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Old 03-27-2019, 06:10 PM   #1
gslenk
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Small Bathroom Remodel

New here, and having a bit of indecision about tile floor substrate outside of the shower. Going with a Wedi shower. Currently deciding Ditra or Wedi board (1/4" or 1/2") under the tile outside the shower. Anyone do either/both or have a preference? After corresponding with a Wedi rep, I think my options are:

1) Ditra with sheet membrane to tie it into the wedi curb (kerdi band or wedi subliner dry +thinset/etc). Wedi sealant is not spec'd to join ditra to wedi for a water tight seal, and I suspect kerdi-fix isn't either.
2) Wedi board either 1/4" or 1/2" thick, and just use the wedi sealant between board and curb. Not too sure if/how this compares to Ditra as far as decoupling performance.

Haven't really found much about a Wedi-Ditra combo, most who swear by Ditra seem to be using Kerdi, or a shower/tub receptor. Have only found a minimal amount of people using Wedi under tile. Although I'd prefer to keep it all Wedi, there doesn't appear to be much real world support/examples of this being successful, especially in comparison to Ditra.

Some more details if it helps:

Remodeling a small 5'x6' master bathroom. 1970's original construction. Removed what appeared to be a 1/2"+ thick mud/mortar/lathe bed under 1"x1" square tiles that were probably as old as the house. I didn't notice any grout/tile cracks, but didn't really look too carefully either.

The subfloor is on 2x8's spaced 16" oc. The span is 10ft from load bearing wall to load bearing wall underneath. Deflecto says I am at 480/L, and I do plan on ceramic/porcelain tile at least 6"x6", and up to 12"x24" (haven't ironed that out yet).

Subfloor appears to be 1/2" ply with 3/8" ply on top, seams are staggered. Total actual thickness measured a little over 3/4" and about 1/8th" thicker than nominal 3/4" plywood for comparison. I wasn't too precise about verifying this, but I am pretty sure it isn't 5/8" + 3/8" as that should be closer to 1" thick. Could possibly be 5/8" + 1/4". All I know for sure is the overall thickness is over 3/4", but not quite 1" thick.

Bonus question(s): Does my subfloor sound good or should I be ripping the top layer off for a thicker subfloor? Any other pointers or considerations? I am not sure what is standard practice in my situation, but it seems I am on the low side of "acceptable" subfloor thickness.
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Old 03-27-2019, 06:26 PM   #2
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I would use ditra outside and tie to the curb using subliner or kerdi.
It makes a big difference on what your layers of subfloor are, you need to be certain before installing over what you have.
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Old 03-28-2019, 02:33 PM   #3
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I'm no pro, just relating what we did: a few years ago when we redid 2 bathrooms w/Wedi showers, we put 1/2" Wedi board on the floor, then under-tile heating, then tile. The foam board helped act as a little bit of insulation to try heating the tile instead of below the floor. I know you didn't mention under tile heat, but that was part of our consideration.
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Old 03-28-2019, 04:33 PM   #4
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So I measured more precisely:
Bottom layer: 5-ply, perpendicular to joists, 15/32. Seems like this is 1/2" nominal. One bottom layer joint under the entire bathroom (2 boards).
Top layer: 3 ply, perpendicular to joists, joints staggered to bottom layer. 11/32", so 3/8 nominal.

Total thickness 13/16" or 6.5/8". Nail spacing on top layer ranges from 4" to 8".

Practically no wood, like chips, or separation/degradation of the top layer, came up when lifting the mortar/mud bed with tile.

Top layer might be possible to remove, however it passes in one piece under a wall that is inside the bathroom (the 2x3 shower water fixture wall), and that would make removal and replacement complicated but not impossible.

The bottom layer passes under adjacent rooms, and seems to be impossible to replace. Either that or I am unfamiliar with common methods.

I will be visiting a tile/wedi supplier tomorrow hopefully and might get some questions answered. But it still would be nice to at least have a goal, and or background knowledge beforehand.
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Old 03-28-2019, 04:36 PM   #5
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Andy, if you can recall, about what size tile did you use for the flooring, and do you know anything about how well it is holding up? Also, do you recall any details on the subfloor like joist size/spacing and subfloor composition/thickness?

