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Unread 04-18-2014, 04:46 PM   #1
Dtraster
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Kerdi/Ditra/Wet Room/thinset advice needed!!

I had planned to use a traditional approach to the floor of a bathroom/shower/wet room: pre-slope; membrane; top mud bed, etc. I’ve been lurking and based on the advice to others here, I’m now thinking that I should use the Kerdi system and wanted to get thoughts from the group.

The bathroom is 8’ by 6’1” and the "preliminary plan" is attached. The main changes are that the door will be 30” wide not 36” and there won’t be a shower head on the wall opposite the door.

I have completely replaced the floor, including the floor joists. It is now double 2x6 joists running parallel to the exterior wall (with the window) and a ¾” tongue and grove plywood floor glued and screwed to the floor joists. I ran the Deflectolator and have a deflection of L/1679. There won’t be a curb. The entire room will be a wet room.

My first question is about Ditra. I understand the need for a disconnect between the tile and the substrate. I also understand that one does not usually use Ditra in a smaller shower. But given the relatively large size of the room, should I use the Ditra to avoid having future problems with the tile?

If I use Ditra, I think it goes like this: mud preslope, thinset; Ditra, thinset, Kerdi, thinset, tile; grout. Is this correct?

If I use Ditra, how does that change the area around the Kerdi drain?

Under either method, how thin (thick) is thinset? A photo showing the floor in the bathroom and in the hallway where the door will be is attached.

I haven’t picked a floor tile yet and I realize that I have to do that before I can take the next step. I want the top of the finished tile floor to be at the same elevation as the top of the hardwood floor in the hallway—4 and 9/16 in the photo. Right?

If I use the Ditra, to figure out the elevation of the top of the mud preslope at the doorway, I need to know the thickness of each layer about it: thinset, Ditra, thinset, Kerdi, thinset, and the tile. Once I pick the tile, I can measure that and I can measure the Ditra. I assume that the Kerdi’s thickness is negligible, but that is an assumption. So how thick (thin) is the thinset/Ditra/thinset/Kerdi/thinset?
Attached Images
File Type: pdf 2013-08-01 A1 Preliminary Plan zoom.pdf (71.8 KB, 150 views)
File Type: pdf IMG_0770 reduced 1.pdf (175.9 KB, 120 views)
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Unread 04-18-2014, 04:58 PM   #2
jadnashua
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Ditra is not intended to be used in the shower...use Kerdi there, and get a copy of their Kerdi SHower Handbook http://www.schluter.com/media/shower...v=201404071927

If you wish to make the entire room a wet room, use Ditra outside of the 'shower' pan area on the floor. If you cover the seams in Ditra with either the Kerdi membrane or use Kerdiband on them, it becomes waterproof as well. If this is a wetroom, you'd use Kerdi on the walls, or at least run Kerdiband along the edges up onto the wall.

Ditra is not specified for use in a shower. It is an uncoupling membrane that can be made waterproof, but it is not designed to be bonded to the Kerdi shower drain or rated for use as a shower waterproofing.
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Unread 04-18-2014, 05:16 PM   #3
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Thank you for the reply.

I have some questions though. You say to "use Ditra outside of the 'shower' pan area on the floor" but it's a wet room. There isn't any part of the room that's going to be "outside" of the "shower pan area" at least as I have thought of it before now. Obviously, the shower is in the corner and water and overspray won't be everywhere, but there are no real barriers.

Before, I had planned to pour the mud preslope sloping from every point in the entire room to the floor drain. I was then going to cover the entire floor and up the walls with 40 mil membrane and so on. That way if the sink or the toilet overflowed, everything would run to the shower drain. (I needed a level area right under the toilet though.)

If I use Kerdi, I will cover the entire floor, up the two walls in the corner where the shower head is, and part way up the other two walls.

My real question is if I don't use the Ditra "uncoupling membrane" how do I uncouple the tile?
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Unread 04-18-2014, 06:11 PM   #4
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If you are mudding the whole floor, then why not kerdi the whole floor and don't use ditra. The mud bed uncouples you from the plywood floor, right? (it is just an oversized shower pan at that point.)
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Unread 04-18-2014, 07:15 PM   #5
jadnashua
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You could use Kerdi on the whole room, depending on how it is sloped and built. As to the toilet, in many situations, to avoid issues with trying to get the toilet to fit flat and steady, they often use a wall-hung toilet. Makes it easy to even take a hose and wash down the whole floor without issues.

If you're planning on plywood and tiling over it rather than a mudbed in part of the room, you'd need either some cbu or this would be a good idea to use Ditra, but Kerdi is NOT designed to go over wood, there, you need either a mudbed or one of their foam pans.

Read the SHower handbook, then come back and ask questions.
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Unread 04-18-2014, 09:42 PM   #6
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The reason for my question is that I'm concerned about uncoupling tile from the substrate. The entire plywood floor will be covered with several inches of mud. The entire mud bed will be covered with Kerdi. How do I uncouple the tile from the mud substrate? Is Kerdi an uncoupling membrane?


