Questions and Advice about plywood shower room preslope

[Kamal]

Hello all,

Thank you in advance for any help and suggestions. In short, I am building a shower room in my home, and would like to explore the option of a ply subfloor preslope to avoid doing one or both mud beds. A few details first:

* Raised foundation home on hill in SF BAy Area with ample room below floor for bracing/monitoring
*5' x 6' shower room, curbless, slope on 6' dimension, joists perpendicular to slope
*5ft wide linear drain at the shower head end of the shower room
*24" x 48" porcelain floor tile, indoor/outdoor rated, slate texture
*Bathroom is up 2.5 flights of stairs, meaning heavy lifting to bring any mud mix
*Never have done any cement work, done plenty of woodwork, no rush on this job

So obviously the joists need to be lowered to accommodate the curbless aspect. I'm afraid to try a 30 sq ft mud bed for my first install, let alone 2, and I can't imagine how many bags of mud it would take to create it. I looked at some deck muds that could be feathered way down, but still the job seems daunting. I can, however, be confident of a stiff and sturdy sloped subfloor. I also believe that an eventual crack in the large slate looking tile doesn't seem to be the end of the world if the rest of the install is otherwise waterproof.

So, assuming I can create the stiff and flat enough subfloor, what are my waterproofing options? Here are my ideas, but would love for an expert to chime in.

Ply Preslope/Kerdi/Tile
Ply Preslope/CBU/Kerdi/Tile

If I can be convinced to do one mud bed, then:

Ply Preslope/Sheet Membrane/mud/tile
Ply Subfloor/Mud Preslope/liquid membrane/tile

These ideas have come from innumerable hours researching this thing, and finally I arrive here to defer to the experts!

I tried to upload a picture. Assuming it shows in the thread, what you are looking at besides the evidence of a pandemic haircut at home, is the shower heads and 5ft wide linear drain at the far end, right side wall is an exterior wall, and the joist run right to left.

 

[wwhitney]

So what fixtures if any are in this shower room besides the shower head?

Is the window staying? Is it a window with water impermeable frame and sashes, e.g. vinyl or fiberglass? If not and it's staying, I'd think you'd need a shower curtain over the window, as there's not really any way to waterproof a wood window.

Your slope length is 6', at 1/4" per foot that's a 1.5" fall. At the entrance to the wet area, what is the height from top of joist to top of finish floor?

And what are the floor joists and unsupported span under the shower room?

If you are able to lower the ~4 joists appropriately, then you could start with a sloped layer of 3/4" plywood, all edges blocked or T&G. Kerdi is not to be installed directly on plywood, so you'd either need a layer of 1/4" cement board on top of that, or a layer of Ditra, in which you prefill the waffle grid with thinset. [Main attraction of that would be to use DitraHeat for a warmed floor.] Then you can apply Kerdi on top of that layer. So it seems like your buildup on top of the joists would be about 1-1/2" in the tiled area.

There's a possibility of recessing the sloped subfloor between the joists for some or all of the bays, and then using a Kerdi substrate on top of that, to save about 3/4" of buildup.

johnbridge.com is a great resource for your questions.

Cheers, Wayne

 

[Sponsor]

 

[Jadnashua]

Working with deck mud is like working with wet beach sand. In fact, you could place it dry, then carefully wet it well with a pump yard sprayer, but that's not an industry approved method. There are companies that will mill a custom foam pan, and then you'd make a simpler flat recess and cover it with Kerdi.

 

[Kamal]

Thank you for your responses!

Wayne, I signed up with Johnbridge about 2 months ago to get this sorted out, but my account was never properly activated, and despite a number of tries, I havn't been able to make it work. Getting signed up here was so easy, and I already have great advice to go off.

To answer your questions:

+The shower is planned with two "stations", either two shower heads in the back wall, or one in the wall and one in the ceiling for rain. No jets or anything that would create a lot of wetness.

+The current window is double hung vinyl. It will be replaced with a vinyl double pane picture window. I wanted to nix the window altogether, but the wife nixed that idea. I'm confident that with only downward facing shower heads, the window sill won't be flooded with water, and standard sill treatment will be fine. This is the master bath, and we understand to squeegee or tamp up any excess water that may build there. And, I will slope the sill to drain.

