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otrex 01-30-2019 11:57 PM

Help Designing a Shower Floor
(posting here based upon advice from another forum)

Hello everyone,

My first time on this forum, and it seems there are a few people who may be able to help me with a shower I am designing.

I live in a 1940s house and it has a tiny bathroom (4.5' x 9') with the door on the short side and a bath/shower at the far end. Recently this shower has started to get a bit old and some tile is starting to look pretty shabby and so we are updating it.

Complicating the situation is that someone about 30 years ago cut a large U-shape into one of the floor joist to allow the toilet drain to pass through, and then didn't bother to buttress that spot, meaning the floor has sunk quite noticeably there over time. When we bought the house the first thing I did was to put a couple of jack posts under this spot. I cannot push the house back up very far, but I did halt the sinking.

I should note here that underneath this shower is the crawl-space which features dirt. So fortunately any leaks downward from our current shower are not ruining any space below.

For the shower, we want it to be a curbless walk-in shower with the same porcelain tile all the way through to maximize the visual size of the space. We're adding clarity 10 glass to divide the shower from the rest of the bathroom, but there is no curb or other solid distraction to make the bathroom smaller (currently the shower curtain, when closed, makes the bathroom look like a phone booth).

All the options for structure are clear to me - we are remaking the entire shower area with Kerdi Board - with one exception - the shower floor.

I have been reading online about the importance on a reasonable slope and the reno provides an interesting opportunity. I am cutting the entire subfloor out right down to the joists and then will sister new joists onto the existing ones. In fact, whereas the old joist are 2x8 , I plan on using 2x10 for the sistered ones in order to allow me to make the above subfloor nice and level again. However, in the shower area, I plan on using a 48" linear drain along the back short wall, and then tilting the shower floor in that direction, meaning I don't need to have multiple angles to the drain - just one slope will ensure all the water goes to the drain. This is also important because I am using tile which is 6" x 3 feet, so I'll simply place these perpendicular to the joist and then having only one slope means this large format tile can be used.

Because of this desired setup, I am simply going to have that portion of the subfloor tilt using the new joist at between 1/4" and 1/2" per foot. Since the shower is no more than 3' wide (if that) this makes for a very low total drop from the highest spot to the lowest.

Now, keep in mind that I wanted this to be a curbless walk-in shower, so this presents a problem. If I follow the traditional 2-3" dry pack mortar plus liner etc setup I end up having a shower pan thicker than the floor outside the shower meaning a step up to get into the shower.

Therefore, I am wondering a couple of things:

i) If I am making the correct slope in the subfloor, can I avoid the mud bed completely? I need to note here that in addition to the new floor joist I will also be building much more substantial support for all the joist to reduce any bouncing.

ii) If I do skip the mud bed, I am thinking of having the subfloor made from 5/8s plywood which is then thinset and then 1/2" cement board applied (with mesh in between for strength/grab). I should note here that my joist are on 16" centers, hence the selection of 5/8ths. I would go with 3/4" if it was wider than that or if I was using a lot of heavy dry-pack mortar, but this proposed setup, I think, will be quite a bit lighter.

Then, in order to have a decoupling substrate for the large-format tile (so it doesn't crack) I would then mortar Ditra on top (since I'm using it everywhere else on the bathroom floor). I have spoken with Schluter and they say Ditra should not be directly-tiled in this manner, and so I propose properly thinset mortaring the Ditra as if I was getting ready to tile, but then applying Kerdi for a nice smooth (if not a bit redundant) finish). Then I would tile on top of the Kerdi as per normal Kerdi procedure.

The goal is to allow for a nice stiff floor but also to have the decoupling advantage that Ditra offers as a substrate (since their normal styrofoam pans are not available in the size I would need). From there it's just a matter of ensuring all the Kerdi is properly sealed to the edges/drain/Kerdi Board walls.

Does anyone see a problem with constructing the "shower pan" this way. Is there anything I can delete from the pan layers?

iii) If this doesn't work, how does one build a completely threshold-free shower without having a shower subfloor portion which is substantially lower than the floor next to it?

