Connecting ABS at <22.5 degree angle?

Question

I’m trying to get my plumbing vents up into my roof joist bays. They’ll run halfway up the house until they hit a beam. I’m then going to level off the vents so they dip below the beam and connect perpendicular. They will be a soffit next to the beam to hide them.

The catch is that to angle them below the joists and maintain the proper 1/4”/ft slope, I need an angle of around 15 degrees. The smallest elbow angles I can find are 22.5.

Questions:

1) do they make elbows with smaller angles than 22.5?

2) is there a different type of connector I should look for?

3) would it be ok to use two 90 degree elbows connected horizontally? I could achieve any angle with those but do end up with a lot of 90 angles in my vents. Is that ok?

Answer 1

My standard procedure for dealing with shallow angle bends when none of the prefab sizes are right, is two of them.

Obviously you can get the bend you want with two 90's; my point is this works with shallower angles too. You simply have the bends in different planes, so they add up to the correct angle in a third plane.