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Shaklee3 02-08-2021 08:29 PM

Cliff's Master Bathroom Thread
1 Attachment(s)
Hi everyone, after doing a couple small projects over the last few years, it's finally time to tackle the master bath in a 1987 house. It's an upstairs bathroom over 3/4" OSB. My plan, like the smaller bathroom I did a couple years ago, is to do ditra on the OSB with 24x12" ceramic tiles on that. The shower will be 16x4" subways. I've attached a picture of the "before" situation. Some questions before starting:

1) I've pulled up some of the old carpet and noticed some uneven spots. The largest I've seen is about a 1/4" incline over about 4'. At first I was thinking that it's not too bad, but with these large format tiles I think it might be difficult to get it the tiles flat. I don't mind a slight slope since that will likely not be noticeable, but I don't want to risk lippage. I'm leaning towards using SLC to even it out, but I've got a fairly basic question on that: I've seen some videos of people sealing between the joints of the OSB with caulking so nothing goes under it. is that necessary? I haven't fully checked every spot, but it looks like all the cracks are pretty close together and wouldn't leak.

2) For deflection, is there a sure way to check the joist length and height without cutting into the ceiling? I measured from the ceiling of the first floor to the floor of the second floor, and it's slightly shy of 12", which makes me think it's a 10" height. I tried using a stud finder on the ceiling parallel with the direction, but I was getting too many false positives. Is the fact that a bathtub is already up there enough to say it can handle the load?

3) We plan to tear out the built-in bathtub and put a freestanding one where the old one is, but on top of new floor tile. I also want to bring the wall down to just a pony wall where the tile stops currently, and have all glass above that. Since the tub will be in the same spot, I'm assuming it'll be easier to leave the plumbing where it is currently in the wall. Does anyone see a problem with this plan? My only fear is there may be plumbing in that wall that I want to be glass, and that would be much more difficult.

4) For the shower the tentative plan is to do kerdi board on the walls with a mud floor and a kerdi drain. My only fear there is a recent post where someone pointing to a youtube video of a ~8yo kerdi drain that appeared to lose the fleece binding and eventually leaked. Do any of you pros see any reason to avoid those, or was that possibly another problem? My main reason for doing the kerdi drain was avoiding the preslope + liner step, but if there's any uncertainty of a leak, I don't mind the extra work.

That's it for now. Thanks!

John Bridge 02-09-2021 09:00 AM

Hi Cliff, :)

I think once you've removed the carpet you could cut out a section of OSB and discover what you've got to work with in the way of framing and sub-floor.

There is no reason to tile under the new tub.

Shaklee3 02-21-2021 12:43 PM

3 Attachment(s)
Hi all, I removed the tub, and it turns out that the builders of the house cut a hole in two places in the subfloor as shown in the pictures. My guess is that since these are tract housing, they left the option open for the drain to be on either side of the tub. Since there are holes here, I was able to easily measure the deflection. With 11" joists, 10' length, 16" spacing, I get a rating of L/1252, which should be more than enough for ceramic.

The wife decided on a clawfoot tub, so as a result I'll need to get rid of those holes there. I also got an 8' level, and I noticed that the floor is almost perfectly flat across all 8' spans, with at most 1/8" drop in any direction. The only spot that was dipped slightly was near where the old tub was in the picture, and I'm assuming I have to replace those boards anyways, so it shouldn't be a problem.

My questions are: as you can see in the pic, this is 3/4" OSB. Reading through these forums I'm seeing mixed messages on OSB. I see that it should be 1.25" minimum if tile are to be installed, but there was a tub on here before which I'd think is much heavier than tile with live weight. It seems the reason for not using OSB is that when it gets wet, it warps or is not as strong. Is that a problem with a waterproof layer like ditra between the tile and OSB? The adjacent room has laminate directly on the OSB, so I'd prefer not to have an extra 1/2" plywood + ditra + thinset + tile for the transition.

Second question is I want to change the wall behind the shower to be a pony wall with only glass, and no drywall above the pony height. It's not clear to me whether this is load bearing, but I don't think it is because there are no joists below it (you can see it's just copper pipes). However, the fact that they used 6 studs in just the left side and that it runs perpendicular to the joists has me concerned. Do I need someone to look at it? Picture is attached.

ss3964spd 02-22-2021 09:20 AM

Welcome, Cliff,

My first guess is the wall probably isn't load bearing but hard to say from here without seeing the roof structure above. Finish removing the drywall up to the ceiling, if you find a correctly sized header up there you'll have to have a closer look in the attic.