I realize that this is not standard shower pan procedure, and I have read a lot in the last few days. However, I am hoping that this still presents a valid way to maintain waterproofing while still keeping the large format tile from cracking.

In any case, I will appreciate any feedback you can provide, and am happy to answer any questions if I have left something out.

cx 01-31-2019 10:53 AM

Welcome, Kevin. :)

Originally Posted by Kevin

Since you have jack posts under there I would recommend you give'em a quarter turn every few days until you've got the joists reasonably close to where they should be. Like with planting trees, the best time to do this was 20 years ago. Second best time is today.

When you sister in your new joists, you want to make the tops in plane to a very close tolerance. Your large format tiles don't give a rat's patooti whether the floor is level, but they care a great deal about flat. The tile industry standard is no deviation from intended plane of more than 1/8th" in ten feet nor 1/16th" in any two feet. That's a very, very flat floor and you'll be glad to have it come time to set those long tiles.

Originally Posted by Kevin
I plan on using a 48" linear drain along the back short wall,..

Perhaps you could post a drawing of your plan? From your description I would have presumed the back wall of your shower to be the longest wall with two shorter walls at each end and your glass wall/door parallel to the back wall.

Perhaps I'm not understanding the dilemma with the floor. Actually, I'm sure I'm not understanding. You plan to put the drain against and parallel to the back wall of the shower? That would have the shower floor slope down from the entry to the back wall? Let's find out if that's the plan before I go any further.

In any case, I would recommend you drop the entire shower footprint sufficiently to make a sloped floor of deck mud from a height allowing you to match the bathroom floor height and sloping to your drain. That would allow you to exactly match heights with only your Kerdi on the shower floor lapping over the edge of the Ditra on your bathroom floor. Not sure why you're concerned with the weight of a deck mud substrate, but it's a non-issue. If your floor structure won't support the necessary mud, you've got another, more serious issue.

For subflooring I would recommend nothing less than nominal 3/4" plywood. Your suggested 5/8ths" plywood is the absolute minimum recommended by any substrate manufacturer. I would personally not consider tiling over such a single layer subfloor under any circumstance. As for the 1/2" CBU, all manufacturers of such products recommend their thinner panels, usually 1/4", for floor tiling. There is no advantage to the thicker CBU unless you simply want to raise the height of your tile installation.

And I'm not sure where the CBU fits into your plan if you have planned to use Ditra as your substrate on the bathroom floor.

I'll stop there 'till we get some clarification on the plan.

My opinion; worth price charged.

otrex 02-01-2019 03:40 AM

1 Attachment(s)
Thank you for your response. Yes, I can certainly shed a bit more light on this build.

For the jack posts and floor, I don't see any reasonable way to get the floor back to its original level without seriously damaging the drywall and other rooms above. It's a 1 1/2 story with two bathrooms (this is one of them) right on top of the most sagged part, and while I would say the drop is no more than about 4" in the worst part, every time I turn the jack posts you hear quite a bit more cracking and substantially more resistance. To complicate matters, some of the upstairs bathroom was built/tiled taking that sag into account, so the more I push the greater chance of having to reno another bathroom. It is still improved from when we bought the house, but I suspect I'll have to cease pushing up very soon and just correct the subfloor as I reno each room.

Regarding the bathroom layout, I'm attaching a really lame sketch of the current and planned layout.

Yes, the plan is to have the drain at the back wall, perpendicular to the spray of the shower head and opposite the main door to the bathroom. The main reason for this is that the 6" x 3' tile will run across the floor relative to the main door (across the short span, matching the direction of the planks throughout the house).

What this means for the tile is at the point where the shower glass splits the room into two portions, this is the point at which I would start sloping the subfloor. That means about a 3' wide shower with 1/4" - 1/2" of slope to the back wall where the linear drain picks up the water (I realize now I'm going to need the drain to run wall-to-wall).

