Sewer or septic gases and odors traced to building drain or vent piping defects:

This article describes how to diagnose, find, and cure odors in buildings caused by leaks or other defects in the building drains or sewer line - leaks that make sewage smells 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.

Building Drain & Sewer Line Defects Can Release Sewer Gases & Odors Indoors

Watch out: Because sewer gas contains methane gas (CH₄) there is a risk of an explosion hazard or even fatal asphyxiation. Sewer gases also probably contain hydrogen sulfide gas (H₂S) In addition some writers opine that there are possible health hazards from sewer gas exposure, such as a bacterial infection of the sinuses (which can occur due to any sinus irritation).

At left is a sewer line leak in a building crawl area.

[Click to enlarge any image]

Sewer gas source: check for a cap left off of a sewer line cleanout:

In the page top photograph our client is pointing out an open sewer line in the basement of a home she was purchasing. The cap had been left off of a cleanout port where main waste line exited the building.

We suspected more trouble than just an open drain cleanout permitting sewer gases to leak into the basement.

Because this "work" appeared to have just been performed, right before our property inspection, we wondered if someone had been asked to attempt to clear a blocked connection between the building sewer line and the community sewer (or private septic system).

Sewer Gas Source: inspect the building drains and gas lines for leaks

Plumbing drains and traps may smell regardless, as they are usually a reservoir for organic debris.

Remember that a building drain can be leaking inside of a wall or ceiling cavity without showing up as a wet spot or mold.

But if you notice a sewer gas smell particularly at one or more plumbing drains, the fixture may not be properly vented.

If the plumbing fixture or drain "gurgles" or makes funny noises when it is draining, or if you hear gurgling noises at some fixtures, say a sink or tub when nearby fixtures are draining, we would certainly suspect that the fixture is not well vented or may not be vented at all.

If you suspect that odors are due to a problem with the building drains but you cannot find a drain line leak nor an open drain cleanout cover, take a look

at CLOGGED DRAIN DIAGNOSIS & REPAIR.

The sewer gas odor may be coming not from a wet building drain (discussed just above) but from the dry or vent portion of a building's drain-waste-vent system.

Other Sources of Sulphur Odors & Sewer Gas Smells in buildings may Include Chinese Drywall

CHINESE DRYWALL HAZARDS discusses Chinese drywall odors, sulphur smells, and corrosive outgassing hazards in buildings.

Major costs to remove this product, repair or replace electrical wiring, plumbing, and HVAC components may be involved, and there may be immediate safety hazards due to damaged smoke detectors or carbon monoxide detectors in buildings where Chinese drywall outgassing has caused damage.

Hidden plumbing vent pipe leaks septic gas into building:

Plumbing vents are used to direct sewer or septic tank gases safely above the building.

But on occasion we trace a sewer or septic gas odor to a hidden leak in a drain or vent line passing through building walls or ceilings.

At left our photo illustrates how hidden vent piping leaks may be lurking in an old building, a new building, or in one that has been renovated.

When the installer needed to extend the plumbing vent and drain line to an upper floor of this 1920's home, s/he simply broke open a cast iron vent pipe that was in a lower floor bathroom.

There the "plumber" jammed a plastic ABS drain line into the open cast iron waste or vent pipe.

In the photo above my pen points out that this connection was certainly not sealed, and that sewer gases or septic gases were readily passing up the vent and/or drain line, into the building walls, and into the occupied spaces.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2023-11-30 by InspectApedia Publisher (mod) - steps to diagnose the cause of sewer smells from our master bath sink drains and shower drain f

Illustration above provided courtesy of Carson Dunlop associates, a Toronto home inspection, education & report writing tool company.

@Dave,

Thanks for a challenging "where does the drain odor really come from?" question.

Let's try some diagnostic steps and a few opening comments before we cut out more walls or start tearing the drain system or fixtures apart.

1. Look for improper drain / waste / vent piping:
   
   Your observation that the odor is worse when you are running water into various fixtures and draining them is diagnostic and important.
   
   That suggests that the plumbing vent piping system is either not correctly installed or is ineffective in some other way such as an opening or leak or blockage.
   
   Then when water runs down a drain that isn't properly vented it tends to cause loss of water in the drain trap seal, then letting odors pass up into the building through the drain or through fixture air inlets such as at the upper front or back edge of a sink or at the upper area of a tub drain control.
   
   You have a lot of fixtures tied to a common vent stack - that might be fine if the stack diameter is adequate and the distance from fixtures to the stack is not too great.
   
