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Unread 02-16-2021, 07:00 PM   #1
GaylonR
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Low curb shower on post-tension slab

I’ve used John’s Kerdi Shower Book and the forum to guide me through a previous tiled shower project but wanted to seek advice from the pros on this one.

For a master bath update, I’ll be removing an existing plastic shower pan and cultured marble from 2 walls, the other 2 walls of the shower will be glass with an opening (no door). See my simple sketch.

The new Kerdi tiled shower will be in the same location but increasing the width by 6” to about 5’ x 4’. Planning to move the floor drain slightly to keep it centered. I’m working over a post-tensioned slab and the area around the drain was formed separately from the slab when it was poured so the trap and drain could be adjusted to align to the plastic shower pan at installation. So there is about a 12” x 12” area I can move the drain around without worrying about floor cables.

My plan is to taper the slab for the ¼” per ft slope. So total drop will be about 5/8+” over the 2 ½ ft to the center drain.

I would like a very low curb/threshold. Instead of a conventional tiled curb, I was considering something like a quartz or granite threshold that’s maybe 1 ½” - 2“ thick and 3”- 4” wide set in thinset directly on the floor Kerdi. Since the Kerdi overlap with the quartz is well over 2” and the quartz is impervious to water, it would act as a curb. Depending on floor tile thickness, slope and set height of my quartz, I might be a bit less than the standard 2” from top of drain to top of curb, but close. I would extend the Kerdi under the bathroom floor tile at the shower entrance for extra splash protection. This seems pretty simple and would provide a clean minimal design. Do you think this will work?

I know there will be more questions as I get into the details…..Thanks for your advice and suggestions.
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Unread 02-17-2021, 09:40 AM   #2
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Hi Gaylon.

Things I'll offer up for consideration.

1. Will this be permitted? AHJ may have something to say about curb height relative to drain grate.

2. Technically, Kerdi is not designed for floors, but just because it's not something they recommend doesn't mean it won't work. You won't have their blessing and should recognize that.

3. If rain head is the only shower, you may be OK with no door. Anything with a directed spray I think you'll be surprised by how much water gets outside the shower. No door means no ability to "warm up" the space as well. Perhaps you could experiment with existing by just leaving door open during shower.

The trend towards doorless showers has its own baggage IMHO, and like many trends, isn't always rooted in functionality. Curbs kinda the same. Is your reason for low curb appearance or functionality?
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Unread 02-17-2021, 10:56 AM   #3
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Hi Carbidetooth, thanks so much for your response. Here were my thoughts....

1. I'm presently not planning on a permit. I checked with the local code people and they said if I wasn't relocating the drain or replacing the fixtures inside the wall, they would not require one. Tiling would not require one. I had asked about curb height and they provided code references citing the 2" rule. I somewhat rationalize the lower curb being OK by comparing the volume of my receptor (20 sq ft) at maybe 1.5" to a smaller one with a 2" hgt. I know the rules are there for a reason, but a one size fits all approach isn't necessarily right.

2. Good point, hadn't thought of it that way. It is under the shower floor and I just thought extending it for a couple ft for extra splash protection out the end where the opening is would be good. The bath floor is 20x20 porcelain (existing) that I will be patching into. I wasn't too concerned as we will always have a bath mat there when showering.

3. The no door thing has been a discussion here. Our previous home had a door-less shower and my wife did not always like the cooler aspect, but she is supporting the no door design (today). We really like not having it in the way, less to wipe down after each shower, clean simple look, etc. If we did the door, it probably should, or may be required to be, on the outside wall where the short glass panel is shown (wish we could hang a door on the glass panel). That crowds the space where we were hoping to have a bench. As far as splashing goes, there is the rain head, and that other thing I attempted to draw on the adjoining wall is a hand held on a sliding bar. I do have some concern that there may be some splash out, but I see many photos of similar setups (but maybe they are dysfunctional too)!

The reason for the low curb is probably some of both appearance and functionality. We see lots of the no door/walk in showers in our area and folks like them. Neither of us has issues stepping over a curb, but know that day will come. And it would look nice. I'm thinking I will get the quartz cut to width and polished at a granite/stone fabricator. I could have them build up the quartz thickness a bit more so that I get 2" of height above the drain (probably just more $$).
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Unread 02-17-2021, 11:49 AM   #4
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My mind's eye likes what it sees with regard to the curb, but the pragmatist in me wonders how well one could create a watertight pan. Think flood test or backed up drain. I dislike the idea of caulk being the only line of defense in spite of it being done frequently, but I'm fussy that way. My go-to curb now is 2" Kerdi Board on edge, just high enough to hit the 2" requirement.

There would be some tile fitting puzzles around curb itself as everything is exposed and not buried under adjacent tile.

Yet another consideration would be alignment of replaced floor tile with the Kerdi build-up underneath.