Yes, the CBU was simply to add some depth to the custom shower pan so as to allow the linear drain to have a proper height which to meet the kerdi. I had not planned on using it outside the shower on the bathroom floor since, as you noted, Ditra would be there.

Something I had not considered would be cutting the subfloor (as before) and then taking 2-3" off the joist in the shower pan area all the way to the wall and then buttressing this all with new sistered joist and posts. Is that the traditional way to create a curbless walk-in shower? If I recall from my framing days, such stripping of the joist can only be about 1/3 the way through before it weakens it too much, so on a true 2"x8", I could take 2.75" away at most.

Rather than the weight, my only concern there is that even with the minimum of 3/4" of slope across the 3' wide shower, the highest part of the slope would only leave 1.25 - 2.25" total for the dry pack mortar bed and Kerdi, which in my reading seems a bit thin, right? That's what led me to the idea of using the plywood, CBU, Ditra, Kerdi "stack" to simulate a shower pan in a space that's a little too shallow for dry pack mortar. Of course, I would prefer the traditional installation method if there was a way to do it.

Speaking of a more traditional method, am I correct that this would simply be (from bottom to top) 3/4" plywood, 4mm poly, dry pack mortar (2-3"), thinset, kerdi, thinset, tile? Or am I missing something there? That compares to my revised plan in the shower area (not the bathroom, just the shower) of: Plywood, thinset, CBU, thinset, Ditra, thinset, kerdi, thinset, tile.

Also, this raises the question: Is it the dry pack mortar that acts as the uncoupling substrate in the shower pan? What keeps the tile from cracking in the shower if something moves/shifts? I do like Ditra for that kind of protection, but unless dry pack acts as an uncoupler, I am not understanding how the in-shower large-format tile would be able to resist cracking as a house moves/settles. Once again, combined with the lack of height for a traditional mortar bed, that ability to absorb shifting was what led me to Ditra in the shower pan, considering I am already planning on using it for the nearby bathroom floor.

Noted on your advice to use 3/4 plywood. I will plan to amend that portion just to be extra safe and avoid movement (and also I understand I should put lots of drywall screws into it too, especially since I'm using large format tile).

In any case, I hope this helps explain the layout a bit better. I really appreciate your feedback thus far.

cx 02-01-2019 11:26 AM


Originally Posted by Kevin

Mostly. You would also fasten 2.5lb. expanded metal lath over your poly before placing your mud. The thickness of the mud would be from a minimum of 3/4" at the drain to whatever the required slope brings it at the perimeter. The big advantage to using deck mud in your application is being able to exactly match your necessary thickness at the drain and also match your bathroom floor height.

You might wanna consider a small curb or at least a bit of a hump at the door, which will need to open into the bathroom rather than into the shower. You can make it swing both ways if you want, but it's required at least to open out of the shower and the sweep on the bottom of the glass door will wanna drag on the floor if you don't make allowance for it.

Originally Posted by Kevin

Part of the problem with that, aside from not being able to adjust to the exact height of your drain, is that ANSI standards still, to the best of my knowledge, prohibit the use of CBU as a substrate in an application where the floor slopes to a drain. You might argue that they mean a centered drain and not a linear drain, but that's what it says. Schluter might also balk at that application of Ditra over a CBU, just because they have no specification for such. Not saying any of it won't work, just pointing out that you're wandering into a bit of a wilderness area.

Originally Posted by Kevin

Yes,the mortar bed would be your "uncoupling" medium in that application. Keep in mind that the ceramic tile industry has no definition nor performance criteria for anything called an "uncoupling membrane." None. And when you talk about a membrane designed to prevent cracking of your tile installation with horizontal movement of the substrate, you're talking about a Crack Isolation Membrane, for which the industry does have standards. Ditra is not advertised by the manufacturer as a crack isolation membrane. And if your described "settling" involves vertical displacement in the substrate, you're on your own as far as any tile installation product manufacturer is concerned. KMAGYOYO as it were.