   See those details
   
   at PLUMBING VENT DISTANCES & ROUTING
   
   and PLUMBING DRAIN FIXTURE UNITS DFUs - drain fixture units or DFUs defined & explained + table of DFUs - can tell us if the vent stack ior other vent piping is overloaded
   
2. Look for external conditions that affect drain odors:
   
   I acknowledge that by your report, blockage has been ruled out, but there may be something similar to blockage but more subtle fixture that's too far from the stack and stack that's too small.
   
   You will read in this article series other more subtle conditions such as rooftop downdrafts that impact a plumbing vent can be a factor in odors.
   
   More about preventing downdrafts ( though I don't think this is likely to be your problem) is
   
   at PLUMBING VENT ROOFTOP CAP or SCREEN
   
   In sniffing to identify the exact source of an odor, use a person with a good sense of smell or perhaps even a combustible gas analyzer such as a TIF 8800 in order to sniff around fixtures as well as right at the drains to convince her of cells that we have accurately diagnosed were the odor is coming from.
   
   Now that sort of test instrument is so sensitive that it will respond even to gases in human breath. So we need to use it carefully.
   
   See TIF 8800 GAS DETECTOR  if you need to try this approach.
   
   But if your inspection or sniffing or testing convinces you get the odor sources actually coming out of the building drains then let me say a bit more:

1. Right at a drain or drain trap it's normal for there to be some odors:
   
   In a properly-vented and working drain we ought not smell anything unless our nose or instrument is quite close - just a few inches - to the drain opening.
   
   So if you were convinced of the orders are coming out of drains and you smell that odor several feet away then there is definitely a problem with the drain system. And if it's not black but I suspect that the venting as improper in length or diameter or routing.
2. Dry traps + backdrafting?
   
   In this article series there are plenty of other possible sources of sewer gas smells and odors in buildings such as a loose toilet or a dream that has leaked into a wall or ceiling cavity.
   
   Backdrafting? We often find odors coming from drains that have lost their water trap (again due to a drain vent or piping error) that will be worse if there is an exhaust fan running or another cause of back-drafting.
3. Exactly which fixtures or drains are the odor source?
   
   Diagnostic as well is whether or not the odor problem is common to all building drains at all fixtures which tells us there's a system problem or whether it is only at certain fixtures.
   
   Please take a look at the odor diagnosis & cure suggestions that begin at
   
   DRAIN PIPING & SEWER ODORS - topic home https://inspectapedia.com/plumbing/Drain_Piping_Odors.php - where we will move this discussion,
   
   and be sure to look through the Recommended Articles at the bottom of that page.
   

On 2023-11-30 by Dave - We have been experiencing sewer smells from our master bath sink drains and shower drain for years now.

We have been experiencing sewer smells from our master bath sink drains and shower drain for years now. Many different plumbers have not been able to fix it. The bathroom is closest to the single vent stack.

One plumber went into the attic and ran a camera through the vent stack in many directions and verified it is NOT clogged. It comes and goes and we have not been able to determine when it is worse. It is always bad when the water is running and draining.

I replaced the traps and cleaned the drain pipes as far as I could.

No slow draining issues and no smells from any other area of the house.

I have added water to all less often used drains.

There are five bathrooms in the two-story home all tied into a single vent stack.

I cut out the wall behind the vanity and the plumbers confirmed it looked okay. I am not sure what to do next.

We talked about doing a peppermint test but haven't done that yet. Thanks

This Q&A were posted originally

at ODORS GASES SMELLS, DIAGNOSIS & CURE  - home

On 2018-05-20 by Sam

Yes, that sounds like a good next step. Thanks for your helpful info.

On 2018-05-20 by (mod) - find difficult-to-spot drain line odor leak

If you are sure the pipe is the odor source you may need to have the system pressure tested

On 2018-05-20 by Sam

I should add we have visually inspected the pipe and ran my hand around it and I do see and feel what appear to be scratches on the pipe surface but not obvious cracks......

The pipe that is in the bottom floor cubby that is smelling is a vertical pipe, and above that on the top floor in the access panel, the pipe doesn’t smell. Its very odd.

On 2018-05-20 by (mod) - hairline crack in piping can be an odor source

In my experience you could be right, there could be a hairline crack. If you have a crack in the upper part of a drain pipe that never carries liquid waste it may smell without showing leak signs. You may need to pressure test the line or do a very careful visual inspection.

