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    Why does the water from bathroom sink back up into bathtub?Thanks!

    Last edited by speedball1; Oct 26, 2008 at 08:14 AM.
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    Milo Dolezal's Avatar
    Milo Dolezal Posts: 7,155, Reputation: 519
    Plumbing Expert

    Oct 23, 2008, 11:15 PM

    Basic Law of Physics... Here is why:

    1. Your Sink and Tub drains connect under the floor and continue as one pipe. Somewhere behind this connection, you have a clog.

    2. Now, your sink drain opening is about 24" high. Your tub drain opening is at the floor level. Therefore 24" below sink drain opening.

    3. When the clog occurs in the common drain, water starts backing up. As the water column rises through the system of pipes, it will found the lowest drain opening and pours out through there. The lowest drain is in your case your Tub.
    speedball1's Avatar
    speedball1 Posts: 29,303, Reputation: 1939
    Eternal Plumber

    Oct 26, 2008, 07:57 AM

    Quote Originally Posted by yvettepch17 View Post
    why does the water from bathroom sink back up into bathtub?
    Because you have a clog downstream from the tub. You have options here. 1) you can snake the line from the lavatory roof vent **or**
    2) you can snake from the tub waste and overflow,(see image). Unscrew and remove the overflow plate,(The one with the drain lever) and pull out the tub stopper. You now have a clear shot at the trap. The way you were going in through the drain it ran you square into a tee that you can't get around. Now feed the cable( 1/4" best or 3/3" with a small tip. You can rent a Ridgid K50 sink cable.) About 2' down you will run into the bend of the trap. Crank and push at the same time to work around it. Your blockage will be found in the lateral pipe going to the stack. You shouldn't have to put out more then 10 or 12 feet. I have found tub and shower clogs to be mostly hair and the worm should auger into it and pull it back. Hope this helps and thank you for rating my reply. TOM
    john034's Avatar
    john034 Posts: 1, Reputation: 1
    New Member

    Nov 27, 2012, 11:02 AM
    Why don't the first floor sink back up
    speedball1's Avatar
    speedball1 Posts: 29,303, Reputation: 1939
    Eternal Plumber

    Nov 27, 2012, 03:40 PM

    Hi John and welcome to The Plumbing Page at You are responding to a 4 year old dead thread. Please check the date before you post. Thanks, Tom

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