As for ripping down your joists to accommodate your proposed shower receptor, you simply can't legally do that unless you somehow support the ripped area independent of the rest of the joist structure. You could do that with your sistering if you were to install your sisters such that the tops aligned vertically with the bottom of your rips of if, as I think you're suggesting, you provide a new support under the ripped portion. Neither would provide any leveling or flattening of the joist tops, of course, and you'd still need to deal with that independently if you want it done.

Four inches, by the way, is what I'd consider a moderately huge bit of "settling" and I'd want to consider some serious repair. You're suggesting that some portion of the bathroom floor you're working on is actually four inches lower than other portions?

My opinion; worth price charged.

otrex 02-01-2019 01:21 PM

Thank you, once again, for your response.

Okay, I thought a mud deck needed at least 2" of thickness, but if it's actually 3/4", that does help.

Apologies, when I said "4 inches of sag" in the current house, I was actually meaning "4cm". That equates to about 1.58 inches. Still not great, but not as bad as 4 inches!

In regards to the transition at the shower door, yes, I am intent upon installing a special profile made by Schulter which is called Showerprofile-WS, essentially a semi-circular rubber and metal bump on the floor which takes the place of a sweeper on the door. This will be further complemented by the installation of Schluter DECO-SG alongside, which is basically a profile which accepts 10mm glass installations. This is to avoid having to use hardware for mounting the glass since the goal is to make the bathroom look bigger by having it as open as possible.

In regards to ripping down the joist, yes, I would assume that there would have to be a sistered "deck" of sorts created in conjunction where full 2x8 extend below the old joist and are then buttressed using a beam and posts for support. In this case that would serve two purposes - both a sturdy support for the weakened joist, but also a way to avoid further sagging of the subfloor. Of course, this new structure would ideally be flush to the top of the now ripped joist.

May I ask, other than ripping the joists down, is there any way that you know of to properly accommodate a mud bed for a shower pan? Building up the bathroom floor is not an option for me because the house has lower ceilings (wartime construction). Currently the ceiling in the bathroom is 7'11", and so I would not want to give away too much more height, especially considering the shower area will have a 3-6" drop ceiling. I should also mention that I stand 6'6" tall, so the ceiling already feels close enough (I get nervous around ceiling fans!).

Finally, I should note that I plan on installing a 28" glass door. Any wider and it would threaten to hit the toilet or not be able to open the whole way. As for opening direction, unfortunately I am stuck with it only being able to open inwards as a full swing out will impact the towel rack located on the left wall (the only option for the towel rack since the main bathroom door also swings inward).

Once again, thank you for this feedback.

Raymond S 02-01-2019 04:29 PM

Perhaps you could consider towel hooks instead of racks. I would do that if the door had to swing out.

jadnashua 02-01-2019 05:38 PM

Code requires the door to a shower to be able to swing outwards which would allow access to someone who might have fell and need assistance in the shower. Could you accommodate a slider? Minimum opening by code is 24".

FWIW, industry does call for deck mud to be thicker when installed over a wooden subfloor, but many people have success when done thinner. In your case, since it would be at the inside edge near a wall where you wouldn't be as likely or able to walk.

cx 02-01-2019 07:08 PM

1 Attachment(s)
Dropping the floor is the only realistic way I can see to accommodate your shower receptor given your other requirements.

You could eliminate the Deco SG and get a cleaner look by simply installing your fixed glass in the tile installation. Like this:

Attachment 206772

I think the shower door width minimum is actually a clear unobstructed opening of at least 22 inches and your 28-inch plan is at least 4 inches larger than I think necessary.

I'll leave the swing of the door to you and your code compliance official. You might wanna plan not to prevail on that one.

My opinion; worth price charged.

otrex 02-02-2019 12:21 AM

CX, thank you - just to clarify, when you say "dropping the floor" you do mean ripping the joist as previously described, right? If so, I think I'm going to start with that plan and then modify if my contractor thinks it's going to be too dangerous/difficult/expensive etc.