On 2018-05-20 by Sam

We have a black ABS wastewater pipe in a cubby under the stairwell (bottom floor)and the exterior surface of the pipe is smelling like mold.

Sometimes it smells less, other times stronger, it’s not consistent. There is no sign of a leak anywhere, fittings have been checked, access area to the tub that is hooked into is dry.

All drains have been snaked, clean outs have been checked. We were thinking a hairline crack but plumber tells us they’ve never seen a hairline crack that wouldn’t show leak signs. Do you have any ideas why the exterior surface of this pipe smells of mold, sometimes very strong?

Other ABS pipes in the cubby do not smell at all.

On 2018-04-21 by (mod) - floor drain odors when connected to sewer line

Walsh

Sounds like your floor drain is connected to a sewer line that is blocked.

On 2018-04-21 by Walsh

Black thick water with burnt gas like smells coming from basement floor sewer in wash room .horrible smell. It backs up few times a day.

Question: trouble finding source of a "pond-like" smell & odor transport in tall buildings

(Mar 25, 2013) Overwhelming Pond smell in kitch said:

This pond-like smell has been with us for a few years. When it's there, dishes have to be rewashed in dishwasher. Sink stinks.
Here are all of the steps we've taken, to no avail:

Replaced J trap, snaked main, replaced disposal, replaced dishwasher, replaced airgap, installed Culligan water treatment, cleared roof vents, dug under foundation (found no sump/standing water). Have 5 yr old MicroSeptic septic system that has checked out ok.

Smell does not smell like septic. This smell has baffled 3 plumbers. Not sure what to do now. It can be overwhelming and then gone the next day. Reminds me of a stagnant turtle pond. Any advice welcome. Thanks!

Forgot to mention, J trap is working..always has water when checked

(Dec 12, 2012) Anonymous said:
I would like to read about sewer gas smell in hi-rise, tall buildings.
How building stack effect may influence the sanitary venting system
How tall stacks may affect the venting system

Reply: when and where is the drain odor observed?

Overwhelming & Anon:

It sounds as if you have sewer gas backing up in your drain-waste-vent system, right?

But you don't say just where you observe the smell. Is it from a single drain or all drains, or from elsewhere in the home?

With more details I can offer more specific suggestions like looking for a leaky drain or vent line (perhaps using a pressure test), inadequate venting, or even needing a trap or check valve on the main waste line.

...

Continue reading  at PLUMBING VENT DEFECTS & NOISES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see DRAIN PIPING & SEWER ODOR FAQs - questions & answers about stinking drains & drain piping posted originally at the end of this page.

Or see these

Recommended Articles

• DRAIN NOISE DIAGNOSIS - gurgling drains may be a tip-off to vent problems, partial blockages, & smells
• DRAIN PIPING & SEWER ODORS - home
  • BACKDRAFTING & SEWER/SEPTIC ODORS
  • FLOOR DRAIN / TRAP ODORS
  • HYDROGEN SULFIDE GAS
  • METHANE & SEWER GAS HAZARDS
  • METHANE GAS SOURCES
  • ODORS, SEPTIC or SEWER
  • ODORS SEWER GAS in COLD WEATHER
• PLUMBING S-TRAP CODES & HAZARDS - these traps lose their water seal & often smell
• PLUMBING SYSTEM ODORS - home
  • BUILDING DRAIN ODOR SOURCES - details about odors coming from plumbing drains - clues that may be more likely to appear in cool or cold weather. Also see our broad-scope article on diagnosis and cure of sewer gas and septic odors:
  • CLOGGED DRAIN DIAGNOSIS & REPAIR
  • PLUMBING FIXTURE TRAPS - such as loose toilets, as sources of sewer gas odors & smells
  • PLUMBING VENT BLOCKAGE ODORS - possible odor source due to improper venting
  • DRAIN LINE SEWER ODORS
  • DRAIN & SEWER PIPING
  • DRAIN LINE PIPE LEAKS
  • DRAIN ODOR SOURCES
  • FIXTURE vs SYSTEM DRAIN BLOCKAGE - identify the odor source with care
  • FLOOR DRAIN / TRAP ODORS
• PLUMBING VENT DEFECTS & NOISES - more comprehensive description of plumbing vent snafus
• SEWER GAS ODORS - home diagnose, find, and cure septic tank and sewer line smells.

Suggested citation for this web page

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

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to PLUMBING SYSTEMS

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