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Thread: 2"main stack vent, big enough??

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  1. #1

    Default 2"main stack vent, big enough??

    Currently we have a 4"cast iron main stack/drain pipe. We are putting in a new bathroom, and going to be taking out the old main vent. We are wanting to just use a 2" main vent in the wall since the current walls are only 2x4. All that would be going off this vent would be the toilet, sink, and tub then going to the sewer line. We will be tapping back into this 4" pipe in the attic. Eventually we will be redoing the kitchen and it will have it's own 2" vent going to the sewer and attic. Will this work, or cause problems. Thanks much.

  2. #2
    Master Plumber master plumber mark's Avatar
    Join Date
    Feb 2005
    indianapolis indiana - land of the free, home of the brave....
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    Talking depends on where you live

    I am down with the flu today so I have ben posting on here like a mad man.
    Better than just sitting in bed.

    In indiana, all you need is one 1 1/2 vent going out of the roof to do the whole home....even a 3 -5 bathroom home!!

    they claim its all just air, and the system works the same wether its 1/1/2 or 3 inch

    Its good to have a couple of vents out the roof if it can be done, but its not really necessary

    just call a local plumber and get a free estimate.
    Last edited by master plumber mark; 03-20-2005 at 09:37 AM.

  3. #3
    Forum Admin, Expert Plumber Terry's Avatar
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    Aug 2004
    Bothell, Washington
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    If this were in UPC plumbing code country, you would need a composite 4" venting the home, that could be done by combining four 2" vents.

    You will notice that many homes have several pipes, (vents) on the roof.
    Working with a 3" waste line out of the home, you will often see two 2" vents and one 1.5" vent.
    Will the plumbing work with the 2" vent? Yes.
    When I'm plumbing, I count up the vents leaving the roof, the inpector will too. Ask the inspector first, what he will allow before committing too much time to this.
    If I plumb, I do it to the code book, then I don't have to worry. It's much quicker to do it right, then it is to try to convince someone against it.
    Tub 2.0 fixture units
    Lav, 1.0 fixture units
    Toilet, 3.0 fixture units
    Total, 6.0 fixture units

    A 2" pipe will handle 24 fixture units, that's like four bathroom groups.
    When I'm plumbing condo's and apartments, I may have eight or ten four-story groups that look somewhat like this but taller.


    Last edited by Terry; 03-20-2005 at 10:40 AM.

  4. #4


    Thanks! I think we'll end up doing the 3" vent. From the calls I made sounds like that's the code. Guess I better do it then


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