For the door swing, not sure what the code is here in Ontario, Canada. I'll have to check. I don't mind it swinging out, but it's complicated by the fact that there is PVC beadboard and chair rail right up to the glass door. The chair rail could be beveled a bit, I suppose to allow for "emergency out-swing". I do not wish to have a slider as it has too much hardware and makes the bathroom look too small. Clarity 10 glass is great but only if it doesn't have hardware everywhere. In the current bath tub layout if you dare close the shower curtain you reduce the rest of the bathroom to a phone booth! So the new design calls for long lines into the shower as if the shower isn't even there (tileable drain, no glass hardware etc). Even the wall coverings continue unabated right into the shower (noting that they are waterproof - in case someone thought I was going to use MDF or something!). The premise is that your eye should forget that a shower is even present until you need to take one, thereby maximizing the visual space of the room.

For the Deco SG, I only tend to want to use that because of the size/weight of the glass going in. It is planned to be floor-to-ceiling, and I would be nervous if it was just tile and silicone holding it in place. I'll show your photo to the tile installer and the shower glass installer and see if they prefer that route.

As for the width of the door, yes, I might ultimately reduce it to 26", but I'm not wanting to go too much less than that just for comfort (again, having the towel rack on the left as the only valid spot means it eats up a little of the door when you "comfortably" walk around it.

This work starts around February 11th, so if anyone is interested I can probably post pics... maybe in exchange for a couple more questions as we go along. Though I have a contractor (my brother-in-law) involved some of the work and most of the planning falls onto my shoulders.

Thank you, everyone for your comments, and I'll be pleased to read more if you have other thoughts.

cx 02-02-2019 09:50 AM

1 Attachment(s)

Originally Posted by Kevin

The tile and silicone is all that will be holding the Deco SG in place, too, Kevin.

The tile bonded with thinset mortar will have far, far more shear bond strength that your glass could ever apply without breaking, even though tempered glass is quite strong. And if you're wanting to go full ceiling height with your glass, installing it with only one side of the glass tiled will make that feasible. Gonna be difficult to do with a fixed channel on three sides. Like these:

Attachment 206775

Entirely up to you, of course.

otrex 02-03-2019 02:55 AM

CX, I understand what you're suggesting, and it creates a couple of questions:

i) First, if my floor tiles are running across the short distance of the room (perpendicular to the joist), when I get to the place where the shower glass should go, do I just leave a 10mm gap for the 10mm glass to slot in? That is to say, the bottom of the glass is held by the surrounding porcelain tile (and silicone, of course)? This gap would be matched in the door area immediately next to it with the Schluter floor profile meant to hold back water in the door space, so that should be nice and even all the way across the floor. I assume, like the walls, the porcelain floor tiles should be employed to hold the glass if I'm not using the track profile rather than just sitting the glass down onto the floor tile. Then, I just leave the last row of wall/ceiling tile until the glass is installed, and then complete the tiling with one layer on the bathroom side of the glass to lock it into place?

ii) On the fixed glass side wall and drop ceiling, if I leave the 10mm space there, as you suggest what do I do on the door side? For example, on the drop ceiling of the door side there is nothing to slot into the ceiling, in order for the tile to line up that would mean I'd need a 10mm grout line? That seems pretty massive. The same is true for the wall on the door side. Since the door hardware is screwed into place right through the tile there, that means no gap in the tile is necessary to install, but that would mean that tile doesn't line up well either, unless the huge grout line is also employed there. What is the standard solution for this? Is it simply to rip the tile on the fixed side down by the difference in the gap in order to make the grout line on the door side the normal size?

iii) Since the walls in the shower area are to be made from Kerdi board, if I employ your suggestion and simply leave a gap, will the orange of the Kerdi board show through the side of the shower glass? If so, how do you stop that from happening? Same question on the floor, though the glass will be right on top of the Kerdi band which covers the line between the Ditra bathroom floor and the mud pan, so I assume it would be orange too and I'm just curious if that's going to show through.

otrex 02-05-2019 12:01 AM

In addition to my above questions (post above this one), I had another come up today.

