Is there really supposed to be standing water in a p trap?

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I am asking this specifically regarding my upstairs shower, which always has about 2 inches of standing water in the drain. It drains, except for one day when it backed up a little bit. My local Home Depot guy recommended that I treat it with something similar to Drano, but after two treatments, it still has standing water. Another trip to HD and a different guy later, I am told that there should always be some water in the P trap. Can anyone tell me about this?
  9 answers
  • Yes, that is the purpose of the trap. It prevents air from migrating from septic waste lines into the house. Not only is sewer gas repulsive to smell, but it often contains methane which is explosive. BTW, that store claiming "An expert in every aisle" is guilty of false advertising. Most of them are great BS artists.

  • KMS Woodworks KMS Woodworks on Mar 15, 2012
    The water should be in there and about 1 1/2 to 2" deep.. One issue in our arid west for seldom used showers in guest baths etc is the water can evaporate and allow the sewer gas to enter the room.

  • Don't ever use Drano or Liquid Plumber or any type of chemical cleaner in your drains. Manually clean the clogs...others may disagree with me but the use of these chemicals will damage the drains...especially if they are pvc. There should always be water in the "p" trap which blocks sewer gas and keeps the place from smelling!

  • Plumber26 Plumber26 on Apr 03, 2012
    I agree w/ handyandy.... always manually clean drains w/ a snake..... if you DO use drain treatment products use the ones that are enzyme-based (bacteria) like Rid-X or Drain-Care..... these you mix w/ water and it's supposed to keep solids broken down and flushed as opposed to letting it build up on your pipes..... Draino and Liquid Plumber are a BIG waste of $$$ and I'm surprised there have not been lawsuits about their claim that they are "safe for all pipes". 9 times out of 10 when I see drain failure on a certain drain I see Draino under the same sink. (2+2=4)

    • See 1 previous
    • Chad Chad on Jan 24, 2016
      Sorry correction, it does NOT appear to be leaking

  • KMS Woodworks KMS Woodworks on Apr 04, 2012
    Sodium hydroxide in the most common ingredient in drain cleaners...some even throw in metal "shavings" that combined with the heavily exothermic reaction "provide scrubbing" like effects. In my book that is a chemical "bomb" with shrapnel. I will pull the p -trap fish out the hair ball (95% of the time it is a hair ball) and be back on line in about 10 min.

  • Comet Comet on Jun 03, 2015
    "Sodium Hydroxide" I believe is LYE---a highly toxic and dangerous chemical that can BURN esp in the presence of liquids. The best way we have found to clear a hair clog--and we have three females with long hair here and dogs who have to be bathed in the tub---is womens hair remover for legs etc---we manually remove as much as possible--we even have a clever device with hooks on it looks like a long zipstrip==and then add a few squirts of the NAIR or similar to get rid of the rest. WE are on a septic and have had no issue with this. For the "P" trap==yes you DO need to keep water in there for reasons above.

  • Cd Cd on Mar 12, 2016
    There should be water in the P trap to stop sewer gasses. Thtas doesn't mean there should be water standing in your tub or tower pan. You've got a clog situation. I also vote for less caustic cleaners. Besides snaking dump some enzymes in your drain at night and let it do it's work without harming your pipes and our water supply.

  • Theresa Adams Theresa Adams on Oct 05, 2017
    My Plummer said my kitchen pipes water is not draining out to the sewer, he said my pipes in the basement go down then a pipe shape like the letter U stops the water from draining out to the sewe, he also said that the pipes in the basement are for bathrooms he doesn't know how this pasted inspection, he said the fix the problem would cost $450, please let me know if this is correct or not?

  • Michele Bryant Millican Michele Bryant Millican on Oct 21, 2020

    Should the water be up to the drain where you can see and touch it?