I am re-working my laundry area. Previously, it had only one device connected to the drain line - a single sink, into which the washing machine (and everything else) drained.

Now I would like to add a separate stand pipe for my washing machine (and others) to drain into, and a second utility sink in the garage.

I understand that each device needs its own trap, and each trap needs its own connection to the vent stack.

In the image below, the area above the top line is my (attached) garage. The area to the right of the right line is outside.

There is an existing 3" ABS drain line coming down from the second floor (bottom right), connected directly to the sewer line. This line already has a T in it, going to 2" ABS for the previous drain setup. This 3" ABS pipe is connected directly to the vent stack - I am not positive, but I think it goes straight up the whole way to the roof.

There is a separate 1.5" ABS pipe for a vent connection near the center of the right wall - at the ceiling. It was previously connected directly to the 2" ABS drain line.

What is the right approach here? Do I just run the 2" ABS drain line to the upper right corner, and with a right angle go left to the farthest sink? Each device needs its own trap, but can I get away with that 1.5" vent pipe coming straight down and connecting to the drain line in that one place, or do I need to extend it so that it also runs to the upper right corner and then out to the left, connecting to the 2" ABS line before each trap?

enter image description here


2 Answers 2


Look at "circuit venting" in the IPC (or your applicable code - IPC is conveniently available on-line.) There are multiple acceptable ways to get this done.

You don't need an individual vent riser for every trap.

  • Thank you - this saves me a ton of pipe and frustration. :)
    – negacao
    Aug 3, 2023 at 18:08

Here is how I would pipe this:

enter image description here

You need to vent the fixtures, not the drain pipe.

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