September 21, 2023

How to install cement board on a shower curb and not pierce the waterproofing

This technique can be used over a PVC liner that is wrapped over a 2×4 stud curb. This technique is the same as when a form curb overlay is used, referred to as a preformed curb on page 258 under common shower configurations in the TCNA handbook , the difference is that the overlay is made by you. While there is nothing wrong with foam overlays, they can be a little tight and may develop a twist as they are installed. This technique makes it so the board can be customized when dealing with the liner. If the liner is bunched or tight, you just need to make an adjustment in the size of the top face. A cement board curb is slightly easier to adjust when correcting for level and slope than a foam overlay is.

Lath and mud is still a good alternative but the preformed method can actually be easier to correct for level and slope. Mud and lath requires a slightly higher skill set than the method here, but mud and lath has no structural advantage, in spite of what some pros may say.

Thinset mortar in this application makes a mechanical bond, not an adhesive/chemical bond.

As we all know and all the internet warriors concur, thin-set mortar does not stick to (adhere to) plastic or PVC. In this instance the thin-set mortar is used to complete the mold that is essentially being made when the curb is wrapped with the cement board. Think of it like this; If I took a bucket of thin-set, turned it upside down and put it over your head and let it dry, you wont be able to get it off. Is that because the mortar stuck to your skin? No. It is stuck because the mortar dried in the form of a mold that contoured the entirety of your head. When someone goes over a liner with lath and mud they are doing the same exact thing with different materials. Essentially creating a mold that will hold similar to an uncoupling system used on floors. This is essentially an uncoupled curb.

Topical or bonded waterproofing membranes should still be used over the curb as well as the wallboard following the dried application of this overlay.
Topical waterproofing does not protect screw penetrations from water intrusion if the screw is installed on the curb.

Read this reply that I posted to a comment:

There was a time when I used to put screws on the top of the curb to hold the board down. I thought I had to. I would put the screws in an area that I felt was outside of the shower door line and I would put topical waterproofing over the board and screws on the curb.

I found out years later that it didn’t work. I never had a complaint, or had to return because of a problem. But on one job a customer wanted to do their other bathroom. I looked at the one I did twelve years earlier. Everything looked good except the curb. The curb wasn’t horrible but I could tell something was going on. I offered to repair the curb. When I took it apart I could see that the water found a way. It managed to deteriorate the curb and was retaining water.

How did it happen? The glass enclosure screws. And I don’t mean the glass enclosure screw through the curb, I mean the screws mounting the hardware on the walls for the panel.
The penetration from the glass shower panel screws allowed water to pass through my topical membrane via the surface tension along the screw, through the board, down the wall and into the waiting liner, where the curb meets the wall. The water then traveled horizontally along the curb liner, mind you, under the board, until it found the screw penetration which had fastened the board to the curb. The topical membrane could do nothing to stop this. The glass panel hardware that was attached directly to the curb may have also contributed to the curbs deterioration, but I could definitely see where the water had entered through a wallboard screw on top of the curb.

This is real, and most customers don’t complain when something starts to fail. You wouldn’t even know that you had been doing something wrong until one day you stumble upon it.

Trust me, no matter what anyone says, screws in a curb will one day screw you up.

Now think about this…

Tile guys are a stubborn bunch and every one of them thinks they’re the best. To those of you that feel you’re the best, I’m sure your mother agrees with you, send her my best!

Please note; The shower in the video is a TCNA B-415 cement board traditional/water in-water out receptor. Cement board can be installed on the curb lower than the height of the shower pan mortar. Fiber cement board cannot be installed below the height of the shower pan mortar. This goes for coated glass mat water resistant board as well (think Dens-Shield). The only other board in the handbook that states the board may be installed below the height of the shower pan is Cementitious-coated extruded foam. This technique can be done with foam board as well, but 1/4 inch cement board is relatively inexpensive and makes for a very strong curb because there is no give, or deflection once embedded, which foam may have.


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