"Nowadays we have thinset that bonds the tile directly to the substrate (substrate is a fancy word for concrete or cement board, etc). The problem is that the concrete will expand and contract at a different rate than the tile. This puts stress on the tile installation and can lead to cracking. Even with the flexibility that today’s thinsets have, they still are no substitute for the uncoupling layers of old."
http://www.tigermountaintile.com/201...ing-membranes/
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Unread 04-18-2014, 10:10 PM   #7
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Welcome, David.

If you'll visit our FAQ you'll find a brief tutorial on how to post and attribute quotes here on the forums. Very simple once you see it.

Even if Ditra were the best product for your application, you'll find that you'll have a very difficult time making it conform to the compound slopes you'll have in that room. If it's all to be deck mud and if it's all to be a wet area, I agree with those who are recommending you simply use Kerdi in the entire room, floor and walls. But, considering that you intend to use a traditional pre-slope, liner, and mud bed, using Kerdi isn't gonna be an option anyway. You'll need to choose one method or the other and in your application I'd recommend the bonded waterproofing membrane (Kerdi in your case).

The second thing you'll find is a real problem trying to install over those compound slopes is your ceramic tile unless you are planning to use some pretty small mosaics.

If you're serious about closely matching the height of adjacent flooring, I recommend you make up some test samples using the actual products you intend to use installed with your setting materials and technique. You can guess at the various thicknesses, but the only way you'll get an accurate measurement is to build the sample and measure it. Once you find the thickness of everything that goes over the deck mud you should have no trouble making the deck mud the correct height at the transition area and work from there to the drain.

I'm also gonna question your joist deflection calculation. You have doubled 2x6 joists of Doug Fir or SYP on 16" centers and they are supported from below every six feet or less? Is that correct?

My opinion; worth price charged.
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Unread 04-18-2014, 10:20 PM   #8
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Kerdi will work fine over your mudbed - you'll need either the standard round Kerdi drain (or more than one), or the use of a linear drain(s). The linear drains come in multiple widths, and may make the room design easier since you could make the whole room one flat, sloped floor. Making the floor for a central drain means multiple slopes closer to a bowl, and can be much more of a problem getting all of the tile to sit flat without lippage.

Over a wooden subfloor, a reinforced mudbed essentially floats, so you do not need an uncoupling membrane. A mudbed bonded to a slab is a different thing.
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Unread 04-19-2014, 09:00 AM   #9
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Thank you so much. This is very helpful.

A few clarifications:

1. Traditional method OR Kerdi, not AND Kerdi. I don't plan to use both Kerdi and the traditional method. I was going to use the traditional method but now am seriously considering Kerdi instead.

2. Hanging Toilet. I like the idea of a hanging toilet and thought about that before but didn't follow up before roughing in the drain line for the toilet. Could redo that and will give that some thought.

3. Mosaic Tiles. When I started, I wanted to use larger tiles on the floor but with the compound slope I have to use pretty small mosaic tiles or change the slope and go with a linear drain. I like the look of a linear drain and spent some time studying those. I ultimately concluded that I didn't want to spend the extra money. So, I definitely plan to use small mosaic tiles on the floor.

4. Floor Height. Should I not be “serious” about closely matching the height of the tile to the adjacent floor? I’ve thought about a marble threshold and like that look which would give me a little bit of flexibility. I have always just assumed that there needed to be a close match. I’ll do some more lurking on that issue. And I have The Kerdi Showerbook, Tile Bathroom Remodeling: From Tearout to Trimout, Parts 1 and 2, and the Kerdi Shower Handbook. Haven’t read all four cover to cover, but maybe the answer is in one of those resources.

5. Either way, I will definitely do some test mock ups. Great suggestion.

6. Deflection. I have doubled 2x6 floor joists. The span is just under 6’. They were purchased new from Menard’s or Lowe’s immediately before installation. They say “Hem Fir” so they’re not “Douglas Fir or SYP” but I had assumed that they were comparable since I did not buy the cheapest lumber that was available. When I ran the calculator again this morning, “Unknown wood, but in good condition” gives me a deflection of 1284. And even “Unknown wood, kind of cracky or knotty” (which is not an accurate description) results in a deflection of 987.

Thanks again for the great advice. You guys are awesome.
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Unread 04-19-2014, 02:51 PM   #10
jadnashua
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Personally, knowing that neither tile nor grout is waterproof, putting the required waterproofing layer immediately underneath the tile and thinset just makes so much more sense than a conventional shower build. Deckmud is porous for a reason...water will get down there, a little at a timer with each shower, and it needs that porosity to allow it to percolate to the drain. I much prefer to contain it right underneath the tile - you can still use deckmud to shape the surface, but then it is only a support structure, not also a wet media. There's more than one surface applied membrane, both in sheets and liquids...I just happen to prefer Kerdi. Properly installed, any of them can work. The ease of installation varies, and the reliability of the result can vary based on learning curve, but Kerdi is not that hard to install once you understand a few basic concepts.
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