Sewer gas odor traced to dry floor drain connected to sewer line (C) Daniel FriedmanFloor Drain Odor FAQs
Questions & Answers on Cause & Cure of Odors From Floor Drains

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Floor drain odors & smell troubleshooting FAQs.

This article lists common questions and answers about floor drain odors: diagnosis & repair. :

This article series lists solutions for stopping sewer gas odors & sewage backups through un-trapped or un-sealed floor drains and lists sources of trap seals, trap guards, and check valves used for that purpose.

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Floor Drain or Drain Trap Odors: Leak & Repair Diagnosis Suggestions

Open sewer line cleanout odor source (C) Daniel FriedmanOur photo shows an open floor drain connected to sewer piping: it may be an odor source, and it may be both unsanitary and unsafe.

These questions and replies were originallhy posted at FLOOR DRAIN / TRAP ODORS.

On 2017-02-20 20:56:41.362665 by (mod) - another self-sealing floor drain


David said:
I enjoyed the article but you cant forget about what works.

mod comments:
The GreenDrain touted by David is a floor drain trap that allows water to pass from the floor into the drain system while blocking emission of sewer gases.

Other self-sealing floor drains are described in the article above on this page.

On 2017-02-07 03:58:27.727148 by Johnnylakis

I want to thank you for having an easy to use site. I had been dealing with a "smell" in my home that came and went. I have a cat that occasionally brings in a mouse,bird or larger critter that will pass on while in the house. This was the type of smell.

Death. No simpler way to put it. I looked under all the furniture, moved the washer and dryer, no little dead critter to be found. Turned my attention to the septic system. I was just getting ready to call a septic company and thought I would "Google" my issue and see what I could find. After reading really vague and useless page after page I came upon your website.

Dry drain for me too. Running a hose for a few minutes does the trick. So glad I found your website.

On 2017-01-23 21:28:01.216743 by Stan Mavroftas

I put a fernco on vanity drain because someone sawed the edge off making it shorter. Can I use a Fernco there? I'm getting a wet smell at the drain in the vanity. If the roof vent is partially block. Could a admittance valve work if gasses are seaping in sink?

On 2016-12-15 22:25:57.788971 by norma baldit

floor map sink no craw space how to repair sewer odor

On 2016-11-27 05:48:04.542849 by Anonymous

Hi,I have a question. It is strange to me but maybe someone will not think so. I have a smell in my bathroom that smells like sewer gases. The plumber found a leak underneath the house and week be back to fix it.

Question is, there is also mildew spots that keep popping up on my shower ceiling. Is this problem related?

On 2016-11-03 20:13:27.491642 by Dennis

Is it ok to cover the basement floor drain with a cloth , so no smell or odor comes from the drain ?
Also to limit radon ? Would this cause any serious problems ?

On 2016-11-03 20:12:29.966219 by Dennis

Is it ok to cover the basement floor drain with a cloth , so no smell or odor comes from the drain ?
Also to limit radon ? Would this cause any serious problems ?

On 2016-11-03 20:12:06.485579 by Anonymous

Is it ok to cover the basement floor drain with a cloth , so no smell or odor comes from the drain ?
Also to limit radon ? Would this cause any serious problems ?

On 2016-05-17 11:53:17.894200 by (mod) finding a drain clog downstream from the trap


Most likely the drain line is clogged downstream from both the toilet and the floor trap. You may be able to unblock the clog with a plumbing snake or by hiring a company to do so. If pipes are broken and collapsing they'll need replacement.

Stacy: it sounds as if your sewage line is also backing up to the floor drain - as I just informed Hussein Ali.

On 2016-05-16 17:19:43.900749 by Hussein Ali

Every time i flash my toilet,the waste comes out through the floortrap,this problems just started recently..why is it so and how can i fix it?..Thank you

On 2016-04-18 23:40:22.825732 by Stacy

We live a home on a concrete slab. Today I have noticed multiple times the smell of sewage. I found the source to be in our mechanical closet, which has the water heater and furnace. The smell is coming from the floor drain.

I pulled a tube out of it to find a mud like substance filling the end of this. Do I just need to clean this out, and how should that be done? Thank you.

On 2016-04-14 20:00:10.824879 by Hans Janke

Thank you, appreciate the response!

On 2016-04-14 14:04:12.059712 by (mod) - drop in water level in a drain trap


It is normal for water level to drop in a plumbing trap when no more is running through the drain.

But if water level drops so low that sewer gases can pass back up the drain line past the trap then something is wrong: a venting system defect may be causing siphonage of water out of the trap, or the trap itself may be leaking.

On 2016-04-14 07:25:31.099893 by Hans Janke

Sorry, not sure about terminology. Had very bad smell in the house. Checked, fixed and plugged EVERYTHING inside. Only remaining problem: Outside, where kitchen sink waste pipe enters the system (little gully), the water level in the trap recedes over time and then smells appear when it is too low. What can cause this? Level can stay OK for a few hours, but at some point starts to go down

On 2015-08-24 23:34:45.895697 by (mod)

Anon I'm confused by the question you post too. Sewage or other ground water can indeed leak into a well piping system depending on the system type and condition.