Since it appears your joists are spaced 16" OC your 3/4" OSB should be fine for ceramic. If you wanted natural stone you'd need 1.25" of structural subfloor. If you plan to cover the OSB with Ditra it should protect the OSB just fine as long as the other details, like the shower curb, are appropriately addressed.

Shaklee3 02-22-2021 09:33 AM

Thanks Dan! I actually found a structural engineer that was willing to help via email, and indeed with pictures from the attic showing the truss running parallel to the wall.

I spent the last day or so going through old JB posts to find opinions on OSB, and it's definitely a contentious topic! My takeaways for people possibly reading this post later is:
  1. You cannot tile directly onto OSB. No manufacturer will allow for it
  2. You CAN tile ditra directly to OSB, and it's allowed per Schluter. However, you will not find a thinset, other than Schluter All-Set, that says it's ok for installing on OSB
  3. If installing ditra, most modified thinsets are fine, although you will not find any datasheets that say so

My only worry right now is trying to patch up those two holes with new wood. Since I may as well replace that entire area, is it best to just find some 23/32 plywood and install it with a 1/8" gap next to the OSB? That area would be the wettest area anyways since it'd be right under the tub.

ss3964spd 02-22-2021 10:21 AM

If the trusses are running parallel to that wall it's a pretty safe bet the wall isn't load bearing.

Yeah, I think the TCNA "recommends" plywood instead of OSB but people tile over OSB alla time. Schluter won't recommend a mortar of any kind other than their own. It's not that theirs is magical, it's that they won't test any others. But, directly from their site is this:

What type of thin-set mortar should I use when installing Schluter-DITRA?

The type of thin-set mortar used to install DITRA depends on the substrate material. For example, to set DITRA over plywood or OSB, a modified thin-set mortar meeting the requirements of ANSI A118.11 is used.

Shaklee3 02-23-2021 09:36 AM

Hi all, as the materials are arriving I'm mapping out what to do in my head. As above, this is a bathroom with about 200sqft of flooring that I'm putting 12x24 tiles over, a clawfoot tub, and re-tiling the shower with subway tiles. For the shower pan I'm planning on mud bed with a kerdi drain. Here is my general thinking of the order to do it in:

1) put kerdi board up all sides of shower and do the appropriate patching on all seams.
2) Put kerdi bench in corner and seal per directions
3) lay curb down in a mortar bed and wait for it to dry
4) Lay down a piece of 1/2" plywood over the existing 3/4" OSB large enough for the shower pan (not the curb). This step was because it did not seem common to lay a mud bed on OSB. Is this needed?
4a) Lay 4mil plastic over subfloor with galvanized lath on top
5) Build mud bed and set kerdi drain
6) Apply kerdi membrane after step 5 sets, and also seal all corners and edges.
7) Tile shower floor (I know the order doesn't matter, especially with caulking the joints, but it seems the floor should be strong enough to withstand anything going on above
8) Tile walls of shower
9) Grout (At this point the shower should be done and I can do a soak test while doing the next step)
10) Apply ditra on OSB to floor of bathroom
11) Tile and grout floor

Does this all sounds reasonable?

For thinset I wanted to try to find a modified Mapei product I can get at a big box store since All-Set is $35/bag where I live, and that's going to add over $1000 to the project if I used that. My plan was to use Mapei modified (not sure which) between the OSB and ditra, and mapei unmodified coupling membrane mortar above the ditra.


cx 02-23-2021 10:30 AM

4. There is misunderstanding there. The only negative about OSB in tile work is bonding to it. Perfectly acceptable to install a mortar bed over it with a cleavage membrane and expanded metal lath.

But were that my project, I'd patch the holes in the OSB subfloor and install a second layer of half-inch plywood over the entire area that will be tiled.

4a through 9. I would not even install the drain or consider placing the shower floor or curb until the walls (and ceiling?) were tiled and probably grouted with the exception of the bottom row, or rows depending upon tile size.

