How to install a vapor barrier for use with a mud pan?

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Nick Evans

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I have been going through lots of threads and videos on the topic of vapor barriers in use behind backerboard, but I'm still coming up with some unanswered questions.

We are doing a cultured marble shower. The installation of the cultured marble will be hired out, and they will be doing the pan (mud pan), and the marble panels. I am doing the rest.

I have the shower torn down to the studs right now, and am looking at putting in the vapor barrier, then the Hardibacker. I have read so much back & forth on the use of vapor barriers in use with Hardibacker, but I'm just going to do it. What I don't understand, is how to install the vapor barrier when using a mud pan.

The guys that are going to do the pan and panels told me that I need to have the backerboard installed before they do anything, which would mean the vapor barrier would need installed before hand as well. The information that I've read has been around installing vapor barrier with the pan already in place (acrylic or fiberglass). In those instances, they brought the vapor barrier down into the pan, and cut it basically at the top of the pan. If I'm doing a vapor barrier before a mud pan though, where does the bottom end? I'm using a 3 piece drain, which clamps down. Does the vapor barrier come down the wall, and tuck into the drain somehow to be clamped down? (this seems logical, but I can't find any info on it). Basically I'm trying to understand how the vapor barrier terminates at the bottom for use with the mud pan. Thanks for your help!
 

Jadnashua

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Some pans require a waterproof liner underneath, and that is not technically a vapor barrier but a true waterproof liner. With a 3-piece clamping drain, the liner gets attached there. Plumbing code requires the waterproof layer (in this case the liner) to be sloped to the drain. There is no good way to seal a 3-piece shower drain to a solid surface pan.

But, if those panels are installed like a huge tile, you'd need that liner, and code says it must go at least 2" above the top of the curb all around the rest of the shower. THat would be installed, along with a preslope, before the panels were installed. But, most of those, if it's one-piece are designed to fit on a flat, level floor, and preslope would mess things up.

So, how does that actual pan seal to the drain, and are you sure you have the right type drain installed?

If the manufacturer calls for a safety liner flat on the floor, without slope to it, how would it ever drain?

Do you have the model of the pan you're interested in to install? Can you let us know?
 

Nick Evans

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So, how does that actual pan seal to the drain, and are you sure you have the right type drain installed?
-I installed what the cultured marble guys said they use, based off of a picture they sent.
Is there a difference when you say 'base' and 'pan'? I thought the 2 were interchangeable, but now I'm not sure.
I understand your statement that there may need to be a waterproof liner below the mud (base\pan?), but is there then additional vapor barrier coming from the wall down over the mud to the drain?
 

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Lots of ways to build a shower. Referring to a conventional, tiled shower, from the original floor, the layers going up typically are:
- preslope
- waterproof liner
- setting bed
- thinset
- tile.

They usually make the preslope and the setting bed out of deck mud, a mix of mostly sand with enough cement in it to hold it all together. The preslope is literally sloped to the drain, so when the liner is installed, moisture will flow verses pool. In a conventional shower, the liner is the waterproof layer, and being waterproof, it does not need a vapor barrier beneath it. The vapor barrier on the walls would typically be lapped over the pan liner so if any moisture got behind the tiles, it would be directed into the pan, and eventually flow out of the drain. TO accommodate any moisture that did get beneath the tile, which is below the surface by typically at least an inch or more, the top of the clamping drain has weep holes to let water drain out. The vast majority of water flows out through the grate even with the top of the tile. The setting bed is an even thickness and the top surface is sloped because of the preslope with the liner on top of it.

A cultured marble pan often does not have a liner (some do), and they use a different type of drain since it does not need or want weepholes (they would allow moisture going down the drain to flow out onto the floor beneath!). The pan is sealed to the drain often in the same manner as the drain is installed in a tub. Most of them are designed to sit on the floor (some may require something there to help level them), but they are waterproof on their own, and do not need a vapor barrier. Those coming down the wall typically get sealed to or lapped over the lip of the pan.

So, while I'm certainly not aware of every product or method, it does not sound like you've got the right drain IF the pan is designed to be sealed like a tub. Without knowing exactly what you are intending to install, can't really go much further.

FWIW, one industry survey found that over 80% of installed tiled showers were not installed per industry standards. It isn't all that hard, but it is very exacting, and without knowing what was intended and tested and approved, can't say how yours should be going up. Now, if you want a conventional shower and many others, once you've identified better what you want, someone may be able to direct y0u better.
 

Nick Evans

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The marble guys told us to install a 3 piece clamping drain so I did that. But I do have some concerns now that I've read some more.
I watched this video on preslope and 3 piece clamping drain.
In the video, he has a 3 piece clamping drain installed as well, but his does not sit flush to the subfloor. Mine had screw holes so I attached it to the subfloor directly as shown in this picture:
7B4N7cd.jpg


Now I'm concerned that I goofed up and it shouldn't be flush with the subfloor, so that I can build a preslope to it, and then the liner goes on top of that to guide moisture to the weep holes.

