Plumbing trap schematic (C) Carson Dunlop Associates Plumbing Trap FAQs
Q&A on plumbing traps

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Plumbing trap installation, inspection, repair, defect, hazard questions & answers:

Frequntly-asked questions & answers about plumbing traps or interceptors used on plumbing fixtures: sinks, tubs, showers, etc.

This article series describes plumbing traps (interceptors) and how to diagnose, find, and cure odors in buildings including septic or sewage or sewer gas smells or "gas odors" in buildings with a focus on homes with a private onsite septic tank but including tips for owners whose home is connected to a sewer system as well.

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Plumbing Traps (Interceptors) Questions, Answers, FAQs

Open indoor trap (C) Daniel FriedmanQuestions & answers on improper plumbing drain traps can release sewer gas odors indoors, posted originally at PLUMBING TRAPS & INTERCEPTORS - topic home.

[Click to enlarge any image]

Our photo (left) shows a plumbing trap that was abanaoned in a building basement.

On 2017-05-16 by (mod) re: Are S-traps on plumbing fixtures really safe and legal?

Iggy, for space and to permit authoritative citations and links to documents, I have moved your defense of S-traps and our reply into PLUMBING S-TRAP CODES & HAZARDS - it may take 24 hours for the updated page to appear on your server: remember you may need to clear your browser cache to see the information.

Basically, S-traps are prohibited by all of the model plumbing codes because of the health and safety hazards they present because of the fact that traps of that design risk siphonage and loss of their water seal.

Thanks for the discussion.

On 2017-05-16 by Iggy

Yikes! Talk about nonsense claimed to be research, fact or even relevant. A guy that built places with S-traps. Another guy that simply noted there's bacteria floating all around us. Some random guy that isn't even found on the internet. And the sole reason that S-traps were just remove stink.

I have no problem with change and embrace any improvement, but S-traps have NEVER been the terror laughably claimed by inspectors. P-traps, ONLY WITH VENTING, are an improvement! P-traps, WITHOUT A VENT, are VERY MUCH worse than any S-trap! Calling out an S-trap and not a lack of venting should be criminal!

Venting is the ONLY "defect", patently not the type of trap! That these guys and gals walk into a 100 or 50-year old house and fail the S-traps unbelievable fear-mongering. I'm the exception? You do realize we're talking about a likely still 100-million buildings, not just homes, that are still HAPPILY S-trapped out there.

On 2016-04-20 by (mod) re: wind effect on trap siphonage

Thanks for the question JCP. I don't have a researched answer, but I think that wind is less likely to suck water out of a trap (it'd have to lift that water very high to pull it up from a building trap to above the roof line) and it's more likely that if wind is affecting your drains and giving an odor problem, wind might be blowing DOWN the vent pipe system and pushing water out of sink traps. I've heard a few complaints from readers along those lines.

Ask your plumber about installing a movable wind-resistant cap atop the problem vent. The cap is basically a rotating 90 degree cover that faces its opening away from the wind. Let us know if that solves the trouble.

On 2016-04-19 by JCP

When we get a strong wind -North- and the vent pipe is on north gable, we get a sewer gas odor from the vanity sink. Ques. Can wind suck water from the p- trap allowing gas in?

On 2015-09-30 by klyvarrobdo

Plumber told me sulfer oder from me sink faucet is coming from surrounding neighbor's sewage due to the shower not ever running Sounds like bs

On 2015-09-08 by (mod) re: installing a main house waste line trap


You can install a main house trap at the point of exit of the waste line from the house

On 2015-09-08 by DavidT

I believe we have fitted a WC with a faulty S trap as gas from septic tank enters house via cistern. Can we fit a second trap in the waste pipe? Your advice much appreciated!

Question: can a dried out floor drain trap cause a sewage backup?

Can a dried out floor drain trap cause a sewer line backup? - Daryl 12/27/12

Reply: directly, no, indirectly ... maybe

Can a dry trap cause a sewer line backup?

