p trap plumbing question. pic included.

Extend up, add a santee and then revent at 42" to the other vent.
Going from vertical to horizontal requires a long turn 90 el.
The vents can use med 90's or vent 90's
Medium 90's cost less then a true vent 90, so I mainly carry and use the medium in those locations.

aharami said:

Yes, that's right, no other fixture upstream of the shower wye. Thank you for explaining.

Now the question is: can I take the shower ptrap left to right and then make a u into the 3"main using a 3x3x2 wye? See picture
View attachment 22749

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As long as the wye is laying down as seen in the photo above, but you may run out of room using the approved fittings. Tilting the wye above the drain or ofsetting down into the wye would require a new vent to avoid s-trapping. It's not allowed for 90s to be installed horizontally on drainage, so 45 degree bends will have to be used to make 90 bends. Some codes also do not allow a 135 degree bend for a fixture drain. It seems that you're trying to get the drain on center of the shower for a nice appearance, but is there any way to adjust the design so that the trap can flow more directly into the 3"?

I just went up to the attic to check how everything is vented, and this 3" pipe is the main vent stack. But from this pipe, there is a 2" vent offshoot going into the second bath. The kitchen and downstairs half bath are below the second bath. So maybe the 2" vent is venting both second bath as well as downstairs.

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So you're thinking that the other bathrooms are vented by a 2" up past this floor, and tied in at the attic?
The 3" you are tying into for the shower is used for waste on the downside of the wye, all the way down and ties in downstream of of the other fixtures?
That makes more sense.

quote; What you are trying to do is to pipe a fixture into the house stack. You can't do that.

And why not, since it appears he already has TWO existing drains connected to it. As for the original question, if you run the drain horizontal as necessary to avoid a separate vent, the "P" trap will probably have to hang through the ceiling. IF it would work then a "combo" in the 3" line and a pipe to the "P" trap should work perfectly. You would rotate the P trap's "J" bend to line up with the shower drain location.

I want to say that this thread has been very educational. Thank you to everyone who participated and helped me out. I have decided to re-plumb everything and connect the sink pipe to the toilet pipe thus eliminating the wye that was getting in my way before and connect the shower ptrap properly to the main stack without doing any uturns or funky stuff.

This thread made me realize that the toilet wasn't vented properly. 3" arm after flange was going down before meeting vent. So I'm going to create a vent via a 3x3x1.5 sanitee and connecting across the joists as shown below.


But I have one question. Does a vent pipe always have to have a pitch like drain pipes, or can a portion of a 1.5" pipe flow downward to meet the main vent stack?

Would something like this be legal for venting?

Trap-to-vent distance is determined by how close the horizontal drain connection is to the vent connection in relation to the trap. Where the vertical riser is has no bearing on the 8 ft. limitation. For example; The 3"x3"x2" wye going off to the shower looks like it will have a trap to vent distance of about 18"-24". The vent starts at the connection of the wye. Additionally, the toilet is vented through the soil stack. Even with an offset on the drain before the connection to the stack, this is allowable by most codes for toilets only as long as another toilet isn't connected upstream....which there isn't.
If you did add a vent for the toilet, 2" is code. I understand that it would be difficult with larger fittings to get it done, but because the toilet is stack venting it's a moot point anyways. An additional 1 1/2" vent on a DIY project couldn't do any harm as long as it isn't looped over with 90s as shown in your picture. A horizontal vent must always continue with slope/fall as cacher_chick mentioned and then go vertical to allow condensation to drain or wastewater in the event of a clog.