Plumbing Vent Question

[ahansel8]

Long story short, I had to jackhammer out a portion of my slab to fix a failed sink drain. I figured, while I was in there, go ahead and jackhammer out a little extra so I could install a drain line for a sink outside on my patio. One of the shower walls is an exterior wall.

The sink drain line is 2", and it ties into the shower drain line after the shower trap. The shower line is also 2". The vent that services the shower also services a toilet, and is 1.5". The vent stack is located roughly ten inches past the sink/shower junction, and is five feet from the sink trap.

Now that the specs are out of the way, is it appropriate for the existing vent stack to service the new sink? I would just put in a new vent jack, but for several reasons routing of a new vent in the exterior wall is going to be, as they say, a total bitch. Thanks in advance for your answers.

 

[Terry]

A 2" trap arm can go 60" to the trap, as long as the grade is 2%, or 1/4" per foot.
If you have any bends on that line that raise it, then you need to add a vent. You can use an AAV for that sink if needed. Air admittance valve

 

[Sponsor]

 

[ahansel8]

Thanks for the quick response! My question however referred to the vent line itself, and if the setup I described (probably not very well) was legit. Attached is a diagram I sketched out. I was going to route a vent line for the new sink itself, but a my neighbor told me the existing vent (show in the diagram) was sufficient. I'm not 100 percent convinced, and since I'm going to be laying tile on the wall that will have to be ripped out if he's wrong, I want to make absolutely sure. From the research I've done it seems fine, but a master plumber I am not. Anyone see any potential issues with this?

Thanks MUCH in advance!

Sink line maintains a 1/4" per foot slope
All drain pipes are 2"
Vent line is 1.5"
Distance from sink trap to vent line is 60"

 

[Terry]

The sink isn't vented.
The vent comes off the top of the trap arm, not below.

 

[ahansel8]

Darn, I was afraid something just wasn't right in there. So once the trap arm makes a 90 and goes under the slab, it's not longer considered trap arm? The vent stack does indeed come off the top of the drain line after the sink/shower junction, if that makes a difference. This is also a single-story home. Here's a picture of the actual project. Pipe on left is for the outside sink, and it joins the shower drain line, after the trap, about ten inches before the vent pipe you see on the far wall next to the outlet.

I can't vent going straight up from the sink drain line because there's a window above it. Zig zagging a vent line around the window is doable, but drilling up through the top cap presents a problem in clearance between top cap and roof (the roof rests on 2x4 trusses) and there's almost no room to work. I guess I could cut a section from the exterior wall to gain access to the corner of the shower wall shown and make a 90 and tie into the vent line, but that would be a last resort and I'm not even sure it would work. Guess I should have done my homework on this one BEFORE starting!

Any suggestions welcome.

 

[Terry]

We typically do have to find our way around window.

Normally the vent doesn't go horizonal until it's 6" above flood level.
42" in many cases.

 

[ahansel8]

Apologies for the confusion, but as it stands there is no horizontal venting. The vent line comes down into the drain line after the junction of sink and shower drain lines, which is within five feet of sink fixture. Bottom line, that vent line is not sufficient as is, to service the sink fixture? If not, why? Looking to learn, not argue.

 

[hj]

The bottom line, since I cannot interpret what you are describing, is the the drain CANNOT turn downwards until AFTER the vent connection. In many cases the vent could be 6" from the connection/sink fixture and still NOT be a proper vent for the drain.​

 

[ahansel8]

That settles it then! Thanks for the help.