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Thread: Basement floor drain - vent required?

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  1. #1

    Default Basement floor drain - vent required?

    Excellent forum! And thank you in advance for any insights.

    I am making plans to add a bathroom to my first floor by utilizing a floor drain in the basement. The current drain, 3-inch, is trapped but not vented, and is approximately 15 feet from the main stack in the house. I intend to remove the trap, install a 3-inch PVC going up-stairs to feed the new bathroom, which will be vented, etc from there. I would like to install a new floor drain in the basement, tying into the 3 inch line using a wye. Question: does the floor drain need to be vented? Iím thinking: yes, or the rush of water in the 3-inch line from the bathroom upstairs would siphon out the floor drain trap. But Iím also thinking no: that a 3-inch line is large enough so this wonít happen. If it is vented, what would be general practice: to use an AAV or to have a vent head up through the floor into the bathroom, where it would tie into the bathroom vents and ultimately go through the roof. That would mean another hole in the floor, not impossible, but not great either.

    Block diagram is attached as a pdf.

    Thanks for any info.

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  2. #2


    If I were going to put all that sewage producing stuff above that floor drain I would seriously consider putting a backflow preventer on the floor drain branch. If it should clog downstream of the floor drain then you gonna have a basement floor full of s**t. If possible/practical, add a trap primer to the floor drain. If you want to do it right, add a vent to the fd branch also.

  3. #3
    In the Trades kordts's Avatar
    Join Date
    Oct 2005
    exurban Chicago


    Each fixture need to be vented. Even if the trap was vented, you couldn't use it to vent fixtures.

  4. #4


    Quote Originally Posted by kordts
    Even if the trap was vented, you couldn't use it to vent fixtures.
    Why is that? Something I'm missing here... and I'll admit, my understanding of venting is questionable.....

  5. #5


    That does not exactly make sense to me as it is written. Assuming I install a vent as described, each fixture would be vented, but all the vents would tie together at various points ultimately passing through the roof as a single vent. The vent from the basement would remain separate from the drains the entire way, but would tie into the vents from the bathroom fixtures some 42 inches or so above the floor in the bathroom. Then that single vent pipe would proceed up through the second story and through the roof.

    I am really thinking I require a vent downstairs for the floor drain now, although in reality I do not think it would make a difference if the vent were not there.

  6. #6


    Interesting point on the backflow preventer. I'll have to think about that.


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