Floor trap odors and cures Carson Dunlop AssociatesBasement or Crawl Space Floor Drain Odor Problems
Cause & Cure of odors traced to floor drains

  • FLOOR DRAIN / TRAP ODORS - CONTENTS: Sewer gas sulphur odor leak discovery, cause, & repairs where a basement or crawl space floor drain or drain trap is the problem.How to diagnose basement or crawl space sewer odors due to a floor drain. Diagnose& repairing cast iron drain connection & leak problems. Diagnose sewer gas odors traced to plumbing drain deterioration
  • POST a QUESTION or READ FAQs about how to track down and cure basement floor drain odors & smells

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Floor drain odors & smell troubleshooting:

This article explains how to track down and cure a sewer gas odors that appear to be due to basement or crawl space floor drain or drain trap problems.

We describe the likely sources of sewer gas or septic smells that seem to come from a floor drain, and we explain how to complete the diagnosis & cure of these odor problems.

The sketch of a plumbing trap primer system shown at page top was provided courtesy of Carson Dunlop Associates.

Green links show where you are. © Copyright 2015 InspectApedia.com, All Rights Reserved.

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

Floor drain odor source (C) D Friedmanand DMOur photo at left shows use of a floor drain that may be an odor source. Draining air conditioning or heat pump condensate in this manner can solve two problems at once. Avoiding a dry plumbing trap and disposing of A/C or heat pump condensate.

The articles at this website will answer most questions about water supply & drain piping, wells, & water tanks as well as many other building plumbing system inspection or defect topics. Reproduction of this web page electronically at other websites is prohibited.


I am having a heck of a time finding where the septic odor is coming from. We have a 3 story house on a septic tank and drain field.

The odor seems to be strongest and most often present in a mechanical room in the basement.

The only plumbing in the room is a floor drain [photo at left from reader]. No other drains are under the concrete slab. I know because I built the house.

I suspect that drain, but the water level in the trap seems to be full and I even sent a metal feeler down to make sure the bend in the trap was below the water level.

The concrete floor does have settling cracks. Is it possible that the plastic drain pipe could be slightly cracked or loose glue joint down stream from the trap, caused by the settling concrete?

Would plugging the 4 inch drain just outside the house and filling the drain with water up to the top of the floor drain give enough pressure to any leaks thereby lowering the level of the water in the drain?

Assuming all the other drains downstairs are slightly higher than the specific drain in question.

Drain piping (C) D Friedman and DMOnly other thing I can think of, is the roof vent plugging from leaves etc. going down a 2 in. vent stack. If that plugged, would that allow gas to come into the building?

[Photo at left (from reader) shows a portion of the drain-waste-vent DWV piping in this building.]

This house is 30 years old and this problem is about a year old.

This question is a tough especially with all the floors being finished with tile or carpet.

I was an electrical contractor so I do have tools and abilities for any of your suggestions, i.e.: shove a metal fish tape down the drains and then trace the tape to verify the drain path under the concrete slab.

Any help is appreciated. Thanks, - D.M.

Reply: Floor drain and trap odor diagnosis suggestions

Rooftop plumbing vent as odor cause

It would be uncommon for leaves to block a roof vent unless the vent projection above the rooftop were very short and the vent was covered with leaves (or snow). But we have indeed seen roof vent pipes blocked by frost if there is a shower or laundry or other source of hot water that sends a lot of steam up the vent in very cold weather, esp. if the vent is small in diameter - say under 2";

We have also found roof vents blocked by wasp nests, and once even a frog.

If you can safely examine the vent from rooftop without falling off due to snow or other slippery conditions, it's worth taking a look.

Other plumbing vent failures that send odors into buildings include a vent or drain line that is disconnected inside the building due to an unglued joint or broken drain line; These should show up as an odor or when you perform a pressure test of the vent line.

Of course if your floor drain piping is actually used to drain a plumbing fixture (such as a washing machine) and the drained fixture is more than 5' from any plumbing vent, then depending on what waste flows through that drain line, it could on occasion siphon water out of the trap and allow odors into the building.

It is also possible to siphon water out of a remote but un-vented trap as waste passes down the joined drain line, but that problem would have been present from the time of original construction whereas your problem has just recently appeared. And you've confirmed that there is water in the basement floor trap so that's not our first suspicion.

Drain line under the floor as odor source

Indeed we have had cases of under-floor drain lines that were leaky, sending sewage odors into the building; that problem is more common when the under-floor drain is carrying sewage.


If you have easy access to this same drain line from outdoors and can temporarily block the line at that point it would be a good diagnostic tool since you are sure that there is no other plumbing connected to this drain. If you block the line from outdoors (and at a point before it joints any other sewer piping) then if that's the odor source, the odor ought to stop.

