Schluter curbless shower with recessed floor, how to make up height

[cx]

Changing the subflooring will not improve the joist deflection. Well, theoretically it might, a tiny bit, but in practice it won't make any difference.

The boring you have is already what you've got. Those metal plate things might strengthen a joist when too large a hole is bored in the center, but if you cannot move your existing drain pipe lower in the joist, I don't see those metal plates being at all helpful. But I've never seen one. And while they might actually strengthen the joist in the bore area, I don't know that they maintain the overall joist rigidity. Not saying they don't, just saying I see no indication that they do.

The problem with running drains perpendicular to the joist structure, if the run exceeds three joists, is that once you've accommodated the required fall of the pipe, you run out of vertical room on the joist for an acceptable bore location much of the time.

I think your choices boil down to changing your plan for a ceramic tile floor, or just taking your chances with what you've got. Were I asked to do your planned installation for a customer, I'd likely hafta decline.

My opinion; worth price charged.

[Kyras]

Myself, I would add a layer of 1/4" CBU and laminate it to the previous layer with thinset, staggering joints. I don't trust thin plywood. At least around here, the humidity makes thin plywood swell between the screws and generates movement. You just can't put enough screws in it to stop that effect. CBU will stiffen the floor. My 2 cents.

[PC7060]

CBU is a decoupling layer which is important for tile but does nothing for the stiffness of the floor.

[vinmassaro]

I believe Schluter also does not recommend installing DITRA-HEAT over cement board.

[cx]

Originally Posted by Vincent

The tile I am looking at is a porcelain 12x24 that is 9mm thick. My understanding is it's harder than ceramic...

Porcelain tile is ceramic tile, Vincent. And it makes no difference in your application.

Originally Posted by Kyras

At least around here,..

That sort of comment is a lot more meaningful if you'll add a geographic location to your User Profile, Greg.

[vinmassaro]

Since I need to raise the floor to 1-1/8 total - what would make for a stiffer floor, installing 3/8" over the existing 3/4", or remove the 3/4" and put down new 1-1/8" subfloor?

[cx]

The double-layer subfloor would win because you can cover the joints in the first layer with the second layer and have no place where a subfloor joint extended from the joist top all the way through the subfloor.

I would personally not install anything thinner than nominal half-inch plywood for the second layer, though.

My opinion; worth price charged.

[vinmassaro]

@CX: thank you - I'm getting back to this project now trying to plan the plumbing. The drain falls right on top of a joist. I can either put it to one side and make the shower around 32.75" wide, or would need to head off the joist and frame it with double 2x10s on each side like the photo below. This way, I could make the shower about 36" wide and have no obstruction for the drain. Thoughts on this? Negligible difference given the situation already?

[cx]

The trouble with heading off the joists in that photo, is you've got the doubled members in the wrong place. When you remove a joist, or a couple joists, the problem is that you shift all the load from the removed member to the two adjacent members. Those are the members that would need reinforcement. Those two short members in your photo do not need to be doubled, but the adjacent joists might need reinforcement, depending upon your situation.

Up to you whether the modification in framing is worth having a centered drain in your shower. Don't see how that would determine the size of the shower, though.

My opinion; worth price charged.