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-   -   My first (maybe curbless) Kerdi shower (https://www.johnbridge.com/vbulletin/showthread.php?t=128483)

dappelha 12-31-2019 01:44 PM

My first (maybe curbless) Kerdi shower
4 Attachment(s)
Hi all, I've appreciated the help in the past and lurking on the forums. Now I'm working on a total renovation of a main floor 3/4 bathroom and could use some help critiquing my plans.

About me

I've done a couple of redguard tub surrounds, a custom niche, a handful of ditra tile floors and a bunch of backsplashes, and lots of drywall taping and mudding, but I have never worked with kerdi or done a shower pan.

The Layout

Attached is a picture of the current bathroom and a drawing of the layout:

My wife and I want tile floors, minimal or no curb, a shower niche, frameless glass sliding door, tile on the shower walls that extends all the way behind toilet and vanity.

The floor is very level and the shower drain is accessible from below in a basement utility room. The shower floor could be lowered to top of joist height but the waste piping does not leave much room for a very tall board to sister against the joist. There are also return air ducts built into the joist bays.

The plan

1. The Shower floor. The floor is level and the current drain is centered in a 48" wide opening, 16" from the back wall (measuring to studs, not drywall). I bought the center drain 36x48 schluter TT styrofoam pre-formed pan with a 1" perimeter and 1/4" per foor slope. I can still return it, but I was thinking the low profile and pre-formed nature would let me a) get away without a curb and b) work with bigger tiles than I could with a more bowl shaped mud pan. I planned to cut 2" from the back side so that it will fit my drain location (16" from back wall) and extend 18" to where the shower door would go.

We would like to be able to have the tile flow from the bathroom floor into the shower, but I have read about concerns with tiles bigger than 4" contouring with the shower slope. Is this still a concern with preformed foam pans with a shallower slope? Can I use say 12"x12" tiles on the whole floor if I cut a line in the tiles from the drain to the corners of the pan?

2. Bathroom floor. The bathroom meets 3/4" hardwood, so to achieve curbless and meet the 1" pan I was planning 3/4" plywood, 1/4" ditra + thinset, + 1/4" for tile for a total 1/2" threshold change. Is this too much of a height change and am I underestimating the thickness of (porcelain) tile?

3. Shower walls. I plan to use plain drywall (not green or blue) on the walls after putting in the Schluter pan because the pan fits the dimensions of the studs. Then kerdi over the drywall and 2" overlap on all seams. I'll get a preformed niche that will go in one stud bay. Stud blocking behind the glass door mounting locations, planning on getting a door like this one:


jadnashua 12-31-2019 06:42 PM

You're going to want to fill in around the drain pipe...that hole is WAY too big. The hole should be in the order of a 5" diameter one for the Schluter drain.

Adding to your shower pan will require some very careful mudding if you want to use larger format tiles. But, yes, IF you end up with four flat planes with a nice crease from the corners to the drain, you can put large format tile on the base. You may want to reconsider that, though, as one of the things that gives decent grip in a tiled shower is the presence of lots of grout joints. If you choose a larger format tile, you need a seam along those 'folds' in the pan, but also would want one that has a high, wet coefficient of friction, otherwise, you might find yourself flat on your back! SO, there are at least two reasons why smaller tile on a shower pan are the norm: conforming without significant lippage, and wet grip while standing there.

By extending one side, and cutting off the other, given the 1/4"/foot, the thickness of the pan will not be the same all the way around. Functionally, that will work, but people tend to prefer the bottom row to be the same height. When using a single slope to a linear drain, though, two sides are tapered, and the other two will end up at different elevations, so you could consider that in your decision matrix, too.

You'll need to talk to your local plumbing inspector on your plans...they generally want at least a 2" drop from the outside of the shower to the drain to allow for at least a little buildup prior to it overflowing into the room (hopefully, you'd notice that before it became an issue).

ss3964spd 01-01-2020 09:42 AM

Welcome David.

The Schluter pan is bowl shaped, and as such you won't be able to bend 12X12's to conform to the bowl. 4X4's would be better but you'll still end up with lippage. As mentioned, there is the traction issue to consider but many people install large format tile on a flat plane shower floor so selecting appropriate tile will be necessary.

I think you are underestimating the build up. You need to set the pan in mortar on the existing plywood so your pan height will be 1"+ 1/16th, maybe a bit more. 3/4" ply (actually 23/32nd's), installed Ditra is probably a bit less than 1/4", mortar for the floor tile will be about 3/16th's, and you're floor tile will likely be 3/8". So 3/4"+1/4"+3/16"+3/8"=1 9/16". Minus 1/32nd for the ply, and a bit for the installed Ditra, let's call the stack at 1.5", so approx. 3/4" above your hardwood. The ply and Ditra stack ends up just below the height of the installed pan.

Houston Remodeler 01-01-2020 10:56 AM

1 Attachment(s)
To add to what Dan said,

You may find it easier to Ditra the main floor first, then you'll know the final height you'll need for the shower floor.