No Daryl, a dried out floor drain trap doesn't cause a sewer line backup, at least not directly;

A dried-floor trap is simply an empty U-shaped pipe in the drain system. For a sewer line to back up we expect to find either that the drain system has become blocked downstream from the point at which the backup manifests itself, or the entire sewer system is flooded, such as during a storm, hurricane, or area flooding.

But in a more subtle way a dried floor drain trap *might* contribute to a downstream drain blockage if it served as an entry point for animals who nested in the dry drain, or if there is no screen over the floor drain and some fool sweeps dirt and debris into the drain system. Both of those are indirect contributions to a sewer line backup.

Question: hidden plumbing traps

(Mar 10, 2013) Mike said:
If there is no visible trap beneath the bathroom sink, is it possible that the trap may be hidden behind the wall?


It's possible to hide a plumbing trap in the wall, but bad practice since that makes it inaccessible for inspection, cleaning, or replacement when it leaks. If your drain smells that may indicate that there's no trap. But to be fair, there are other non-visible plumbing traps commonly found in some buildings such as the trap below a shower or bath tub that is really hidden in the ceiling below.

If the drain line drops straight down and then makes a 90 degree turn into the wall with no trap it's an improper installation.

If there are other clues suggesting non-standard plumbing in your home there may be other problems that affect both function and safety - I'd ask an expert plumber to take a look.

Question: bathtub drain stopper clog repair

(May 8, 2014) Anonymous said:

My older homes bathtub become clogged, so I removed the drain lever to see if it was malfunctioning and it was old however working properly however tub drain is now completely blocked.

I also noticed that the toilet is now becoming clogged and wont drain properly. I tryed to look down tub piping to see if stopper was stuck however stopper came out when I removed lever. Any suggestions on what could be the problem and what if anything I can repair myself. Thank you


Anon, if you're sure you were able to fully remove the drain stopper assembly then there is probably a debris blockage od the drain or it's trap. Try a gentle use of a small diameter plumbing snake.

Question: getting the landlord to fix sewage smells from the kitchen and bathroom

(Feb 7, 2015) Hulya said:
Help me please. I moved in to a new build flat in 2012. Within 2 months, we began getting sewage smells from the kitchen and bathroom. We also would get food smells from bathroom. Landlord told me to use a plunger.

We also had very bad and strong sewage smells outside the building. The smell worsened in the bathroom, neighbours water gurgling noises from somewhere inside our bath, neighbours bleach smell coming through the bathroom increased to several days every week. Reported these to landlord who repeatedly ignored my concerns as the smell began making my son ill.

Smells strengthened, lasted longer especially schools holidays being every day. 2 years 2 months later, July 2014,landlord acknowledged the smells and blamed water company. water company found several manholes owned by landlord blocked and unscrewed. The manholes were finally drained less than 2 months ago.

But the cause of blockage could not be found, landlord stated they would need to obtain the drawing of the building from council. Landlord promised to have it rectified after Christmas holidays. The smells have been every day for months now.I am on the first floor.

No flat beneath me. Most neighbours have similar problems too but not as strong or long as mine. Ended up calling a plumber myself yesterday. He took off the bath panel and straight away said the fitting was wrong, the u bend thingy. Sorry, I am female so can't remember the terms he used.

Basically the while funny shaped thing which the water goes down, was for showers and not baths. He explained it all but it was too much to take in and understand. He said landlord would need to make a hole so a longer one could be fitted similar to the one in my kitchen.

He also found a long wire with a small plactic box attached, which he claimed was electric. I took photos.

he said he would write out a report so I could hand it to my landlord. please could you offer me any advice because we have to constantly leave home when the smells are very bad as it stinks out the whole flat, as it is quite small.



If you have notified a landlord of an unsanitary, and potentially unsafe (risking methane gas explosion) condition in your home both orally and in writing, and if nothing is happening, you may have little choice but to ask for help from your local health officials or building department. Do keep us informed of what happens.


Continue reading at PLUMBING TRAPS & INTERCEPTORS - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see PLUMBING VENT DEFECTS & NOISES where we explain the role of plumbing vents and give plumbing vent specifications and codes

Or see PLUMBING DRAIN NOISES where we explain the basics of proper plumbing vent piping and how errors cause trap siphonage, odors, and noises


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