If that step confirms that the line is the odor source, and presuming the building is new enough that your piping is plastic not cast iron under the floor, I'm guessing that the bad news is the line would have to be abandoned (sealed off outside) or dug up and replaced.

Before doing so it may be worth asking a plumber who has a drain line inspection camera to send that through the line to see what's going on, on the chance that you can find and dig up just a short section of line that is damaged or disconnected.

Floor drain trap as odor source

Sewer odor and clogged shower floor drain (C) Daniel FriedmanIt is interesting that you found the floor trap filled with water. Often a floor drain whose only function is to catch on-floor flooding in a basement is so seldom used that its floor trap dries out and sewer gases pass backwards into the building.

We solve that problem either by pouring some mineral oil into the drain trap or installing a floor trap that includes a check valve designed specifically to prevent gases from passing backwards up the drain line and into the building.

When a building is more than a year old and the floor trap is always filled with water, and if we think that the drain line is never used, where is that water coming from?

Is that a clue that there is water under the building leaking into the trap (i.e. an open or broken drain)?

Other Possible Sources of Basement Sewer Odors

Open sewer line cleanout odor source (C) Daniel FriedmanIf blocking the drain line off outside the home does not stop the odor there are some of the other possibilities:

  • sewage backed up into and leaked out of the line into a settled cavity under the floor, continuing to create an odor source
  • a sewer line cleanout cover has been left off (our photo at left) or is leaking
  • the odor is not from that drain line but perhaps from another building drain leaking into the basement ceiling or walls.
  • other "sewer-like" odors that might be found in a mechanical room could be from some different source - a dead animal, a chemical spill, or if there are odors in the building water supply, sulphur in the water or if in hot water, bacteria or a bad anode in the water heater tank.

Additional floor drain or trap odor problem diagnostic questions:

  • Are we sure we know the piping layout under the floor, the materials used, and quality of connections?
  • Do floor slab cracks or other signs of settlement mean that slab movement may have damaged the under-floor drain piping?
  • What drains in to this floor drain? Anything? or is it just for water spillage?
  • In other words, are we sure that the drain line connected to absolutely no other building plumbing?

Follow-up from reader about smelly basement floor drain

Whole drain is not 4 in…4 inch is a main drain with taps off of it under the slab. The floor drain in question is a 2 inch line and trap. All abs piping.

Drain in question is just for spillage as is the other one in a different room. The reason water is in the trap is because I diligently keep it full. Have thought of putting antifreeze, like a travel trailer uses, instead of mineral oil. But not until this is settled.

Check all roof vents this morning with a 1/4in metal fish tape. No restriction and a visual with flash light showed no problem. Also, the gas odor coming out seemed consistent at all four roof vents.

Since last writing, I did plug the 4 in drain just outside the home and then filled all the pipe under the slab until the water level was even with the top of the floor drains. I left it for 14 hours and the water level never changed. Therefore, I feel there is not a big leak under the slab. I suppose a real small leak might take more time to be apparent. An air test would be more accurate but would complicate plugging the pipe completely.

After plugging the 4 inch line, I aired out the house and no odor returned. This makes sense because all the lines leading to the septic tank were plugged with water.

Also, this morning I had the tank emptied. It probably could have gone another three years, but because of the problem, I had it cleaned anyway.

After careful review of when this intermittent problem occurs, It seems like the odor follows the draining of one particular bath tub. The tub was used for three consecutive days, and each of those days we had the odor. Is there any type of flushing action that can cause the odor getting through.

Remember, this is a problem that occurs when it is cold out, not neccesarily freezing, and the problem started a year or so ago and we have been here for 20 years..I can’t even think of a place where a nail could have been put through a pipe. No one works on this house but me..

Included four pics; 1. The outside four inch clean out used to plug entire house drain 2. The suspect floor drain [see at top of this Q&A] 3. Misc. piping to suspect tub/lav/ toilet area on first floor, as seen from basement [see at top of this Q&A] 4. Outside vents, all were clear…

What can you tell me about a smoke test? I am looking into plugging all pipes and putting and doing an air pressure test. This would take the talk out of a leak but would not tell me where it is..ugh…

Tip for Dry Plumbing Traps & Floor Drains

Reader Comment:

10 January 2015 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. - Alison 1/10/2015



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.

Here's a tip: for dry floor drains or for that matter any plumbing fixture drain trap that is likely to remain un-used for a long time - long enough to let its water trap evaporate - I pour about 3/4 cup of clean mineral oil into the trap. Like water, the mineral oil seals the trap against odors leaking back out of the drain system and it also lasts longer than water.


Continue reading at PLUMBING SYSTEM ODORS or select a topic from the More Reading links shown 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

Suggested citation for this web page

FLOOR DRAIN / TRAP ODORS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

More Reading

Green link shows where you are in this article series.


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Publisher's Google+ Page by Daniel Friedman

Click to Show or Hide Citations & References