There are companies that make custom foam pans to any thickness. The picture below shows a custom shower pan that is only 1/4 inch thick at the drain. I know a guy ;) Sharpie is for scale.

dappelha 01-01-2020 11:10 PM

Jim--This is in my own home and I'm not planning on having an inspection. There are several threads here disagreeing if the 2" code applies if there is no curb--seems like a gray area and I'm seeing lots of curbless showers in remodels around my area without a 2" drop. I hear the clogged drain and water build up concern--but this main floor bathroom shower probably won't be used very much anyway.

The hole in the subfloor is a 5x6 oval. I read that it should be no more than 5 inches, so I was thinking I would add some blocking or strips of 3/4 OSB screwed from below to close it up more and then fill the 3/4 height difference with thinset when I set the pan. Think that will work?

Paul, the 36x48 pan I got from Schluter is about 1/4" at the thinnest part at the drain flange setting area. Still is 1" at the perimeter. Am I missing a better option?

Dan, thanks for the help on figuring out the height. :bonk:

Sounds like if I match the 1 inch perimeter height of the schluter pan, I should expect a build up of 1.5" which is 3/4 difference from the hardwood. This sounds like a lot to me and a tripping hazard--isn't this too much of a difference?

Sounds like without a big transition height my options are

a) Drop the shower subfloor 3/4 inch to the joist height. Then I could skip the 3/4 plywood under the ditra and have everything meet up nicely. The joists are 2x10, 16 o.c., with the far shower wall of the bathroom resting on a steel support beam so I think ditra alone on the 3/4 OSB t.g. subfloor would be sufficient. This still gives me a bowl shaped pan that will require smaller tiles than we would like for the rest of the floor.

b). Install a linear drain at the shower threshold. Looking at Schluter products I would need a 44" drain to fit the 48" stud to stud rough opening. Anyone have any pictures of how this looks in conjunction with a frameless sliding shower door? The other part I don't understand is the depth of the drain assembly. If I want the finished tile height at the drain to be 3/4" above the subfloor, do I have to notch out subfloor and maybe also joist to bring the drain height down?? (joists are running perpendicular to shower threshold, i.e. drain)

c) Just forget about curbless and put a small curb in, say bricks cut to 3" wide and 2.75" tall. If I still used the Schluter styrofoam pan this should give about the 2" curb to drain height and I can make the main bathroom floor meet nicely with the hardwood floor height.

I like b) and c) because then I don't have all the work of dropping the subfloor, and b) would let us use the same tiles throughout the whole floor.

Thanks for your help everyone. Thoughts?

ss3964spd 01-02-2020 09:43 AM


Let me first say, having recently finished my own curbless shower, I encourage you to pursue doing one of your own if at all possible. I used 12X24's for the main floor, and 2X2's of the same tile for the shower floor.

For what it's worth, code here in Fairfax Va says IF there is a curb the top of the drain grate must be a minimum of 2" below the finished height of the curb. Code offers no details when there is no curb.

Recessing the sub floor will likely buy you a 1/4" reduction in height, and that's because you'll still need to install 1/2" ply over your recessed 3/4" ply to cover it and your joist tops; because no thinset mortar manufacturer is going to endorse applying their product directly to sawn lumber, nor will Schluter endorse installing Ditra over a combination of ply and sawn wood. You could maybe get away with 3/8" ply, saving another 1/8". Further, you could maybe get away with 1/4" Hardie over the recessed floor, as I did under my foam pan, but exactly no one here will recommend doing so. Mine is doing fine so far, but has been in use for only 3 months. Time will tell if the gamble paid off. Another option might be a combination of shaving down the joists and recessing the sub floor, but that's going to depend on the open span of your joists.

I've no experience installing a linear drain. From what I've read here, though, it does seem possible that you can save some height by using one, how much probably depends on the manufacturer. I've read several threads here that recommend installing a small "speed" bump on the other side of the drain to arrest any water that might over-shoot the drain. However, that same speed bump will preclude the use of a large format tile, and your door would need to be placed exactly on top of it. I think a threshold linear drain would need to be exactly the size of the opening or else water will bypass it at the ends. A fixed panel of glass will stop it and will eliminate overshoot, but will be a maintenance issue as that junction will always be wet unless you squeegee/dry it off. The door, of course, isn't going to stop it.

Maybe someone with actual experience installing and using a threshold linear drain in a curbless configuration can expand upon the pros and cons.

Yeah, retrofitting a curbless shower is a lot of work. I wanted one, bad, so I signed up for it.

RichVT 01-02-2020 01:28 PM

Schluter does have a detail showing their foam pan installed over wood joists with a recessed subfloor.


ss3964spd 01-02-2020 01:50 PM

That's interesting, Rich, I wonder if a working assumption/requirement is that the plywood is dead even with the top of the joists.

dappelha 01-02-2020 02:03 PM

3 Attachment(s)
I have seen some videos where the pan was installed over the top of the joists and I think that would be fine, or a 1/4 plywood or hardie to give a uniform surface. As Rich said, schluter even shows this in their material.