You certainly can have a door hinge on glass, but of course the fixed panel needs to be firmly attached to wall and/or ceiling. You may want to consult local glass folks and see what they might offer for advice. Fortunately, I have a good one nearby.

Coated glass has become my glass supplier's standard for showers, with traditional glass being the lower cost option. Clients report easier care, but it's still a piece of glass. Another option is patterned glass, which doesn't spot any less but doesn't show as much. I'm also a fan of using surfactants such as Clean Shower. Again, clients report greater ease of cleaning.
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Unread 02-17-2021, 08:34 PM   #5
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1. I was thinking that the Kerdi on the floor, extending under the quartz threshold 3+ inches, bonded to the Kerdi with thinset would be a water tight joint. Maybe not?
2. Yes, the floor tile would need to be cleanly cut and set to edge of the quartz. It will be visible.
3. You are correct that there could be a Kerdi build up issue extending out to the existing floor. I won’t know for sure until I see how thick the thinset is under the tile when I pull out the old shower.
4. A trip to the glass shop to understand all the options is on my list before we commit to the design.
5. Do you think that tapering the existing slab to the drain will be extremely ugly? My thought was to use a diamond concrete grinding cup wheel on the angle grinder. Worst case is making very shallow slits and chiseling out where the most material has to be removed closest to the drain.

Thanks
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Unread 02-18-2021, 08:48 AM   #6
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1. Gaylon, I was thinking specifically of vertical intersection of curb to wall. Typically that's a substrate wrapped with Kerdi that creates the pan, but you've got a kinda different scenario.

2. Yep. Straight, crisp, fitted cuts will be the order of the day.

3. May be a deal breaker. I guess you don't know till that point. Plan B probably a good idea.

4. Good idea. Some are better than others, but most all have glass cut and tempered by one of a few big guns who specialize in such. Good templates/drawings and installation are the key local elements.

5. I think that's a great idea on paper, but execution may a dusty, miserable chore. Having local concrete cutters cut and chip out a rectangle, then you construct a deck mud receptor to be Kerdi'd might be worth a look. I should mention I have little experience with homes on slabs. Wood framed, suspended floors are the norm around here. Others on the forum may be better resources.
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Unread 02-18-2021, 10:27 AM   #7
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When I read "post tension slab" my very first though was before I attempted any modification to it I'd want to know, very precisely, where and now deep those cables are.

But assuming they won't interfere, yes, grinding out a shallow bowl will probably be as ugly as you envision it, and creating a consistent, straight slope will be tedious indeed.

I feel like that's alotta of work only to still end up with a curb, although one somewhat less tall.

And you need more than 5/8". The 1/4" per foot slope should be calculated from the corners of the pan. You also need to account for the thicknesses of mortar+Kerdi+mortar+tile.

Being an owner/operator of a doorless (and curbless) shower I fully support not having a door for the reasons you mention. Given the 5' length of the shower I don't think you'll have much of an issue with water splashing out of the opening, certainly not enough that a bath mat won't easily take care of. Yeah, no door will make it colder, consider where you can hang towels so they are accessible from within the enclosure if possible.

Consider where to put the shower valve and diverter so they are within reach while in the shower, but also so you can reach them without getting wet. Given you are in AZ perhaps you can locate them in that long wall.
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Unread 02-18-2021, 02:48 PM   #8
GaylonR
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Hi Dan, Thanks for your evaluation.

I really wish it was a normal concrete slab and then I could just cut a chunk out and lower the floor, put in a linear drain and probably even go curbless, stepping down into the shower. If I build up a mud deck for slope, I end up with a substantial curb that I was hoping to avoid. So I may suffer the consequences of attempting to taper the top of the slab.

1. Has anyone used a product that can be tapered thin to establish the slope (I’ve seen Rapid Set has some things like that)? That way I wouldn’t have to go 2” – 3” thick with deck mud. Since I’m going over it with Kerdi, it only has to be solid and bond well.

You are right about the taper depth, I forgot the darn diagonal is the farthest point! So at least ¾” depth needed. I’ve been told if you keep it less than an inch into the slab you’re OK. And good points about the valve/diverter location. We’ll discuss that here.

But Peter is right about the 2 corner joints where the threshold would meet the wall. They would only be waterproof if you were to accept the Kerdi to porcelain wall tile joint and then the end of the quartz threshold joint to that porcelain. Probably not a real sound plan, any movement in those joints and there would be leaking for sure.

I may have to throw in the towel and go with the idea of still tapering the slab and then add a small curb (like a concrete brick or paver slice) to wrap the Kerdi up. Then tile over it and on top of it (or maybe a stone top). If I find that tapering the slab is a nightmare, maybe plan B would be adding a slope on top with a high strength thin concrete product and increase the curb height as needed.

I’ll think about those options and see if anyone else has additional thoughts. Thanks again for your thoughts on this.
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