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Flat vent


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But then again, the drain just below those two may be for that sink. There were no other plumbing fixtures beyond this area. Kitchen was on other side of house.

If it's just a vent, then it should have been configured so that the wye takes off from above the centerline of the pipe.

If it's a sink drain and also functions as a vent for the shower (not allowed in my area but, I believe, allowed in many other areas) then it still has to drain into the larger pipe from above the center line.

If it's just a sink drain with no vent, then I think it's fine as it is, except that then the shower isn't vented.

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Am I seeing this correctly that another pipe enters from the background of the picture, and that there is no vertical vent pipe at the sanitary tee that is just downstream of the wye with the shower trap arm?

Regardless, this thing is a mess. If that pipe to the right is the vent, then the problem with having it "flat vented" is that water in the shower arm can close off the vent, siphoning the shower trap. Anything that causes you to lose a trap seal can allow entry of sewer gases, vermin, and other nasties into the living space.

If that pipe to the right is a fixture drain with a vent on top of it, then you conceivably have an instance of horizontal wet venting, which California started to allow in bathrooms in 2008, except that it is still fubar. The wet vent would need to be sized for all the fixtures for which it is providing venting, and it doesn't seem possible here. The sanitary tee also has to be vertical. What is the other pipe coming in through the combo fitting in the foreground? It also looks like they notched the bottom of the beam.

A sort of general rule to follow on something like this is that plumbers will install pipes vertically or horizontally. They won't improvise something that is in between.

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It's possible that the top of the 3x2 tee is the vent, with a 45 right where the beam is notched, and a pipe continuing back, parallel to the joists, and then probably up inside (or on the outside of) the exterior wall. Very hard to tell from the photo.

That 3" pipe is likely coming down from the toilet. No vent there.
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