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Unread 12-01-2019, 12:10 PM   #1
bbh03
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Using subfloor slope for linear shower drain pan

Hello,

I am planning a shower replacement project and was anticipating using a Schluter linear drain with LTS prefab pan. I would like to make this shower as curbless as practical. The shower will be enclosed by walls on three sides. The drain is planned for the back wall of the shower opposite the entry side. As an aside, I have a lot of experience with the Schluter system and would like to stay with it for this project.

Today I was checking level of the bathroom floor and discovered that the floor is already sloped dramatically toward the wall where I plan to place the linear drain. Slope is close to code minimum at 3/4" over 4' for a shower pan.

My question is this: What are the downsides/issues with utilizing this slope to my advantage rather than leveling the floor and then installing a pan? In other words, what if instead of using the prefab pan, I install a very shallow pre-slope of mud to bring the slope from the existing ~3/16" per foot up to 1/4" to 3/8" per foot? This would have the advantages of 1) minimizing buildup of materials probably making the shower truly curbless at the entry and 2) saving me the cost of the prefab pan. So, what are the downsides?

Thanks!

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Unread 12-01-2019, 12:21 PM   #2
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Welcome back, Brian.

How much height do you have to work with? I think a mud bed over wood framing needs to be at least 1" thick.

As an aid to identifying other implications of a not even close to level floor, perhaps you could offer us a sketch of the proposed layout.
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Unread 12-01-2019, 12:45 PM   #3
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Hi Dan,

So, I remodeled the rest of the bathroom maybe 5-6 years ago and decided not to mess with the level of the floor at that time. I'm trying to remember, but I believe there is 5/8" original subfloor, 1/2" ply I added above that, a ditra layer (~1/8") and then tile. So, I if were to sink a new subfloor flush with the joists, I would have a depression of almost 1-1/4" (not counting floor tile thickness which should be about same as shower floor tile thickness) at the entry end of the shower. I should add that the shower is planned at approx 48x48". I will try to put together a sketch and take a photo of the existing. Thanks!
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Unread 12-01-2019, 04:36 PM   #4
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Here is a photo of the existing. New will come out further to be 48" square. Sketch coming.
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Unread 12-01-2019, 04:49 PM   #5
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Sketch:

May extend the partial wall to eliminate the glass corner.
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Unread 12-02-2019, 07:08 AM   #6
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So the floor slopes down towards the back wall of the shower. Is that slope uniform all the way across, including the toilet area? Does the slope dip towards one of the shower rear corners?

Couple of thoughts; if you intend to use your currently built in slope, and you then add in the slope of the pre-made pan, where will you be? Also, gaining 3/4" by recessing the subfloor between the joists will help, but you will still need a layer of 1/2" ply over the now exposed joist tops if your new pan is foam. You definitely don't want the joist tops to be higher than the recessed subfloor.

Which way do the joists run? If you don't intend to recess the subfloor for the entire bathroom you'd need solid blocking to pick up the unsupported edges where the recessed and non recessed subfloor edges meet.

I love my recently finished curbless (and doorless) shower, but it was definitely a challenge to build.
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Unread 12-02-2019, 12:47 PM   #7
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It is pretty uniform and includes the toilet area. It is pretty close to level side to side. The reason for the slope is that the main beam of the house has sunk over the years. The middle of the house and that beam are very near, and parallel to, the back wall of the shower. The joists run in the direction of the slope out from the back wall of the shower.

If I were to add the prefab foam pan, the shower slope would be considerable. I do not know the lower thickness of the pan (cant locate in their specs), but the higher edge is 1-13/16" Adding this to the existing slope would be a considerable slope I think. That's why I'm considering making a mud base that adds to the existing slope slightly. I am not sure what the minimum thickness of such a base can be.

I hear you on the blocking as I will only be recessing the shower area. I will slip blocking just past the recessed cut flush with the edge of the exisiting subfloor. Then there will be recessed block along and across the joist spaces down 3/4".
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Unread 12-02-2019, 01:38 PM   #8
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At 1 13/16" I'd think that particular prefab would be a no-go; you'd have to build the main floor up at least the same amount. Wait, more since you'll need 1/2" ply under the prefab.

My 42"X84" center drain foam pan is 1" thick at the perimeter, Slope is barely adequate in the long dimension, but it still works.

Looks like mud to me. You might be able to build in some additional slope when you recess the subfloor.
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Unread 12-02-2019, 01:56 PM   #9
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That’s what I’m thinking. Once recessed the 3/16 per foot will still exist. Then I will mud it to 5/16 or 3/8 per foot. Mud bed would be the thickness of the linear drain pan at the thin side and therefore about 1/2 to 3/4” thicker than that at the high side.

Is the extra 1/2 ply necessary in the case of mud? Is the 3/4 recessed ply sufficient? I may need it anyway to bring the shower floor level to the existing ditra to get a truly cut less shower but can’t be sure until demo


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Unread 12-04-2019, 01:53 PM   #10
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I still have this question of how thin can a deck mud mortar bed be. I've done some additional research here and found that thinner (than the traditional 1-1/4" thickness at drain) can be used when the deck mud is adhered to a loose thin set slurry on the subfloor. So far I cannot find a recommendation on exactly how thin.

Further, Schluter seems to be advocating that the deck mud may be minimally at thin as the linear drain support (this dimension I cannot find in their documentation), which would be a plus for my situation. See here:

https://sccpublic.s3-external-1.amaz...ess-shower.pdf

So, if I can have a minimum thickness of the support, I am proposing a screed that is probably about 1/4" to 1/2" thicker a the high side (to add to the existing preslope of the subfloor) to yield a final slope of 5/16"-3/8" pf.

Can anyone opine on:

1) using a minimum deck mud thickness of the this channel support (unknown but seems to be ok by Schluter).
2) the need to adhere the mud to the sub floor with thin set.
3) the need for additional plywood (say 1/2") over the 3/4" subfloor that will be placed flush to the joist tops?

Thank you!

Brian
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Unread 12-04-2019, 02:33 PM   #11
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They state about 1in thick at the perimeter, and 1/4in thick at the drain. Keep in mind that is just some article from 2013 which is not written by schluter. I would not cite it as authoritative by any means.
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Unread 12-04-2019, 02:37 PM   #12
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Hi Mike,

The only reason I cited it as possibly being authoritative is that Schluter has the link directly on their product page and the article has been branded by them.

https://www.schluter.com/schluter-us...E/p/KERDI_LINE

Yeah, I cannot find a reference to the drain support thickness either. I may call and see if I can get to a product engineer.
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Unread 12-04-2019, 02:52 PM   #13
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As someone who works for a corporation with marketing, engineering, customer service, and legal, let me assure you more times than not all those divisions have no idea what the other is doing. In fact, marketing (and social media) typically is the least reliable source for technical information.
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Unread 12-04-2019, 04:37 PM   #14
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I hear you.

I just spoke w a Schluter rep. For anyone wondering in the future, the Kerdi line drain is 7/8” for the channel piece and hence their trays have the same thickness at the minimum location.

I described the application and was given the number for my local territory manager. Apparently that person can give a go/no go to a nonstandard application such as a sloped subfloor. I will call tomorrow and find out.


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