+Current framing is 2x10, 16" OC, 14ft span. Current flooring I put in is 3/4" Brazilian Cherry right on top of the original 1978 5/8" ply subfloor. I plan to add a beam under the joists at the left side of the original pic, making the effective span only 5'. Flatten the hill, place blocks and post jacks. The new picture shows the space to work with under the floor. The shower will border the exterior wall and foundation. Because I can add a beam, I can easily add joists to reduce the OC. I had planned for 2 layers of 5/8" without additional joists, but I don't really know one thing from another, so your ideas are helpful.

+Because I will add a beam, I can cut the joists way down. With this in mind, any suggestions?

+ I have plenty of Ditra leftover from my kitchen tile install. So prefill it, and then thinset the Kerdi to the dry filled Ditra? Pic of my kitchen added. That was my second tile job ever, and its really nice. Despite adding another layer of ply and ditra, one of those huge tiles cracked, but nobody can see it except me! Since then, my tiling has only gotten worse! The smaller the tile, the worse the result has been. This shower will be tile job #6 and 7. Tiles will be 24x48 for the floor and 32x32 for the walls, so I'm expecting good results. I'm a Psychologist by trade.

 

[Jadnashua]

There are some very different techniques needed when setting large tiles and if not executed well, it's really unlikely that you'll get it embedded properly. Any gap could result in a potential point load failure.
Try again at www.johnbridge.com. What is your username? I can talk to one of the moderators for you.
You should also look at a slant-notch trowel. The earlier technique in setting a large format tile called for the use of something like a pad sander (without any sandpaper!) to vibrate the tile into the mortar, and required burning in a layer on the back prior to setting it. I haven't been to a class recently, and the tools and techniques may have evolved.

 

[wwhitney]

Current framing is 2x10, 16" OC, 14ft span. Current flooring I put in is 3/4" Brazilian Cherry right on top of the original 1978 5/8" ply subfloor. I plan to add a beam under the joists at the left side of the original pic, making the effective span only 5'. Flatten the hill, place blocks and post jacks.

I'm slightly skeptical of that method. Let's see, if we call live + dead load 50 psf, and you have a 6' beam breaking a current 14' span, then the tributary area of the beam 42 sq ft. So the load on the beam is 2100 lbs, and on each post would be 1050 lbs. Soils in this area are commonly clay with an allowable bearing stress of 1500 psf. That means each footing should be 0.7 ft^2, or say 10" square.

I thought there was a minimum 12" depth requirement for any footing in the IRC, but looking just now I see that I'm mistaken. So I guess if you get a minimum 10" square pier block and bury it say 3" minimum, that would be a sufficient footing, and you can post up to a beam. From table IRC table R602.7(2), a double 2x6 would be an adequate beam.

So yeah, I guess that works. I'd still be inclined to try to minimize how much the joists are shaved, and to sister the joists with 2x8s (probably that's overly cautious). Also, if notching a joist, overcutting the corners of the notch vertically significantly reduces strength. Best practice would be to avoid a square notch corner, e.g. to do a trapezoidal shaped notch, but that won't work if you are notching to the exterior wall plate.

I'd be torn between recessing the subfloor between the joists to reduce how much joist notching is required, and going with deeper notches to allow a single length of plywood subfloor. In all cases the face grain direction of the subfloor should be perpendicular to the joists. If you want to do two layers of plywood, then I'd definitely suggest recessing the first layer, e.g. 3/4" recessed plus 1/2" on top. But that doesn't gain you much versus just doing a single 3/4" layer on top of the joists.

I have plenty of Ditra leftover from my kitchen tile install. So prefill it, and then thinset the Kerdi to the dry filled Ditra?

I think that's right, check Schluter's literature, I believe they have some info on waterproofing Ditra.

Cheers, Wayne

 

[Jadnashua]

Ditra, if you band the seams is waterproof, but won't work in a shower as the pockets won't drain, thus the need for the Kerdi layer on top keeping any moisture away. Plumbing code says the shower slope needs to be continuous and at a minimum of 1/4" per foot.

 

[Kamal]

Jim,

Thank you for any help with the Johnbridge forum. My username there is Malgyver.

Although I am totally green to tiling, and jumped right out of the gate with large format tile, I think I've done ok. Did 20 18x36 in a shower, that was my first and best tile job. No failures. It was a wall though. Recently did 18 12x24 on a wall too. The floor that "failed" was 15 24x48, and the tile that failed is adjacent to the fridge. It may have cracked rolling the fridge over it upon install and a number of times moving the fridge. And I do believe that there was a dip in the subfloor I overlooked, being totally green. We have danced and jumped on that floor, and even set a temporary post on it while I was working with the beam overhead.

