I'm remodeling my bathroom here in California, and the group has been
very helpful with a few questions I've asked so far. Right now I'm
trying to finalize the layout, and I have a couple questions on sizing
the soil stack and plumbing wall.
The plumbing wall will service a water closet and lavatory on each
side of the wall (back-to-back fixtures). I'm planning to locate a 3"
soil stack in the wall between the two toilets and service the
lavatories with a 1.5" drain/vent that revents to the soil stack in
the wall (the drain will connect in the ample crawl space). So:
1) Should I use a 4" soil stack? 3" is adequate in terms of fixtures
units and I don't have any problem with the aggregate vent area
rule. I've also heard that 1.6 gallon closets perform better with
a 3" drain line.
2) With a 3" stack, is it reasonable to frame this wall with 2x4s? My
understanding is that 3" ABS is 3.5" O.D., but the stack will not
intersect any studs. I assume it is OK to completely sever the top
and bottom plates where the stack passes through. As for the
reventing, the wall is nonbearing, so I can bore a hole up to 2" in
the studs, and the O.D. of 1.5" ABS is only 1.9". Boring for a 45
degree vent run will lengthen the hole required, but the width will
still be 1.9"; I assume the extra length will not be a problem.
Thanks for all the help.
The 1.6Gal closets should not care what the stack size is.
As I mentioned earlier, you may have problems connecting
2 back-to-back closets on 3" stack if using San Cross/Dbl TEE.
Better is 4 X 3 (4" stack/3" branch) and I would ask if they
will even allow this in CA.
Here too, I would go with 2 X 1 1/2" Dbl TEE/Cross (if allowed).
The vent can reduce to 1 1/2".
If you can use PVC, there is a small dimensional advantage
(may not matter).
I don't see any problem boring holes, although you may have
to add stiffeners to the studs.
Thanks for your comments here and earlier. My reading of the 1997 UPC
(operative in CA) agrees with you: back to back drains entering a
stack are allowed using a sanitary cross when the stack is increased 2
pipe sizes (usually 1"). However, my inspector indicated that a 3"
stack is OK if the sanitary cross has some mechanism to prevent the
discharge from one inlet from going back up the other inlet. She
mentioned a "Figure 5". I have no idea what this is and couldn't find
any information on it.
Anyway, I thought I would avoid the whole issue by using two sanitary
tees (stacked vertically), as I have ample altitude in my crawl space.
Any problems with this or with a 3" stack?
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"?
Most importantly, does the 2x4 plumbing wall seems reasonable with all
this going on, or should I go to a 2x6 wall? Obviously I'd prefer to
have the 2" in the rooms.
I've ruled out PVC on environmental grounds, it is the worst plastic
out there from a manufacturing and disposal point of view. ABS is
apparently not much better, but I can't really see using cast iron or
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.
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.
OK, if allowed. Better practice here to use 2".
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.
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
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)?
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.
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
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!
No, that is exactly the situation which would require
reventing the lower closet bend. Vertical distance has
no relevance. And not just closets. It really gets tricky
with combos of fixtures above and what needs to be vented below.
Different where you live too...
Yes, use branch of 1 1/2" and trap adapter to accept 1 1/4" trap
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