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Plumbing: Why upstairs drain water comes into downstairs sink

D107 Member Posts: 1,852
Just discovered this, and heard water draining on floor through it when upstairs sink or shower is used. (Luckily not toilet--maybe toilet waste empties down another side of the main stack.)

Makes me wonder if, with the downstairs P-trap being open --not water in trap--if that creates an inlet for upstairs drain water that will be solved by replacing the P-trap, or if the drainage indicates another problem.

I was hoping to simply replace the P-trap myself (hone my very limited plumbing skills), but aside from only one of the 3 connections being movable so far, I'm thinking best to have a pro look at this if there are other problems present here.




  • Brad White
    Brad White Member Posts: 2,398

    I would suspect a blockage in progress- arteriosclerosis basically. Essentially the same as an undersized drain line. The volume heading down the drain is too much for the pipe to carry so the stacked head of water is forcing the path of least resistance.

    Other things to look at: Blocked or restricted vent (late bird nesting this time of year and by golly they are none too particular).

    Hardly the only reasons, just what comes to mind. Yes, call a pro before you find yourself over your head.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Weezbo
    Weezbo Member Posts: 6,232
    it might depend on who when and how it was 'Piped\"

    some older homes seem to have gone through extensive homeowner interventionwithawrench..likely you have no vent.

    the upstairs drain is the vent .
  • D107
    D107 Member Posts: 1,852
    thanks Brad, weezbo, yes prior owner

    did alot of re-piping, though supposedly by plumbers, but shortcuts are always attractive at the time.

    I see in the attic, the 1" PVC AC waste line goes into a horizontal portion of the main stack before it 90s and goes up through roof. Maybe that connection used to be for the sink. Of course I always understood the top floor was stack vented, and the sink would be covered by the same vent that handles the toilet and bathtub. But when they re-did bathroom floor years ago, maybe connections weren't done right. I recall that in merry old Brooklyn years ago even when they re-did the toilet lead bends they rarely vented them; the old construction didn't allow them room.

    Six months ago after much difficulty I augered out a big blockage from the upstairs sink--hair mostly--perhaps it's making its way down. Also been told that the old galvanized drain lines could be trouble.

    There is a vent connection for the first floor, leading out to the back house wall--protected by a grill, but I agree who knows how well this was all plumbed. And perhaps the 4" stack through the roof should a have a screen to keep out the birds and copious falling leaves.

    I'll have a plumber come around; it may be the house needs a roto-rooter top to bottom. We did have our sewer line cleared of tree roots this summer--we now keep bushes at least three feet from the sides of our house. Ever since I saw roots growing right through the house trap caps.

    Thanks again,

  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
    Teenage Girls

    Agree with Brad. Just had this happen to me. First signs of a problem surfaced about 3 months ago. The waste line backed up, flooded the downstairs bath and hallway. It was tampons.

    If the trap was an issue, it probably wouldn't drain to start with. You described waste water backing up into the trap.
    Sounds like a job for a pro.
  • martin
    martin Member Posts: 144
    Joy of Gravity

    I don't know how many clients I've had say "but my upstairs toilet is fine only the one downstairs overflows." You've got a blockage or obstruction somewhere below the bottom sink.
  • D107
    D107 Member Posts: 1,852
    Found possibly unusual piping

    Thanks Glenn and Plumbaris for your helpful answers. I love learning how this all works.

    While I wait for a plumber sometime this week--even though obstruction seems to have passed-- I checked out the waste lines a bit. Turns out:

    1-Main upstairs 4" waste line is only connected to the toilet upstairs (unusual--or just easier?); sink and tube drains appear to be connected to a separate 1.5" copper line that goes down parallel to the main stack and connects with the downstairs sink and then the main stack. That's why the upstairs toilet flush didn't come through downstairs sink drain. So obstruction as you say was somewhere below the sink in the 1.5" line that connected to a larger copper horizontal line --that also carried 1st floor toilet and kitchen drainage--that Y-ed into the main stack.

    2-Not yet certain but it looks like the downstairs sink waste piping didn't enter the main 1.5" copper line at the required downward angle (45deg?), which helps to prevent downcoming waste from entering the lower fixture drains.

    Thanks again,

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