" (surface applied membrane) such as Schluter's "Kerdi" is installed on the walls and the floor. Simpler and the water only goes to that point and then to the drain. You DO need to use their proprietary drain as it has a "bonding flange" as opposed to using the cheap "clamping flange" type drain.

For more info on the Kerdi System, go to or just UTube "Kerdi Showers.

    Bookmark   August 29, 2014 at 2:57PM
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"Should he have used plywood to replace the plywood that was cut out?"

Yes, and you need a cleat on the left and right joist ends or expose the joist to lip over and butt to the existing subfloor.

Cement board has virtually no flexural strength.

    Bookmark   August 29, 2014 at 7:06PM
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Right now my bathroom floor looks like your floor except with bigger holes; the holes will patched by plywood and another layer of plywood will go over the patched subfloor before the shower and floor are tiled per the tile setter's instructions.

    Bookmark   August 30, 2014 at 1:17AM
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Sorry...double post.

This post was edited by Ranton on Sun, Aug 31, 14 at 17:46

    Bookmark   August 30, 2014 at 1:18AM
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What you have looks like OSB, not plywood?

My contractor did not want to mess with the integrity of my bathroom floor and he accessed all the plumbing from the floor below. Luckily the master bath it is over the garage so no additional mess in the house, but another option to consider for others doing similar remodels.

    Bookmark   August 31, 2014 at 4:16PM
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