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Here is the layout and I am questioning the need to have the purple segment
the green blue selected segment is already in place, the green segment between the shower vent and the sink is what I am building (still dry fitting the pieces but I did not drill the holes for the studs to the left of the vertical green segment
As mentioned the distance between the two green verticals is 4'

I am also considering an air admittance valve enter image description here

Update: Here is why I need to go down and not horizontal with the drain enter image description here

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  • Why are you not taking a vent vertically out and up of the top of the sanitary tee up and then turn 90 to go horizontal to the vent?
    – Alaska Man
    Commented Nov 30, 2020 at 18:28
  • those studs are supporting cabintes on the other side and a hood range. They will also support a floating vaniti cabinet and a mirror on this side so I would drill them as little as I can (only drilling itself is discouraging enough :-)) )
    – MiniMe
    Commented Nov 30, 2020 at 18:36
  • But you are drilling holes for the drain pipe? I don't think holes in the studs for 1 1/2" vent pipe are going to render the wall incapable of supporting cabinets and a vanity. I have seen kitchen walls with cabinets that have horizontal vents in them.
    – Alaska Man
    Commented Nov 30, 2020 at 18:49
  • Don't have that much experience I prefer not to risk (in this case I will have to sets of holes if I do what you are suggesting)
    – MiniMe
    Commented Nov 30, 2020 at 19:00
  • One set of holes for drain and one set of holes for vent is standard operating procedure. Make sure you put a drill prevention plate on the stud where the pipe passes through the stud so you do not drill into the drain or vent pipes.
    – Alaska Man
    Commented Nov 30, 2020 at 19:01

1 Answer 1

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If the slope of the drain is 1/4"-per-foot you can do without the pink vent. Be sure to measure your slope with a level. Don't just measure from the floor.

You don't need an AAV to delete the pink vent.

trap arm size    trap max developed length @ 1/4"-per-foot slope
1 1/4"           5 feet
1 1/2"           6 feet
2"               8 feet

Instead of the green branch drain having a vertical drop, just run it horizontally (with a 1/4"-per-foot slope) over to the corner. That is within the max length before siphoning could occur. edited CAD drawing with red line for branch drain

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    worse come to worse I could install the AAV later correct ? When I drill the holes I draw a horizontal on the studs from where I want the lowest point to be. Then at the other end I calculate (distance times 0.25") and I draw a line at that point. After that I connect the two above with a line on the studs and I drill on each stud where the now diagonal line is (middle of it) That will guarantee me that the holes are perfectly aligned. I do not use the ground as reference as that is misleading
    – MiniMe
    Commented Nov 29, 2020 at 22:53
  • Sure, there's even a product called Magic Trap which is a kit containing a sink trap and a small vent all together. It's easy to add later if you need to. You shouldn't have to based on your plan, though.
    – Jeff Wheeler
    Commented Nov 29, 2020 at 23:05
  • Hmm it seems that your answer is wrong at this is breaking the rule of the weir trap arm buellinspections.com/weir-d-science-p-trap-works The trap weir must be able to "see" the vent take off. So neither of the solutions work, I would need to go horizontal straight from the point where the trap arm enters the wall Am I misunderstanding this ?
    – MiniMe
    Commented Nov 30, 2020 at 6:04
  • If your slope is correct, your trap weir will be beneath the vent tee. That's the reason the developed length of trap arm is based on the diameter of the drain and its slope. The link you posted is trying to explain this. Do not install a flat horizontal branch, it will clog. 1/4"-per-foot is the correct installation.
    – Jeff Wheeler
    Commented Nov 30, 2020 at 16:09
  • yes but the point is that in the trap arm the water could fill in the entire section of the pipe and on its way down on that vertical it will create a suction effect and it will take the water in the p trap with it
    – MiniMe
    Commented Nov 30, 2020 at 16:21

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