You have to have 1/4" to 1/2" slope per foot, so the minimal dimension that you can do curbless is 4', with the steeper 1/2" slope over the whole shower. If you are on crawlspace, you will need to reinforce the joists in order to lower the floor enough to get the required 2" drop from the entry to the drain. If you are on slab, you'll need to jackhammer the slab to do the same thing.

Your plan to place a cantilevered bench at the rear in order to achieve the needed drop to use the trench drain will work as long as you use some steel reinforcement for the bench, and us a membrane system for your waterproofing. The drain shoud cover the whole back wall of the shower, not b a segment, and the entire shower floor has to slope that 1/2" per inch towards that drain. No steeper areas and flatter areas. You will have a break in the plane between the plane of the floor and the plane of the shower at the entrance and your waterproofing should extend out into the bath floor at least 6'', and preferably 12''.

hollysprings,
Thank you for your comments! This is new construction, so by rights, it can be done however necessary to make it work.

One question... You mentioned a required 2" drop from the main floor to the drain. Who actually requires it? I mentioned it to my contractor in passing, and he claimed there was no such code requirement. (However, I have ever read it elsewhere.)

I just want to make sure we don't have a mess.

I'd appreciate comments on what's really the right thing to do, whether it's required by code or not.

Thanks!

Here's IRC, which is the prevalent code. Find out if it applies in your area:

2012 IRC:

P2709.1 Construction.
Where a shower receptor has a finished curb threshold, it shall be not less than 1 inch (25 mm) below the sides and back of the receptor. The curb shall be not less than 2 inches (51 mm) and not more than 9 inches (229 mm) deep when measured from the top of the curb to the top of the drain. The finished floor shall slope uniformly toward the drain not less than 1/4 unit vertical in 12 units horizontal (2-percent slope) nor more than 1/2 unit vertical per 12 units horizontal (4-percent slope) and floor drains shall be flanged to provide a water-tight joint in the floor.

Code can be a little confusing to read and translate. The premise is that if you plugged your drain, the shower needs to hold a minimum 2" depth of water before water cascades over the curb of your shower and out on to the unprotected floor of the bathroom. In a curbless shower, the "curb" is typically the break in floor plane where the floor goes from a flat bathroom floor to a sloped shower floor.

IRC gets revised every three years. That same language has been in the previous revisions of the IRC for...well, seemingly forever.

There certainly are other codes out there. Some have fallen by the wayside, some are still in effect. For example, with a very broad brush I'll write that IBC is generally concerned with the nuts and bolts of how to structurally build a house. IPC is concerned with plumbing; properly sizing supply and DWV lines, for example. IRC sort of rolls everything together into how to build a house that is built well, plumbed well, and that has systems that properly relate to one another.

If code does not apply in your area, you can still use it as a guideline for the minimum standard on how to construct a house. Or a shower.

If your contractor is unaware of these basic code requirements, then I recommend you also read up on capillary breaks. A capillary break on a curbless will prevent water from wicking from the waterproofed shower floor to the unwaterproofed bathroom floor.

Good luck!

Multiple slopes in a wet environment is a recipe for an accident. I'd be a bit on the worried side that your contractor didn't know or understand the 2" drop requirement. It's been standard construction for a zillion years. Be sure that you understand all that goes into making this actually waterproof. And that your contractor understands it. Double check. Triple check.

AHMIowa_NJ:

" Still not sure how we'll arrange a glass wall/panel to shield the vanity area from the water, but the first question is how to drain."

I strongly disagree. You must think comprehensively. Figure out the panel and/or door first and it may help you figure out the drain.

Aletia, let me post an alternative.

In large showers, it's best to keep things as simple as possible. It can help minimize mistakes.

In large curbless, I've done a couple designs like the following; both in residential and in commercial.

It allows you to have your 1/4" slope per foot.

It allows you to keep a single plane of tile on the floor instead of having the breaks you have.

It eliminates the too steep 1" per foot drop you have under your bench.

And, it can save you the cost of a linear drain.

I would only do a shower like yours using a topical membrane. A trowel-on along the lines of Hydroban, Aqua Defence, or RedGard, OR a sheet membrane like Kerdi.

Anyhow, the shower has a single plane, sloped at 1/4" per foot, that leads to a trench at the wall. The two sides of the trench floor itself are sloped to the drain.

I don't recall the exact dimensions of your shower, but for simplicity of math I'm going to say it's 6' wide and 5' deep. And I am going to simplify this:

Your main shower floor will slope down 1/4" per foot along that 5' depth, from the entry of the shower to the trench at the back wall, for a total drop of 1-1/4". Your trench along the back wall is 6' long from left-to-right, with the drain centered in the middle. Essentially breaking the trench into two 3' long sloped sections. If each 3' run is also sloped 1/4" per foot, that gives an additional 3/4" of drop to the drain.

Your 1-1/4" drop from the main slope, plus the 3/4" drop within the sloped trench? Add them up and you get your 2" required elevation between the curb and the drain.

I'd shape the sloped floor and the trench in deck mud, then cover it with one of the aforementioned membranes like Hydroban, etc.

With the membrane being on top of the sloped mud, that also satisfies the code requirement that your membrane be sloped to the drain. Hopefully your contractor is aware of that code requirement.

The drain needs to be compatible with the membrane.

If you did this design, the "open trench" can be left open, as it'll be covered by the bench. Or you can cover the trench with a grate of some sort. I've done typical looking metal grates as well as tile-on grates to cover the trench.

Good luck!

Thanks, Trebruchet, for your views about looking at the shower pan and the door as a whole rather than piecemeal - I've been hoping to do that as well, but have been struggling to come up with something that will minimize the complications. My goal is to find a drain arrangement that will allow some flexibility in any door (or if we go without).

Mongoct - I really appreciate your explanation of an approach to refine what we were thinking about, with a sloped trench at the rear wall of the shower - it makes great sense, and even saves the cost of a trench drain assembly!

I'm always amazed at the experience and knowledge of Gardenweb-ers...and now the illustration skills! Seriously - the diagram really helps to understand your concept - and the point about the implementation of whatever membrane system we use is important as well.

This might actually work after all....

Thanks,
Aletia