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Unread 11-17-2021, 11:21 AM   #1
NCKyle
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Mud bed shower pan over plank subfloor

Hi all,

I'm renovating a 1950s house that has 1x8 diagonal plank subflooring, over 2x10 joists 16" oc. I'm putting plywood over the planks in the bathrooms under the ditra/tile layers, but was wondering if this extra plywood was necessary under the mud bed shower pan I'll be building (with the standard metal lathe and dual layers of deck mud, etc.), or if the mud and lathe add enough rigidity to make that not absolutely necessary?

Avoiding the extra height there would be nice, and it would make a couple other things a little easier, so figured I'd check with y'all. Thanks.
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Unread 11-17-2021, 12:30 PM   #2
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You can't technically do anything directly over that sawn board subfloor in your application, Kyle. If your boards were perpendicular to the joists, you could use a reinforced mortar bed of a minimum 1 1/4" thickness, but with the diagonal orientation that would not be industry acceptable.

Even with a second layer of plywood subflooring a minimum of nominal half-inch thickness, you'll not be in compliance with any industry recommendation, but I would personally be willing to do that if I were sure my diagonal boards were at least 3/4" thick and in very good condition. T&G edges would be a plus, but I'll wager that's not what you have.

Continue your second layer of plywood, preferably nominal 5/8ths" or better, through the shower area and be sure your mortar bed is at least 3/4" thick at the thinnest point would be my advice.

My opinion; worth price charged.
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Unread 11-17-2021, 12:47 PM   #3
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Gotcha, thanks CX. Yep, not T&G, unfortunately. They do seem in good shape, at least - I've patched any questionable areas and don't feel any noticeable give when I jump on them.

Curious about the industry standards - I was planning on 1/2" plywood to minimize height change, given that Schluter allows for that thickness over 3/4" plank with Ditra: https://resources.schluter.com/media...20Handbook.pdf Does that contradict another standard from elsewhere though?
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Unread 11-17-2021, 01:30 PM   #4
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While it is not specified in the ANSI standards, I've always presumed the requirement was that the boards must be oriented perpendicular to the joists as is always specified for engineered wood panels in those standards. What is actually specified is that the boards be 1x6 with T&G edges, which I've regularly managed to overlook, correctly or incorrectly. What is also not specified, but presumed on my part, is that the added plywood is to isolate the tile installation from the instability of the board subfloor, rather than for additional strength or rigidity.

I still favor the additional thickness of plywood when the boards are not perpendicular to the joists, not T&G style, not 1x6, etc, but that is my personal opinion only and carries no weight at all. You're comfortable with the nominal half-inch plywood, you're free to use that.

As for Schluter's recommendations for subfloor under their products, that has nothing at all to do with what we're working with here. As a tile substrate product manufacturer, they can specify anything they want below their product and that specification trumps any other recommendation or standard.

My opinion; worth price charged.
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Unread 11-27-2021, 01:49 PM   #5
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Hello again! I always think I'll just have a single question, but I suppose this should be renamed to a project thread once again...

As mentioned, I'll be doing a much bed here, but will be using a linear drain for the first time, so I wanted to double check that I'm planning the preslope and tile portions properly. Unfortunately, the linear drain is not wall-to-wall, nor is it flush with the wall alongside its long side. There will be about 4" of floor at each end of the drain, and about 6" of floor between the long side of the drain and the wall.

Do the attached pics (with exaggerated slope for clarity) show the proper way to slope the various planes around such a drain? I'm assuming:

A (6" portion between long side of drain and wall) = flat plane sloped (at least) 1/4" per foot from wall to drain.

B (4" portion between short side of drain and wall) = flat plane sloped 1/4" per foot from wall to drain.

C (large portion between long side of drain and far wall) = flat plane sloped 1/4" per foot from wall to drain.

I'm also assuming that, in theory, if I'm using larger-format tile, I can use a single piece for each plane (limited only by the size of the tile, of course), given that they each should be flat planes. They have some funky intersecting lines, but I don't think that should matter.

Anyway, just a sanity check here to make sure I've got all this right. Thanks!

