Wayne Whitney wrote:

In article , Speedy Jim wrote:

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.

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.

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.

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?

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.

Here too, I would go with 2 X 1 1/2" Dbl TEE/Cross (if allowed).
The vent can reduce to 1 1/2".

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".

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 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.


If you can use PVC, there is a small dimensional advantage (may not
matter).

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
DWV copper.

Thanks,
Wayne