Just for example, a bad foot valve in a well can allow water to drain backwards (thus under negative pressure) into the well from the water tank; if at the same time we have a leak in well piping that an draw unsanitary water into the piping system.

I'd ask for help from an experienced plumber, as with just an e-text and the data you've given, more would be perhaps too much wild speculation.

On 2015-08-23 21:41:38.338026 by Anonymous

the drain from my water tank has sewer smelling water in it
also there is a slight leak in my main drain sewer and slight amount of mud in it so the big ppe commming from the upstairs toilet is the one that has a slow leak and showed when a load of laundry was done and the water tank drain smells like sewer what can I do to figure this out?

On 2015-08-11 18:27:27.527612 by (mod)

Pools of water on the floor does not sound good. And if water leaked under the laminate, worse, risking mold, rot, insect damage. And odors.

Is the water really just from condensation? If so you want to both run a dehumidifier and look into any sources of un-wanted moisture such as leaks from plumbing or from the exterior into building cavities.

I'd want an analysis whose results I trust before I'd want to pay for any more costly "solutions" - perhaps a home inspector

On 2015-08-11 11:16:03.120826 by Valerie - flying colors on the home inspection (sic) followed by puddles on the floors

My daughter and son-in-law recently bought their first house, a raised ranch built in the 1980's. Home inspection and septic system passed with flying colors. Recently went to paint a room in basement with laminate wood flooring. It was a humid July day and there were pools of water on floor and the same on upstairs kitchen laminate flooring.

Returned approx 2 weeks later to do second coat of paint. Kitchen floor was dry. Pools of water on basement floor were gone but floor felt clammy and noticed a foul, not musty, smell. The room across from this one is carpeted and no odor is noticed.

Basement has a sump pump. There is one window in this room and this side of house is surrounded by trees. Backyard has areas of moss and kids have noticed occasional toads.

Home is in the city. None of us are mechanically inclined. Can you advise any simple things we can do to try and determine cause of sweating/odor and how to eliminate, or what type of worker to call for help. Thank you.

On 2015-07-08 15:28:46.466680 by Mark

Good stuff keep it going!

Comments: thanks for diagnosing a smell that came and went

(Jan 9, 2015) Thank you, thank you, THANK YOU! said:

I want to thank you for having an easy to use site. I had been dealing with a "smell" in my home that came and went. I have a cat that occasionally brings in a mouse ,bird or larger critter that will pass on while in the house.

This was the type of smell. Death. No simpler way to put it. I looked under all the furniture, moved the washer and dryer, no little dead critter to be found. Turned my attention to the septic system.

I was just getting ready to call a septic company and thought I would "Google" my issue and see what I could find. After reading really vague and useless page after page I came upon your website. There was the answer, plain as could be. Dry drain.

We have a shower that hadn't been used in a few months, along with some very windy conditioned dried the plumbing. I turned on the shower and let it run for a few minutes along with pouring a few gallons of water down a floor drain in my laundry room. Smell left almost instantly!

I wish I had found your website first. Your information is so very much appreciated, and the time you took to put it out there for us to use.
Most sincere appreciation.



Thank you in return for such a generous note. We work hard on our information and are thus thrilled when a reader finds it useful. We also welcome criticism, content suggestions, or question.

My cat, one of them, used to bring snakes into the basement - they were not always dead either.

As it will help other readers I've added your comment and a tip from us about keeping odors from backing up through dry floor drain traps into the article above. Let me know if that's unclear of if you have other suggestions.

Question: is the odor source a drain odor or a dead animal?

(Sept 11, 2012) Dan said:
I have the same problem with my house and have tryed everything i can possibly do. I've had my vents and main line power washed out to the street.The city has power washed the street line and still stench keeps pouring out the 3inch main line.

only thing i can think of doing next is having smoke testing the lines. I just hope that this will detect problem


If it's a dead animal it's most helpful to find and remove it. An animal could be in the wall cavity nor just under the floor. Or in a ceiling.

If it's a small animal typically the odor will dissipate as the corpse dries.

Question: air quality testing for urine odors?

(July 7, 2014) Rob said:
Do I need to get an air quality test following a urine spill? This is in a commercial building



An IAQ test following urine "spilled" in a building would be unusual but I'm not an onsite consultant nor expert. One would think that cleaning and disinfecting the area would be sufficient. If odors remain either the spill areas haven't been completely identified or a sealant is needed over some absorbent surface.

Question: watch out for improperly installed vent piping

(Nov 9, 2014) TT the Plumber said:
In the FLOOR DRAIN ODORS conversation...It is possible the house was plumbed with a DRY VENT IMPROPERLY INSTALLED.

If an unwashed vent for the tub was installed horizontally it could possible backfill with materials that have no ability to get "washed out" an get plugged over time. Maybe try running a hose through the roof vents to clean out any possible issues as this. Just one thought.