It would certainly put me more at ease if I were to go the wedi route.

Was the under tile heating using Ditra heat or wires embedded in the thinset?
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Old 03-28-2019, 05:20 PM   #6
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Will it fail if left alone? Hard to say, but if I showed up to do an install I would remove the top layer and install a layer of 3/4" exterior glue plywood. Your 15/32"+ 3/8" isn't optimal.
The first layer is supposed to be tounge and groove, and the absolute minimum would be 5/8" for that first layer.
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Old 03-28-2019, 05:45 PM   #7
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Ryan,
So your plan would be to remove the shower wall/framing, water copper supply pipes, remove the top subfloor layer, replace with 3/4", rebuild shower wall, add copper pipes back in (I'm leaning towards removing the existing twin valves anyway for a single mixing valve). Then still use Ditra on top? Or would a sheet style membrane suffice to keep overall thickness down, since the floor will be much sturdier?

Like I said, it will be more work for me, but not quite impossible. I would feel comfortable with the finished surface going up to about 1" above the first layer of plywood since that's about the height of the finished carpet floor in the master bedroom. And I imagine a threshold could tolerate about a 1/4" differential up or down ad still be negligible.
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Old 03-28-2019, 06:08 PM   #8
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I do not remember if the bathroom on the 2nd floor had any plywood added or changed. The joists are engineered using 2x4s at 24" oc. The tile is 12" square and the under tile heating is wires embedded in the thinset, from Warmly Yours.

It was done in 2011 and has lasted fine so far w/o any tile or grout issues. I don't think I knew about this forum at that time, but the store where I bought the Wedi provided a lot of info similar to what I see here & I forced the contractor to change his normal style of work to get things properly waterproofed, use of real thinset (not premixed), etc.

The other one is on a slab so isn't comparable.
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Old 03-29-2019, 11:09 AM   #9
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Attached are some pictures of the area, if anyone wants to give some pointers for the plan of attack. Just came back form the supplier, they seemed to agree that although wedi may be an ok option for the bathroom floor, ditra would likely be better since it is over a wood subfloor. They recommended 1+1/4" total thickness, which seems to mesh with the idea of throwing 3/4" on top of the 1/2".
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Old 03-29-2019, 01:24 PM   #10
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The 1.25" inch thing has been a misunderstanding for some time. I think it goes back to when tiling directly to a ply subfloor was more common. That requires a minimum thickness of 1.25". When installing natural stone you have to meet certain parameters as well. In your case you will be forgetting that the 1/2" is even there. It is doing nothing for you really. If it wasnt there you could get away with a single layer of tounge and groove plywood.
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Old 03-30-2019, 10:49 AM   #11
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Got it. Would you cut around that small wall or remove and rebuild it?
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Old 03-30-2019, 03:25 PM   #12
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Cutting around it is fine, I assume you're just removing the top layer of 3/8?
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Old 03-30-2019, 03:53 PM   #13
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Yes, remove 3/8 top layer, add 3/4 as the new top layer.

The 3/8 has 1/4" spacing around the perimeter of the bathroom, but runs continuously under the shower plumbing wall in the middle of the bathroom.

I am pretty sure the joists are located roughly where the red lines are.

I was wondering if the extra work to remove and rebuild the plumbing wall was worth any potential floor structural benefit?

If not, then this would be a very simple task.

The perimeter walls/studs lie on the 1/2" bottom layer, and I'm inclined to think that would be much more complicated to remove.
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Old 04-01-2019, 06:28 PM   #14
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The 3/8th's top layer is only nailed down, it will come up easy. I think it has been implied to not touch the shower plumbing wall. A voice in my head is saying it will be better to remove/replace the shower wall so that the 3/4" will lay as a continuous piece across joist spans.

I plan to go with 3/4" Advantech OSB, Mapei ultraflex 2, ditra, ditra set, tile. May substitute Mapei for versabond.

Anyway, any final words on whether to remove the shower wall or not?
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Old 04-01-2019, 07:16 PM   #15
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Lenny, if you're fixin' to change out that plumbing (good idea) anyway, I'd remove and replace the wall.

My opinion; worth price charged.
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