Can a plumbing vent start in the horizontal where it connects to drain line?

Question

I'm laying out my shower drain and it initially was going to look like this:

Fairly straightforward. Shower into p-trap, vent coming out of top of drain line over to wall to be vented, all connected into the main. This is the TOP drawing below.

The catch is that the total height from the top of the vent elbow to the bottom of the p-trap is much taller than the height I have to work with in the joist bay.

Can I do what the BOTTOM drawing is showing? Instead of the vent coming off the drain line vertically, it's horizontal over to the wall, where it then goes vertical. This would shorten the total height allowing me to raise the whole shower drain assembly up. I can then connect that to the main run with a 45 degree connection.

In researching this, I've seen some informal mentions that this is OK as long as that vent branch is also sloped the requisite 1/4" per foot like any drain line. And I don't see a physical reason why this wouldn't work. But I can't seem to find a definitive "yes, this is OK" from a code perspective. Is this OK code-wise?

Answer 1

The only thing I see in the code is “A dry vent connecting to a horizontal drain shall connect above the centerline of the horizontal drain pipe.”, so as long the wye is angled up (which it would be to get the slope you described), I think you would be good.

I am not a plumber, so I advise waiting on some more experienced advise.

Answer 2

Could you try angling the wye 45deg (at most) between the trap arm and drain, then using a 45 or 22 to connect at the vent?