My floor tile will be the same throughout the bathroom and right into the shower. Its 6" x 3 foot porcelain which looks like reclaimed wood. I should note here that it is rectified tile.

After reading the big tiling thread here about gaps for grout, though I wanted a 1/16th gap initially, it seems like most here woud encourage me to use a 1/8th. Fair enough. But based on what I'm reading, you're not supposed to use sanded grout on rectified tile. Trouble is, I'm not sure unsanded grout for the floor of a shower is such a great idea based upon what I'm reading about possible contraction.

Is this a situation where I should use an epoxy-based grout? I know it's harder to clean but at least I can say that I plan on using a dark brown grout to match the surrounding "wood" floor.

Answers to this and the questions in the post above are greatly appreciated.

cx 02-05-2019 01:32 AM

i. Mmmm, after a couple readings I think that's a yes. I would not make the space for the glass the same width as the glass thickness, though. I leave the better part of a 1/8th" gap on each side, which is then filled with a color-matched sealant.

ii. Again, if I understand the question. I would simply not cut the ceiling tiles where there would not be fixed glass and just leave the normal grout joint that would have been on the rest of the ceiling if there were no glass installed.

iii. I've thought about that when I first did an installation over Kerdi, but found it's not a problem. I do not ever set the glass directly onto the Kerdi, but use clear spacers to support the glass and I think that helps. The ones I prefer are little round clear spacers about the size of small Tiddly Winks unless I nut need more of a gap under the glass to make it fit the rest of the installation. You want to make the measurements very, very carefully for your glass. You can get whatever style and thickness spacers you need from the glass supplier.

iiii. I would not recommend epoxy grout for your shower floor.

I have no idea why you would not use sanded grout with rectified tiles if you can get it into the grout joints.

The think the very narrow grout joints look good on the wood-look tiles, but you're gonna find it difficult to maintain those joints when setting your tiles unless your tile are very well rectified and you are well skilled at setting the tiles.

My opinion; worth price charged.

otrex 02-18-2019 11:04 AM

Just wanted to post an update along with a couple new questions.

As per advice here, I will be ripping my joists down to create the necessary depth for a proper mud bed. As noted above, I'll be sistering some new joist on to the ripped joist for strength, and then jackposting this new platform down to the ground to help reduce movement.


i) If my mud base creates the slope to the linear drain and then Kerdi is on top of that, I don't need the traditional extra layer of mud on top of that, correct? Just thinset the Kerdi and tile, right?

ii) For the Dry Pack Mud Base, I know a 5:1 ratio is appropriate (5 parts sand, 1 part Portland Cement). But is this ratio based upon volume? For a 40Kg bag of Quickcrete Portland Cement, what would be the appropriate amount of sand?

iii) The Schluter Linear Drain requires a dry pack thickness of 15/16" to meet flush with the drain body. If I have a 30" wide shower sloping 1/4" per foot, I come up with a total dry pack thickness of 1.6" . That would mean a 60" x 30" shower area with a minimum dry pack thickness of 15/16" and a maximum of 1.6". Is this thick enough to be sturdy? Cleary I will need to have the subfloor as solidly built as possible to avoid deflection.

iv) I'm noticing a couple different grades of Portland Cement, such as "10F" and "GU". Any particular type I should be using?

Thanks again, everyone. Should have photos soon as the subfloor is being ripped up at this very moment!

cx 02-18-2019 02:09 PM

i. Yes. The Kerdi is what is called in the industry a Direct Bonded Waterproofing Membrane. You bond your tile directly to it.

ii. You can measure it either way, but volume is generally a lot easier on the job site.

iii. Done correctly it should be.

iv. I got no eye-dee what 10F might mean, but GU (General Purpose) will be fine. What you really want is plain ol' Type I and most of the bagged Portland available in my area is marked "Type I or II" and is also just fine (the Type II has a moderate heat of hydration, about which you couldn't care less in your application).

It would be helpful for some questions if you'd put a geographic location into your User Profile.

My opinion; worth price charged.


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