The current floor will be 2 or 4 human feet on 12mm Porcelain tile adjacent to a huge glass wall, I'm fairly confident I can set the tile well enough to avoid a point load failure, and that we won't be jumping around in there like its a nightclub.

Wayne, the ground on my property is quite rocky. Some of the home foundation is actually built upon protruding rock, and right above me is the apex of the hill with a huge city water tank on it. Just on the other side of the foundation wall where the bathroom sits, we tried to drop posts using a two man auger, to shore up a retaining wall, and we couldn't dig. It was all rock. So we made a block wall instead.

I will try to dig the blocks as deep as I can, and had intended to use post jacks so I could be sure to monitor the preload on the posts. It would take me literally 30 seconds to get up from the computer right now, and walk under the house to where the posts will be, in case I had to snug them up.

In terms of the joists, I'm wondering if it would help to not only sister the current joists, but add joists in solely for the shower floor, which wold reduce the OC to about 8". In that case, would the single layer of 3/4" ply become stiff enough? Perhaps in that case, I can ditch the subfloor altogether, lay the tile on the joists, and simply create a drainage catch under the house.

In terms of how much lowering I need....The top of the floor is currently 3/4" hardwood plus 5/8" ply = 1 3/8". The new floor will be 7/16 tile + thinset + Kerdi + Ditra + 3/4" ply = I'm guessing 1 5/8, but I'm no tile guy. So that would suggest that the first joist has to be notched about 3/8", and the last joist has to be notched about 1 7/8".

Slowly coming together. I'm attaching a snippet of the floor plan. The shower floor is the highlighted area that used to be the toilet and old shower. The rest of the 14' span is a double vanity, single vanity, a toilet, and the bathroom floor. And the roof is a truss roof with only perimeter load.

 

[wwhitney]

intended to use post jacks so I could be sure to monitor the preload on the posts.

I think a typical post jack is not meant for a permanent installation (not certain). There may be an adjustable post type that is intended for permanent installation. I'd be more inclined to use a KD 4x4.

In terms of the joists, I'm wondering if it would help to not only sister the current joists, but add joists in solely for the shower floor, which wold reduce the OC to about 8". In that case, would the single layer of 3/4" ply become stiff enough? Perhaps in that case, I can ditch the subfloor altogether, lay the tile on the joists, and simply create a drainage catch under the house.

A 3/4" plywood subfloor on joists 16" o.c. is sufficient for a ceramic tile installation, and I wouldn't suggest a subfloor thinner than 3/4" plywood, so I don't see any upside to joists 8" o.c.

As to omitting a subfloor, I guess it is possible to find a substrate that would be a durable shower floor and would span 14" (or 7") between joists. But how are you going to provide a waterproofing layer without continuous support for that waterproofing layer? You don't want water running over your joists. So I would say a subfloor is mandatory.

Cheers, Wayne

 

[Kamal]

Wayne, I was joking about no subfloor...but great answer nevertheless!

Your thoughts on the amount the joists have to be notched? I’m figuring 1/4” on the first one and 1 7/8 at the last one. Seem reasonable to you? Would you sister them all or just the last one?

And I can do 3 posts w/o jacks to hold up the additional beam if you think it’s wiser. The material cost and complexity is no issue.

 

[Kamal]

Hello folks,

This project is moving forward, but I have a few tile questions. In terms of the work completed, I did notch the joists, and create a post and beam system underneath using 4x4 and 4x6, with some lateral bracing. The pier blocks were sunk into reinforced concrete. The soil was mostly rock, which is what the whole house is built on, and a number of the joists that had the greatest notching were already sistered due to some design features on the other side of the span. I've placed 3/4 T&G with the proper slope, and the floor is flat and very solid. I plan to do kerdi over ditra for the floor, and Kerdiboard or similar for the walls.

My question is about the strength of the waterproof foam boards for large heavy wall tiles. I have done some research, but have not come up with anything definite. The tiles are 32x32 Emser Porto II, which weigh about 32lbs each. Are the foamboards strong enough to carry that weight? Wedi/Kerdi/Go Board.

Many thanks!