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?