411 plumb The unsung hero of your sanitary drain system is no doubt the P-Trap.  The p-trap is an inexpensive device found on every drain except the toilet in your home or, business that serves as a barrier between your nose, and the foul gases, and, diseases that lurk in the sewer system of your town, or, city.

The P-trap is a “U” or “J” shaped bend that holds water in that shaped section providing a liquid seal between you and those sewer gases. When water goes down the drain the excess water is water over the dam so to speak allowing the drain water to enter the sewer system without the seal ever breaking.

In the past less effective methods were used to provide the seal. Those methods included bell traps, s-traps, drum traps, and a few others. These all had limitations in the sense that they either failed to provide good reliable drainage, were difficult to clean, or, were subject to the trap siphoning due to not being vented properly because of the design. In 2003 an outbreak of SARS in China was traced to ineffective traps.

A common mistake by DIY’ers is to not install the drain pipes under the sink properly, or,  to remodel a kitchen installing a deeper sink and garbage disposer without paying attention to the height of the drain in the wall. This results in the trap being installed in a manner that alters the depth of the water seal on the trap. This results in poor drainage, odors, and possible early failure of the disposer. The depth of the water in the trap is a set depth and must not be altered..

Today’s p-trap properly installed and vented is very reliable at draining and provides excellent sealing. There are rules at to how it is installed and vented. Generally the wall arm coming out of the trap is horizontal with a pitch of 1/4″ per foot minimum downward into the wall. Once inside the wall it can either turn down into a vertical drain or continue on a pitched horizontal run. However in any case it must have a vent before the drain turns downward. There are also limitations of the total developed length of the horizontal run before having a vent. Consult your local code so your installation will meet the code in your area. Failure to properly vent the p-trap can cause siphoning of the water out of the trap allowing sewer gases to enter your home.

When your sink becomes clogged it is a good idea if you can remove the trap and inspect it and the pipes from the sink drain to the wall to see if the blockage is in any of those pipes. Use a bucket under the trap to catch any water remaining in the sink and drain pipes to keep the mess under control. If the  blockage is not in those pipes but into the wall you would probably be best served by calling a plumber to clean the drain line.

Posted in How To Plumbing Tips, Plumbing Parts, Suppies, & Components Tagged with: , , , , , , , , , , , , ,
54 comments on “P TRAPS
  1. Sherri says:

    I live in a older house and there is a foul odor coming from the bathtubs. I have an almost impossible house to climb under to fix anything, but I am sure that the trap that is supposed to block the odors is missing! Is there anything I can get that does not cost an arm and a leg to get to fix this problem?

  2. Jack Stroud says:

    My son is redoing the bathroom. He would like to reinstall his original vanity but he wants to center it. The vanity was originally installed off center because it is designed with three drawers which intefere with the p trap.

    Is there a solution for this?


    • Redwood says:

      Hi Jack,
      Yes, there is a solution, all you have to do is figure out where the supply and drain pipes need to be located in the wall to fit the new vanity and move them to the new location. A much easier job before the vanity is installed. After the fact sometimes an 1/8 or 1/16 bend elbow may be used to line up the drain but that depends on the clearances allowed by the new vanity. It’s always better to do it right before, than trying to patch it later.

  3. Andrea Giba says:

    We lost an earring down the bathroom sink. We removed the p-trap and got it out, but then we couldn’t put it back together without leaking at the 1st connection coming from the wall to the p-trap. No matter what we do, we can’t get it to stop. It wasn’t leaking until we took it apart. Can you help?

    • Redwood says:

      Hi Andrea,
      Are the pipes metal or, plastic? Have you replaced the rubber or plastic washer at the connection if there is one? Some pipes just use a beveled connection with no washer if this is what you have is it cracked? A telltale sign of a crack on a beveled connection is the more you tighten it the worse the leak gets. Do the pipes sit together naturally and straight without the nut being connected or do you have to bend them to make the connection? You may just have to start out with new pipes to fix the leak. They aren’t expensive.

  4. tammi says:

    Trying to clean out p trap, only 1 part of the trap has a screw, there is no back screw everything is glued into place. Ughhhhhhhhhhhh what can I do………..

    • Redwood says:

      Hi Tammi,
      In some areas of the country Glued Trap Outlets are required by code, if the plumbing code in your area allows it you may be able to cut off the existing trap and replace it with a slip joint connected trap.

  5. Ron says:

    Do they make 2″ rubber P traps to replace plastic ones under a shower ?

    • Redwood says:

      Hi Ron,
      I’ve never even looked as it is not a product that I would even consider using or, recommending.
      What is wrong with the p-trap that you are looking to replace?

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