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From above:

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Unread 11-28-2021, 06:18 PM   #6
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Just a quick bump to see if anyone has thoughts on the above. Thanks!
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Unread 11-28-2021, 06:37 PM   #7
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I'd want to use mosaics on that floor. Keep in mind that you want the perimeter of the shower floor to be level if possible. Since the distance from the drain to the walls differ, the shorter distances will have a steeper pitch that the longer distance.
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Unread 11-28-2021, 07:29 PM   #8
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Hmmm, okay, thanks. What is the reasoning behind wanting the perimeter to be level - just aesthetics of the bottom line of wall tile, or something more?
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Unread 11-28-2021, 09:39 PM   #9
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That's correct, Kyle. Otherwise you'd start with full tile at the lowest point and gradually cut them as you work your way around the walls. It can be done, but I wouldn't do it because of how it looks.

You didn't mention the size of your wall tile, but if it was a smaller tile, you could imagine cutting them more and more as you get to the high side of the walls and just have a sliver there. It might not be quite as noticeable with large format tile.
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Unread 11-28-2021, 09:54 PM   #10
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Perhaps I'm missing something here, but with a linear drain at one end of the shower and a flat, single plane slope for the majority of the floor, I'm not seeing how a fella'd make such a floor with a level perimeter.

There will be a couple strange little pieces at the ends of the drain, and a much lesser rise from the drain to the end wall, but aside from that it's just a single-slope linear drain application with a necessarily sloped wall on either side.

What am I missing?
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Unread 11-28-2021, 10:38 PM   #11
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Thanks KMan. My wall tile is 10" high, at least, so hopefully it won't be too glaring. I've planned the rows so that it shouldn't be too sliver-y at the base.

But CX, I did wonder the same thing - I was thinking Davy maybe meant "as much as possible", in that it would look better to slope plane "B" (in the pics) at a slightly steeper rate so that it met plane "A" in the shower corner, and at least maintained an even perimeter between planes A and B (e.g. below), rather than the sort of janky transition between those planes that I showed in the original pics. That does seem like a smart move.

(Then of course, the perimeter would slope up from the drain on plane C, as you suggest.)

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Unread 11-28-2021, 11:23 PM   #12
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I have not done an installation such as you have there, Kyle, but I do know that it's those little side pieces, your Plane B, make things difficult. Seems you'd need at least to have the sides of Plane C a little higher than flat, at least close to the drain, to have any slope at all on the Bs. I still don't see trying to make the entire perimeter of the floor level, though, and having severe slopes at A and B. But maybe that's the only way folks make those linear drains work when they don't fit wall to wall.

Maybe someone who uses the not-wide-enough linear drains will 'splain us both.
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Unread 11-29-2021, 09:50 AM   #13
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Are you using large floor tiles or small ones? Linear drain seem to go hand in hand with large tiles; If you're using small tiles, why not use a conventional drain and preslope versus a linear drain?

For a linear drain, maybe consider shrinking your plane "A" width to about 1/2" so it's almost invisible with the wall tile overlap only? Also, consider shrinking plane "B" width to about 2" so it's negligible? Those changes would more closely match Schluter shower linear drain pans and their intended tile pattern. Their foam pans dont have all those planes either, just one slope plane and a flat drain plane. Here's a photo of the tile pattern for linear drain from schulter: theres no multi plane trapezoidal tile pieces anywhere, just simple cuts. Your design may be more functional vs the style design from Schluter.
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Unread 11-29-2021, 10:48 AM   #14
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Hi CX,

Hmmm, I'm having trouble following you there. I agree that plane C has to be steadily sloped rather than flat (included at the edges), at least!

I'm not sure why you'd need to do something funky to have a slope in plane B, though. The concave valley between B and C would run diagonally, so that it only reaches the perimeter at the point where plane C's slope makes its perimeter height naturally match the perimeter of plane B. I could be misinterpreting your comment, though?

Phil, thanks for the thoughts. That picture is interesting - I would think that you'd get pooling in those 2" portions, given that they're not at all sloped towards the drain! I actually was planning on using larger-format hex tiles, and was hoping that would allow me to plan the layout in a way that would make the diagonal tile breaks less glaring.

Unfortunately, my shower and drain dimensions are more or less set at this point, so I don't think plane B can shrink. In retrospect, I should have done what you say re: plane A - I was (incorrectly) informed by my plumber that that was inadvisable, and regret not pushing back on that. Give some logistics, it wouldn't really make sense from a cost/timeline perspective to reseat the drain, so I think I'll just have to make the best of it this time around.
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Unread 11-29-2021, 06:35 PM   #15
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Kyle, my bad, like the others said, you probably won't be able to run the perimeter level. For some reason I thought the drain was close to the middle of the shower.
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