Thanks TT for the helpful comment. I agree that odors are sometimes traced to either improper or incomplete venting or even open or disconnected plumbing vents somewhere in the building. We'll merge it into the article above.

Question: maybe the plumber who snaked the drain broke the line? I smell sewer gas

(Nov 28, 2014) Anonymous said:
A plumber came out three days ago and snaked the drain in my laundry room. (I've had back ups in my laundry room in the past if it isn't snaked in the fall.) The next day I smelled sewer gas. I thought at first that it was from him snaking the drain but the odor is still there.

Could the plumber have broken the line when he snaked it. I called the plumber today, Friday, but didn't hear back. The odor isn't real strong but I'm wondering if it is dangerous to have these fumes in that area. I have a game room right off the laundry room area.


Having to repeatedly nsnake a drain suggests an underlying problem such as broken piping, roots, inadequate slope.

Unless you can find and clear a blockage in an otherwise intact drain piping system, sooner or later you'll probably have to replace the line. Scoping the drain with a camera might tell something useful, also an experienced plumber can get a feel for drain condition from the snaking process.

Watch out: if the drain line is clogging because it's an older galvanized iron drain with a thick accumulation of scale or rust that reduces its interior diameter, piping replacement is in order. Snaking such drains is not effective.

Watch out: Methane gas can be explosive if it accumulates.

Question: horrible odors when we run the clothes washer or from sink or washing machine

(Feb 23, 2015) Linda said:
We bought a 62 yr old home about 3 yrs ago and we have had a horrible odor every time we run the washer. This is constant, year round and seems like it might be getting worse.

There is a floor drain just a couple of feet from the washer and drain tub which seems to be where the smell comes from. We have tried keeping it filled with water, and pouring Clorox in it, nothing helps. The smell is unbearable. Where do we go from here? Thanks

(Feb 24, 2015) Nnana said:
Hi we bought a house four months ago, nd it was empty for 16 months, its not a new house.

The problem is the bad smell that comes from sink nd washing machine pipes. We tried to fill it up with water but still the smell occupie the whole house. Please help



I'd be looking for
- a failed septic system if your home is on private septic
- blocked drains
- blocked or defective vent piping

Question: field report of a smelly S-Trap on plumbing

(Apr 3, 2015) Walter Kraslawsky said:
In our case, the "S" Trap is in a master shower used daily, and the trap never goes "dry."

We are hoping someone can help identify a specific cause and resolution for an unpleasant odor coming up out of our master walk-in shower drain. Looking into the pipe, it appeared to be a solid black PVC pipe. However, after using a “bottle” brush (actually a long wire-handled dryer lint brush), I was able to scrub it with liquid soap (Selsun Blue shampoo to help kill any fungus, mold or mildew) until it was clean and white.

The wire brush handle is long enough, and flexible enough, that I could feel it push around the bend at the bottom and back up. However, I was unable to get it to make the second turn to head back down even though it is long enough to do so. Anyway, the smell was almost entirely gone – with me thinking there was simply just residual odor in the air that would dissipate. I thought the problem was solved.

Just to be sure, I poured about half a cup of Selsun Blue into the drain in hopes it would kill any remaining odor source.

However the smell quickly returned. Several more attempts had exactly the same progress and result. So I used a flashlight to look down the pipe and the water at the bottom appeared grey and oily. I thought it might be the Selsun Blue, and it only took a second (without scrubbing) to pour a bucket of water down it and look again. Now I could see clearly down through clear water to the clean white bottom of the "S" Trap. Great! Problem solved again. Right? NO!

As I admired the clean water, it started turning grey and oily again right in front of my eyes. So I repeated the bucket of water, saw clean water and the clean white bottom of the pipe again -- and watched the water turning grey and oily again immediately. This time, watching carefully, it looked like an underwater oil slick coming through the side of the pipe just below the water line. I did it again, and saw it turn oily again. WOW!

Since the "S" Trap pipe will be accessible through the ceiling of the identical shower in a guest room directly below, it should be resolvable. However, access from below will be expensive. Like the master shower, the guest shower below is fully tiled including the ceiling. So before destroying the guest shower ceiling to open up the space I want some assurance that this is the best and only reasonable approach.

My confusion is because there is no conceivable source for this oily substance to leak through the “S” trap pipe inside the 12+ inch gap between the master level concrete floor and the reduced height guest bathroom ceiling for pipes, vents, ducts, etc. I fully expect that space will be open air with nothing touching the outer surface of the PVC pipe.

It is also important to note that there is no odor coming from the guest shower drain or any other drain in our 3-level condo, which takes up the top three levels of a 7 level condo building.


Walter I've thought about this a bit but remain uncertain about what's happening. If there were actually a drain leak then leak stains ought to show up in the ceiling below.

Perhaps what you are seeing is a partial drain blockage and back-flow into the trap from the rest of the drain piping system.


Continue reading at FLOOR DRAIN / TRAP ODORS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see CAST IRON DRAIN LEAK, ODOR, REPAIR for a case history of diagnosing sewer odors due to a broken under-floor sewer line



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