The main design problem is that you have to then find small tiles that match the larger tiles, so the tile choices goes down quite a bit. Do you guys cut your own small tiles? How do the edges look when grouted?

Lowering the subfloor also is not straightforward, I would have to figure out how to re-cut and attach the sheet metal that is forming the hvac return ducts. Here are some pictures of what I mean. I would have to cut the sheet metal to fit a sistered board and get a tight seal (can use some tape and duct seal goop to help). This is doable.

I also wonder about sistering at the spots where the waste line comes through one of the joists and another spot where it runs along another joist. The waste line is only about 2-3 inches below the current subfloor which doesn't leave much vertical space for a sistered board. Where the waste line punches through the joist, I could just notch the sistered joist, but in the other picture along a joist the waste line runs 3 inches from the subfloor for the whole length. Maybe some angle iron instead of a sistered board would work in this spot?

Houston Remodeler 01-03-2020 10:09 AM

1- There are times when making our own floor tiles is required. Sometimes its easy, sometimes a first class PITA. Depends on your equipment and how easy the tile cuts without chipping.

2- Of course all our projects look spectacular :)

3- Some company makes hat shaped metal pieces which fit over joists and holds 3/4" plywood. Dunno the name. The coffee hasn't taken effect yet. Perhaps another Pro can chime in.

4- Mylar coated foam board makes easy ductwork

Spencer84 01-03-2020 10:11 AM

You do not have to sister the full length of the joists to get the benefits of sistering.

In your case you do not have to get the sistered joist onto the metal beam. So the air duct could stay as is.

The joists with the waste line looks like it is already doubled. That could be a good thing or it could cause more problems.

One of things I didn't know when remove subfloor to sister the joists for my curb less shower is that you need a plan for supporting the walls that are now floating.

dappelha 01-03-2020 11:23 AM

On my last project I upgraded from a $20 craigslist saw to the beautiful dewalt 24000 saw, so cutting my own small tiles without chipping is doable.

I know I don't need the sister board to get on the metal I beam, but I only have room for about a 2x3 for most of the length of the shower on the joist which has the waste line running along it. I'm thinking I could use angle iron on this joist. This joist is doubled up, and is directly below the load bearing shower wall above it

About the unsupported walls. I guess you mean the case where the wall falls between two joists and runs parallel to the joists? I guess I'd have to add some blocking there to help support it and also to support the end of the dropped subfloor. For walls that run perpendicular to the joists, I think they are fine, and for walls that have a joist directly below them I would also be able to just sister for the new dropped floor. I think that leaves one wall where I might have to add more support--fortunately the shower is 48 wide with 16 o.c. joists which leaves both walls very close to supporting joists.

ss3964spd 01-03-2020 01:17 PM

Unless your joists are insufficient for meeting L360 you don't need to "sister" them to add strength,. So you'll basically only be adding cleats to the sides to give you something to support the ply and give you something to screw into. 2X3's will be fine, even if you have to notch them, so long as the notch isn't too deep and is of a short length.

Correct about a wall that falls between, and parallel to, the joists. Gotta add some framing to hold it up.

dappelha 01-05-2020 08:13 PM

I have spent most of the weekend at the tile stores. We love marble but know it is hard to maintain. Would I be crazy to use 4x12 marble for the bathroom floor on 7/8 subfloor + ditra? This rarely used main floor 3/4 bath seems like the perfect place to wow with marble while also never using the shower (and therefore not worrying about maintenance).

I have a solid 13/16 or 7/8 subfloor. The deflection is hard to calculate. I have an 11 foot span, then a floating basement wall, then a 3 foot span (under the shower area is the utility room of the basement). One of the joists runs directly under the wall of the shower and is doubled up, and another is at 13 oc instead of the 16 of most of the rest. I guess I have L680 ish?

Would I be crazy to try 4x12 marble on additional 3/8 inch plywood + ditra? Doesn't acceptable deflection depend on the size of the tile as well?

cx 01-05-2020 08:29 PM

David, a 7/8ths" subfloor is uncommon. You have 7/8ths" of what? Sawn boards? Engineered wood panels? Other?

As for your joist deflection, you're certainly free to install your stone over whatever you feel comfortable with, we can only tell you what the ceramic tile and stone industries recommend and where the smart money is betting. Will your tile installation over your current structure fail? I dunno. We can't guarantee failures any more than we can guarantee success. If you think your structure is adequate, you can tile over it.

I would caution you about that nominal 3/8ths" plywood, though. I know Schluter likes it as their minimum requirement for a second layer of subflooring when one is required, but the 3/8ths" plywood I usually find available today generally looks more like a stack of potato chips than a stack of good structural plywood. Sometimes you can locate some decent stuff, but I generally find that the nominal 1/2" plywood is in much better condition. Can be really tough to get that thinner stuff to lay sufficiently flat even with a pretty tight fastener schedule.

Originally Posted by David
Doesn't acceptable deflection depend on the size of the tile as well?


My opinion; worth price charged.

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