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Venting a combined basement bathroom / laundry room into existing 4″ vent / drain stack

m2ew | Posted in Building Code Questions on

I’ve had a few quotes for installing a new 4 piece combined basement bathroom / laundry room (dryer, shower sink toilet) as apart of getting our basement finished.

Currently the main 4″ stack serves as a combined vent / drain for a 3 piece washroom (2nd story), and as a drain for a (1st story water closet – 2 piece, that is separately vented with a 2″ vent through the roof)

Our hope was to tie the new bathroom / laundry room into the existing 4″ stack without having to run an separate additional 2″ vent. Is this possible? The bathroom will fixtures are all within 10 feet or less of the stack. If venting is need, would running 1 1/2″ branch vents from the fixtures to the 4″ stack suffice?


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  1. GBA Editor
    Martin Holladay | | #1

    Plumbing code requirements for venting are complicated. When in doubt, consult a plumber.

    Q. "Our hope was to tie the new bathroom / laundry room into the existing 4-inch stack without having to run an separate additional 2-inch vent. Is this possible?"

    A. As far as I know, the answer is no (although I welcome input from GBA readers who are plumbers). Attached is an illustration from a Fine Homebuilding article called "Plumbing a Basement Bathroom" to give you an idea of what is usually required.


  2. DIYJester | | #2

    The main issue you are going to run into is that you're trying to tie your vent in below the water line of your highest fixture's drain from what I can tell. In other words, it would be possible to send drainage from an upper floor down the stack and into the vent.

    If the stack is vented, which many times they are in the basement, you probably can just tie into that stack.

    As Martin says, plumbing codes can be tricky to read, but are necessary. There are also limits on how far a vent can be from the P-trap and are based on piping size. You also have to ensure you have the proper size vent for the number of DFU (drainage fixture units). If you have a 2" main it would be good for 24 DFU's. My house has three bathrooms and has two 2" vents and a 3" vent. Most of the reason for the number is due to distances from drains so they are all not running all the way across the house.

  3. Expert Member
    Michael Maines | | #3

    The IRC rules are pretty confusing, but worth trying to understand:

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