Wye/45 combo on vertical drain pipe

Is this acceptable/allowed or not? Or should it be a tee? Of course, this graphic shows it’s unacceptable, but it seems like from what I read, it is acceptable?? TIA.

Plumbing - tees and wyes.png

The graphic is correct. Do you understand why the graphic is correct? See venting restrictions.

I see that. But everything I read, says that trap arms should use tees here. I can’t find anything in writing stating that you can’t do it on main drain like this, or maybe I’m reading it wrong. Do you have any written documentation you could share?

Hmm, not sure. I’ll have to take a look later today. I will post here if I find anything. In the meantime, consider this. It is required for trap arms thus the needed venting for the trap to operate properly. The venting “air” needs to travel from the vent stack all the way to the trap. If there is an improper fitting downline of the trap arm, air will be restricted. Visualize the ‘path’ that the waste/gray water takes at a downslope. Is there an unhindered path for return air?

Does that help?
(Haven’t had my coffee yet).


706.2 Horizontal
drainage lines,
connecting with a
vertical stack, shall
enter through
combination wye and
1/8 bend branches.

A wye is not allowed as in your graphic from a trap which is up steam of the vent which in theory can cause the trap to siphon. Your picture depicts a drain which is down steam of the vent.

Did all of the DWV system components function properly during your inspection?
Did you detect any backups, sewer gas odors, leaks or gurgling? If not, I would not foray into the realm of a licensed plumber.