In article , Speedy Jim wrote:

Go he http://www.codecheck.com/plumcode.htm Scroll down to
"back-to-back fixtures. Fig 5 is apparently a cast iron pattern.
But the "double fixture fitting" should be available in ABS. I
think that's what she intended.

Hmm, so "double fixture fitting" is different from "double sanitary
tee"? I guess I have to find an appropriate "double fixture fitting",
as UPC 704.2 states:

Two fixtures set back-to-back, or side-by-side, within the distance
allowed between a trap and its vent, may be served by a single
vertical drainage pipe provided that each fixture wastes separately
into an approved double fixture fitting having inlet openings at the
same level.

BTW, under "Change of Direction", codecheck.com says "Double sanitary
tee - two pipe sizes larger than inlets, horiz. to vert. [706.2]".
I assume this applies to the situation of two branch drains joining a
stack (with a double sanitary tee), while 704.2 applies to two
fixtures joining a stack (with a double fixture fitting)?

I don't see a problem with the 3" stack. But if you use stacked
TEE's (WYE for the lower one), the lower entry must be revented
since the flow from above will cause trap siphoning.

As above, I guess this isn't allowed. But I'm curious about your
statement that the lower entry must be revented. When does this
apply? For example, I assume it doesn't apply for an upstairs toilet
above a downstairs toilet, both entering the stack with a sanitary
tee, as the vertical distance is so great.

My understanding is that a lavatory requires only a 1 1/4" drain, and
that a common drain for two lavatories must be at least 1 1/2". Using
a sanitary cross here would require the outlet to be 2" as above, but
with stacked sanitary tees, 1 1/2" should be OK. If I do this, is
there any reason to oversize the drain to 2"?

OK, if allowed. Better practice here to use 2".

OK, I'll use a "double fixture fitting" with a 2" drain outlet. I
belive you mentioned a 1.5" vent would be fine. Should I use 1.5"
horizontal branches from the lavatory traps to the "double fixture
fitting"?

I don't see a problem with the 2 X 4 wall, but double check the
O.D. of 3" ABS fittings; I suspect there will be interference with
the wall covering.

Ah, that is an excellent point about the fittings. I checked that the
3" ABS pipe has an O.D. of 3.5", but the hubs on the fittings will be
even bigger. Since I'm going to revent the sinks in this pluming
wall, I can't fit everything in a 2x4 wall. Many thanks!


Cheers,
Wayne