I understand that since I don't know the specifics of what's going to be happening in the full shower install, it's hard to give any input on this.
I found an image on floorelf.com that I think is an accurate representation of what should happen.
shower_cutaway.gif


If the picture above is acruate, then I goofed up on installing the drain, and it should have sat higher so the preslope could go under it slightly. It would in this instance go:
  1. pre-slope
  2. liner
  3. top mud bed
  4. vapor barrier that would be over the liner, and would drip onto the mud bed.
I'm going to have to get in touch with the marble guys and see if I can get more details from them on what they do. I was going to higher them to do basically, the pre-slope, liner, mud bed, and marble panels. But depending on what they say, I may have to rip out a lot of work to remove the drain (there was no wiggle room to begin with, so if I have to cut it out, this is going to become a whole new project!).
 

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If the marble panels are being set similar to a big tile, then yes, you have your drain at the wrong height. And, keep in mind, as I said, the waterproof liner is REQUIRED to go at least 2" above the top of the curb AND pass a leak test. What you have shown is a conventional shower construction.
 

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If the marble panels are being set similar to a big tile, then yes, you have your drain at the wrong height. And, keep in mind, as I said, the waterproof liner is REQUIRED to go at least 2" above the top of the curb AND pass a leak test. What you have shown is a conventional shower construction.
 

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The panel's are going to be one solid slab per wall, so I guess my drain is the wrong height :(
The liner going 2" above the curb doesn't seems totally do-able... is there something about this that I'm not considering, or is it just an FYI?
You mentioned that I'm showing a 'conventional shower construction'. Is that just based off the way I installed the drain, or is there something else that makes this a 'conventional' shower that I need to change to work for the cultured marble?
 

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Preslope, liner, setting bed, finished surface (regardless of what it is made of), is a conventional shower!

Plumbing code and industry standards call for the liner to be flood tested, and extend 2" above the top of the curb. So, the normal way to construct this is install the preslope, then the liner, then the vapor barriers on the wall, then the cbu on the wall (no fasteners lower than that 2" above the top of the curb, which means it can flop around!), then, the setting bed is installed which locks the bottom edge of the cbu in place. Then, you can install your panels. Once the preslope is in and you've installed the liner, you perform a flood test. IT should NOT leak anything!

Note, there are a bunch of products called cbu or cement board. The procedure differs slightly depending on the class. There are two, those that are essentially cement based, and those called fiber-cement based (HardieBacker is a fiber-cement board). A cement based one can be buried in the setting bed, a fiber cement one CANNOT, and thus, since you can't screw the bottom edge in place, can be problematic since there can be NO fasteners below the 2" above the curb. One of the biggest mistakes people make is driving screws through the liner into the curb (often to install cbu there, which is NOT the way it should be done!)...there should be NO holes, and it takes a bit of skill to get the curb so that you can install something on it where it is stable and mortar will stick (it will NOT stick to the liner!).

Unless the cultured marble pan is one-piece and has a curb and lip all around it with built-in slope designed to attach to a drain, thus making that assembly waterproof on its own, you need to build your shower as if it were going to be tiled, and when you talk about a preslope, that precludes installing what you're talking about unless the setting bed is installed flat. That is likely to make the 3-piece clamping drain too short to fit - the top part only extends so far.

Normally, if the pan is preformed with its own slope in it, it is designed to be installed with a different kind of drain assembly that is either clamped to the pan and attached with a nut and big gasket, or something similar. That would have NO weep holes, which is a characteristic of a clamping drain and a tiled shower pan. Now, if what you've planned has passed plumbing certifications doing it another way, fine, but you'd really want to see and verify the instructions before you proceed with generic instructions. IOW, you really need the instructions for the pan you are having installed as to what the prep work is required, and exactly how to install it.
 

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Thanks for all that info. I reread my last post, and realize I wrote something wrong. I meant to say, installing the liner 2" above the curb didn't seem like a big deal. I did not however realize that this means no nailing at that point (from base to 2" above curb). I also didn't know hardi-backer couldn't be embedded in the setting bed (mud?). The marble guys recommended Duroc which I am now reading is a CBU that can be placed in the setting. So I think I'll go with that.

I reached out to the Marble guys because I'm realizing that I simply don't know enough about what will be going in to seek the info I need here on how to do it. Hopefully they can provide information on how they do the pre-slope, liner, mud, and pan as it seems like those things dictate how the CBU goes in. If/when I hear back from them, I'm certain I'll be back to discuss it further.
 

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The issue with fiber-cement boards is that some of the filler (and it also makes them stiffer and less likely to crack) is that, at least in the case of HardieBacker, it can contain up to 15% cellulose (i.e., wood fiber!). Now, that is encased in cement, but, by design, the top layer of mud in a typical shower pan reaches some equilibrium of moisture content, and remains at least somewhat damp. FOr that reason, you do not want a fiber-cement product embedded in there. While there is a method described in the TCNA Handbook (Tile Council of North America) that describes building a proper shower with both types of board (along with numerous other methodologies!), it DOES make a difference in how you proceed for a reliable, long-term solution.

Personally, I prefer a surface applied waterproof membrane, then, you don't have a drain with weep holes, and only the preslope...the membrane goes on top of that and then your tile. Having waterproofing directly beneath the finished surface means there's much less to dry out, too. Moisture is one leg of the triangle that enables mold (the mold spores themselves, moisture, and food). Surface waterproofing could be used with solid panels, but the drain is entirely different (and probably overkill). 3-4 different companies make 'systems' that use that technique including Laticrete, Mapei, Schluter, and NOble. FWIW, to get an idea of what I'm talking about, check out one of the Schluter Kerdi shower systems videos on their channel. The other companies offer similar things, but don't have as good videos to get an idea of the concept and methodology.
 
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