I'm not familiar with the MAPEI mortars available at the home centers, but I'd find a decent modified one and use it for the entire project. That would eliminate a Schluter warranty, but I wouldn't expect that to apply in any case.

My opinion; worth price charged.

Shaklee3 02-23-2021 12:07 PM

Thanks CX!


But were that my project, I'd patch the holes in the OSB subfloor and install a second layer of half-inch plywood over the entire area that will be tiled.
My biggest concern is with doing that there will likely be an almost 1" step from the other room which is < 1/4" laminate, and with the plywood + ditra + 2 thinset layers + tile it's going to be very high. I assumed that given I had such a high deflection rating that the OSB being non-ideal isn't as much of an issue because it won't flex as much. With the ditra + kerdi band on some walls that might get water, I also assumed there's no chance water would get to it anyways.


4a through 9. I would not even install the drain or consider placing the shower floor or curb until the walls (and ceiling?) were tiled and probably grouted with the exception of the bottom row, or rows depending upon tile size.
Good idea! I'll do that. The ceiling is not being tiled, so nothing to worry about there.


I'm not familiar with the MAPEI mortars available at the home centers, but I'd find a decent modified one and use it for the entire project. That would eliminate a Schluter warranty, but I wouldn't expect that to apply in any case.
If I'm interpreting this correctly, you are saying the schluter warranty would not apply to what I'm doing? I thought everything here was per their warranty instructions, but maybe I missed something. Mapei has several modified thinsets they just called "porcelain/ceramic mortar" that I figured would be good under the Ditra, and their "uncoupling membrane" mortar for above since that's the only unmodified one I've seen at home stores. This is from the blue store (not sure what the nickname of that one is around here).

MAPEI - Technical Service 02-23-2021 12:26 PM


It may be a good idea to give Schluter a call just to make sure you are abiding by all their terms to keep the warranty intact. Unless they have recently changed course, our Porcelain Tile Mortar (modified) should be suitable below the Ditra, and Uncoupling Membrane Mortar (unmodified) should be suitable above the Ditra to install tile.

cx 02-23-2021 12:28 PM

Your joist deflection has nothing at all to do with the rigidity of your subfloor, Cliff. Two separate and distinct issues. The added half-inch subfloor is to reduce the between-joist deflection and compensate for the patched first layer. Entirely up to you what you are comfortable tiling over.

I'm saying that any failure you might have with the Schluter products will be an installation error and not a product liability issue. See my warranty information below.

My opinion; worth price charged.

Shaklee3 02-23-2021 01:15 PM

Thanks CX and Holden, I called Schluter and they said that thinset is fine since it meets A118.4 and ANSI A118.11. CX, thanks for the info on the difference between the rigity and deflection.

Shaklee3 03-01-2021 12:15 PM

3 Attachment(s)
Hi everyone, I've made a bit more progress on the demolition and I'm almost ready to start the walls on the shower. I had a couple more questions that came up during the demo.

1) as you can see I cut the wall that was previously there to be the height of a pony wall now. obviously it's nowhere near as sturdy as it was before going all the way to the ceiling. I've read in a few places that I should brace the pony wall underneath the floor to make it sturdy enough for a half tile wall plus glass on top. Is that needed for this size wall or is there any chance I wouldn't have to open up the sub floor?.

2) attached is a picture of the 32-year-old drain from the original shower. The screws on top are entirely stripped, so I don't know if it's worth trying to remove it as is. Or just getting a view from below and cutting it. What would be best given the state of the drain?


cx 03-01-2021 12:27 PM

Cliff, that drain hasta come out to install your Kerdi drain, so I'm not sure I understand the question. Probably wanna replace the trap as well.

The short wall hasta be braced sufficiently for the intended use. Running one or more of the studs down through the subflooring and attaching to a floor joist is one good method of doing that. Were it my project I'd most likely remove that wall entirely and start over, building what I needed there.

My opinion; worth price charged.

Shaklee3 03-01-2021 12:32 PM

Thanks cx! On the drain question, I was mostly wondering if I should just cut out the grate, and just use One of those internal pipe saw drill bits to cut the pipe from the top. From your description it sounds like I'd probably be better off just opening up the ceiling and doing it from there.

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