# Can a 2" pipe connect vertically to a 3" pipe before it hits the vent?

I have about 2' of 2" pipe coming from my shower. It's well within the required 5' before the 3" vent stack. Can I tie it into the 3" pipe coming from my toilet with a sanitary tee?

In my head I feel like this setup could syphon the trap from my toilet, but I haven't been able to find any code that prohibits this. Am I missing something?

• Is there a problem with connecting directly with the vent/drain stack? Jan 4 at 0:25
• It's much less convenient to get to @crip659 Jan 4 at 0:44

That arrangement should be fine, provided the shower has an individual vent (see sketch 1). The other option is to connect to the stack above the tee for the toilet with a 3 x 2 tee (see sketch 2). This would make the portion between the two tees a wet vent.

• Thanks for the diagram. That confirms what my intuition was telling me. I'll have to figure out how to get a vent in there. Jan 4 at 4:43
• My eye sees the shower drain flowing up a vertical pipe on its way to the stack in drawing 2. I'll overrule it and interpret that as a long horizontal bend. Jan 4 at 4:56
• Yes, that is a two 45° elbow horizontal change of direction. This is one negative of isometric drawings.
– pdd
Jan 4 at 16:50

It's well within the required 5' before the 3" vent stack.

But the toilet pipe is NOT the vent stack. And therefore code does indeed prohibit that.

The fact that you mention the distance of the shower pipe to the vent stack implies that the shower is not vented off your picture, and what will most likely happen is that the toilet will suck the shower trap dry when it flushes. Then the bathroom will have a sewer gas issue.

No, your vent is no good.

The purpose of the maximum p-trap to vent distance is to keep the p-trap's weir elevation below the top of the drain pipe elevation at the vent, where that constraint prevents the p-trap from getting siphoned like an s-trap. For a 1-1/2" drain, for instance, and the 1/4" fall per foot spec, the maximum p-trap to vent distance is (1-1/2")(1ft / 0.25") = 6 ft. Under the IPC anyway.

• UPC and IPC have different trap arm lengths, (so 5 feet for 2" is UPC, while IPC allows 8, IIRC) but given the trap arm is not connecting to the vent in the proposed arrangement, that point goes out the window. Jan 4 at 1:04
• @Ecnerwal, thanks. Jan 4 at 1:16
• That logic makes a lot of sense, thanks for explaining the "why" of it. Jan 4 at 4:40
• @sallf, sure. See codes.iccsafe.org/content/IPC2021P1/… if you're interested in the IPC's exact language. Under the IPC, you can still get rejected for too much fall even if the trap arm meets the maximum length spec. Jan 4 at 4:51