The vent would be the 3" pipe I'm connecting it to. That 3" pipe runs to the right, then goes up to vent out of the attic.Where's the vent?
The pipe from the trap to the vent connection, wherever that is, MUST be horizontal, (other than the slope due to pitch), It cannot turn up OR down until then, and your pipe appears to turn downward into the "Y". Also, if there is a toilet 'upstream" from the shower connection, that is also illegal unless the shower has its own dedicated vent.
I see what you have there now. What you are trying to do is to pipe a fixture into the house stack. You can't do that. You need a separate stack that runs all the way to below the fixtures on the first floor.
What is the reasoning behind that? As long as everything is joined properly, vent is vent, no?
No vent is not vent. That stack serves the fixtures below it. If you use it the way you are trying to use it, you would need to re-vent all the lower fixtures to a point above the flood level rim of the 2nd story fixtures. In short, you can't tie into that 3" pipe with waste water. You need to run a separate 3" pipe all the way to probably the basement and tie into the waste there. Absolutely do not continue to do what you are doing or you will have big problems.
Uh-boy. Dude, study this for some help. Helpful Plumbing Hints for Residential Construction by Bert Polk Plumbing Inspector Lincoln County
Then, look at p. 9 where there is a picture of a correctly-vented tub p-trap.
Also, p.3 of this guide may be helpful to you: Plumbing-and-Trenching-for-Homeowners---2018 (klickitatcounty.org)
Also look at p.6, and p.4 and 5.
It's okay to pipe the shower into the stack. That is the main soil stack and fixtures can drain into it as long as they have their own independent vent. Any fixtures connecting to the stack below should already have their own vents.
If that 3"x3"x2" wye is the highest connected fixture, then the remainder of the soil stack becomes the stack vent and will also serve as the vent for the 2" shower p-trap. Meaning this...it looks like upstream of the shower the 3" 90 bends vertically and may go through the roof from there. As long as no fixtures flow in from another higher level than the shower.
This is awkward, but...
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