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Unread 05-11-2022, 08:07 PM   #1
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Shower Pan Cement Board Gap

Hi Everyone, I’m new to the site and amazed at the amount of info here. I’ve recently begun a full bathroom reno including shower and may have taken a wrong turn or two on the shower prep, but let’s see. This is the first shower I’ve done. Details are as follows:

30x36 dimensions
3 2x4 for curb with a 90 deg angle
Kirb perfect
shower pan with Mapei 4:1 with the pvc pitch sticks
Oatey preslope under liner with weepguard
3/4 +3/8 ply subfloor
Wonderboard lite on walls

I’ll attach picks shortly.

I set the pan over the weekend and left an approximate gap of 3/4 inch between the cement backer and the pan. The tile is 3/16 thick so I figure about 3/8 inch total additional height when tile is thin set to the pan, leaving a gap of 3/8 inches between the unfinished wall and finished pan. I believe I read somewhere on the forum that the board should be 1/8 above the pan pre-tile and that joint caulked with silicone. Is there a way to increase the height of the pan with additional mortar? Or should I add fiber glass tape the joint with mortar and then Redgard that joint when regarding the walls? Or leave as is, have the pan tile run under but not touch the cement board and have the wall tile come to 1/8 inch of the pan tile and apply silicone to that joint?

Thanks for your consideration.

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Unread 05-11-2022, 10:04 PM   #2
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Welcome, Matt.

Don't think I've ever seen anyone use one of those Oatey foam pre-slopes. Waiting for the photos.

I'd really like to see pictures of your waterproof liner installation, especially at the curb junction and the ends of the curbs. Did you slope the tops of the curbs to the drain?

What you really wanted to do with your CBU walls is embed them into the final mud bed. Otherwise you have no way to anchor the bottom of your CBU panels as you cannot have mechanical fasteners lower than two inches above your finished curb top. Depending upon your drain, you may be able to remove your existing mortar bed and make a new one that's high enough. If you do that, I'd very strongly recommend you throw away those plastic dividers and that you use a 5:1 sand/Portland cement mixture rather than the MAPEI 4 to 1.

My opinion; worth price charged.

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Unread 05-16-2022, 07:30 AM   #3
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Hi CX,

Thanks for the welcome, and advice. Sorry the pics took so long - see attached. I've included images at the various steps, but all corners are dam sealed with the Oater PVC sealer. The 90 deg angle was tricky. I hospital cornered the inside and then sealed with a dam, then cut a second piece to cover the remaining top of curb to outside (also sealed with oatey liner). I'm probably not describing great, but I've attached a pic which hopefully shows better.

As far a ripping out the bed, definitely a lesson learned here as this is a summer rental house - so deadline of finishing are going to prevent this and I have to work with what I have at this point. Is there a way to put fiberglass tape at the joint with thin set and then regard over that?

The curb comes with a built in pitch and torpedo level is showing it's right on point for draining.

Thanks again for the insight.

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Unread 05-18-2022, 06:35 AM   #4
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I don't think mesh tape and mortar are really going to do much to strengthen those wall to pan joints, Matt, though adding them certainly won't hurt. And painting those joints with RedGard really isn't buying you anything, either, but also won't hurt. What you don't want to do, given the liner, is paint the floor with RG.
If I recall correctly my memory is excellent, but my ability to access it is intermittent.
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Unread 05-18-2022, 10:35 PM   #5
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What's the size of the wall tile?

Although I normally do this the other way, in your case I would tile the floor first, then (assuming your wall tile are of decent size) I'd set the wall tile down to the floor tile with an 1/8" gap. So there will likely be a bit of the wall tile with no backing, but it'll be really small.

Not ideal, but I can't think of any other way around it without removing your mud floor.

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