Plumbing drain/wall sizing questions in US bath remodel

Hello,
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.
Cheers, Wayne
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Wayne Whitney 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.

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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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 DWV copper.
Thanks, Wayne
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Wayne Whitney wrote:

Go here: 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.

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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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)?

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 fitting"?

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Wayne Whitney wrote:

Yes

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 arm.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.