Horizontal Drains Can Combine Through Wye Laying Flat, Or Can't They?

Question

QUESTION Can horizontal drains combine through a wye in the horizontal position laying flat on its side and not upright at all, Or Can't They? Then see my sketch: Am I doing this right? - Merry

IPC TABLE 706.3 "fittings for change of direction" says wye is okay for horizontal to horizontal.

Are my proposed wyes okay? How about overall? I don't want to go down too low with the drains or relocate much stuff.

BACKGROUND: This is just a draft but in the interest of limited headroom and headache I want the plumbing improvements to not descend too low into the door/passageway below, and I dont want to change the lower level toilet system, an older wall mount model, except just to vent it, without moving the bracket, and to drain the upper level bathroom through a separate soil pipe and vent it. And most obviously there is improper use of a sanitary tee and that needs to be rectified.

Note I used the term "vertical" incorrectly in my sketch, it should be horizontal but i mean the vent going upward there from the wye which is specifically horizontal but not laying on its side, like the other two in question. thank you.

Is the vent on the "vertical" wye even necessary there at the end? I don't know, I was just doodling and it felt right.

Don't you see flat horizontal wyes combining drains within a slab or floor usually?

Some special sanitary situations specifically call for wyes in the vertical position, so i have read, but here we should be good. My state is IPC.

Answer 1

Yes, it is/has been done often. If you can, I personally prefer to put the wye so that the offshoot is angled slightly down into the main run to create a natural direction for flow.

The only thing I'd caution you on, if your code is picky about it, calculate your DFUs to make sure it is acceptable.

Also... I was looking at your drawing, and this is 100% my personal preference only, I dislike taking the first floor vent off of the second floor flow. If possible I would tap the line ABOVE where the second floor fixtures enter the stack and run that as a second line down to the first floor vents. As far as I know there's no code that requires it, but I prefer to keep my vents dry at all times. (Which you have done in the drawing)