I have recommended this idea a couple of times, but American plumbers and/or tile setters seem resistant to this at least in my area. One of them would only agree to do it if there was a curb at the doorway coming in, which kind of defeats the whole purpose. The other we ended up doing it but it still has a roll over rubber curb (like an ADA shower) around the shower area.

Perhaps as houses get smaller again or people work more with what they've to rather than adding on, this idea will take hold a bit more.

The floor has to slope, which can be difficult to do if you can't access the area below the floor.

I assume that the doors swing out when you need the room as a shower, but are normally out of the way for using the toilet and vanity.

Excellent idea.

A small shower has a 2" curb, by code. Extrapolating this to a larger area should lead to a smaller curb, but it doesn't, because "code" as now written does not get as detailed as that. It's 2" and that's it.

If shower water is running, in a small shower, a 2" curb prevents overflow for a few minutes, when the drain is blocked. It might happen in an emergency or unintentionally. Examples are when someone faints and their body happens to fall in a way that exactly blocks the drain. Or, a child or prankster positions someone over the drain to see what happens; usually such a person loses interest and lets water drain out again after a minute. The 2" curb was the height voted as best for the purpose.

In the smallest shower bases that are legal to sell, the volume of water is X cubic inches until it overflows over the curb. (Those are very small showers; most people like showers bigger than that.)

In a wet room or a large shower, the entire room (or large shower) is potentially fill-able, if the drain is blocked. BUT this takes a lot more time because the volume required to fill the "pan" is quadruple X or ten times X or some other multiple.

So, it makes sense to expect to use a marble sill as the wet room threshold, and hope that the next owners deem it to be acceptable. I know people who do this. I did this too. I know people who build recessed showers without any threshold. It's all a matter of showing the volume X needed to fill it. As far as I'm concerned.

Hth

>Examples are when someone faints and their body happens to fall in a way that exactly blocks the drain

Well, you know, personally I'd rather have the water run out into the hall than have it collect so that I drowned in an inch or two of water in that situation.

or let someone know to come running to check on me.

What about ADA accessible showers that have no curb? What if you are doing this for universal access? What if you are using a trench drain?

These are very relevant questions -- does anyone have answers or views?

Im just about to build a zero threshold shower, and would love to know what the issues are too...
What is a trench drain? Is it like long drain like the ones I sometimes see at the bottom of a driveway by a garage door that is below ground level?

Here is one manufacturer

Here is a link that might be useful: Shower trench drains