Photograph of a home made septic tank in snow Remedies for Sewer Odors Caused by Plumbing Defects or Septic System Defects

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Here we provide descriptions of sewage smell sources and list of links to details about steps to take in curing methane gas, sewer gas, or septic gas smells that are traced to these building plumbing or to an onsite septic or cesspool or drywell system.

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Remedies for Sewer Odors Caused by Plumbing Defects or Septic System Defects

Photograph of melting snow indicating septic tank locationArticle Contents:

Also see our broad-scope article on diagnosis and cure of sewer gas and septic odors: Sewer Gas Odors diagnosing, finding, and curing septic tank and sewer line smells.

  • Backdrafting in building causing sewer gas odors: bath or other building exhaust fans may draw sewer gases up from drain piping, especially if there are dry plumbing traps. See
    • Make sure that bath and other vent fans are properly installed and located
    • Make sure that the building has adequate fresh air intake,
    • Check that there are no dry traps or siphoning traps.
    • Add plumbing venting or vacuum breaker vents (where permitted) to stop trap siphonage if necessary.
  • Clogged plumbing traps or drains may result in siphonage of the trap water, then releasing sewer gases into the building.
  • Cold weather correlates with sewer odors or slow, noisy drains: If your sewer or septic odors appear to occur only in cold or freezing weather,you will want to install a larger diameter plumbing vent stack from the roof surface up, or from the attic floor out through the roof (if your attic is un-heated). See
  • Plumbing traps indoors, smelly: Special plumbing traps which include a built-in seal against gas backups are available for use in problem locations. We just pour mineral oil into unused traps to prevent evaporation of the trap seal. See
  • Plumbing vent: Inadequate plumbing vent diameter or height: If it's a plumbing vent diameter problem the solution would be to install a larger diameter vent pipe, eg 2" up through the roof and outside. See
  • Plumbing venting Inadequate: plumbing drain line venting: if building vent piping is missing or inadequate or improperly installed get advice from a plumber on both stop-gap ("V-200" vacuum breaker vents from Oakville Stamping & Bending)* or proper (vent piping) repair alternatives. See PLUMBING DRAIN NOISES for an explanation of plumbing vents, and see Plumbing Vent Defects for diagnostic procedures.

Transite plumbing vent (C) Daniel Friedman

  • Plumbing Vent location, Bad: if a building vent is too close to a window or otherwise is delivering gases to occupants, it may be possible to extend or relocate it. See Plumbing Vent Defects for diagnostic procedures.
  • Plumbing vent discharging into the building attic or roof space - plumbing vents should terminate outside, above the roof level. This defect should be corrected as soon as possible as there are both health and methane gas explosion hazards when sewer gases are vented indoors. See Plumbing Vent Defects for diagnostic procedures.

Odors and Plumbing Drain Line Leaks or Blockage Repairs

Blocked sewer line (C) Daniel FriedmanDiagnose leaks and/or sewer gas odors due to damaged drain or sewer line piping

If a building drain is damaged and leaking the point of damage needs to be found and repaired. If it's an indoor drain the leak can usually be found by evidence of leaks into the building; if it's an outdoor drain leak, use of a plumbing snake, judicious careful probing, or excavation will be needed.

Sometimes simply lifting a section of sidewalk over a drain or lifting a large stone will disclose a previously unrecognized wet area where a drain line has been broken or damaged.

Our photo (left) shows a source of both sewage odors and a blocked main drain line between a house and its septic tank.

The sewer line was broken when a heavy commercial lawn mower drove over the concrete sidewalk below which the sewer line had been installed.

We replaced the entire drain line between the house and the septic tank and our contractor made sure that the new plastic sewer line was properly protected from damage by bedding it in sand.


See SEWER LINE REPLACEMENT for details about installing a new sewer line.

Blocked Drains as a Cause of Septic or Sewage Odors

If building drains are blocked or appear blocked hire a plumber to snake the drains. If all drains are blocked, the plumber might start by snaking out the main drain. During this process an experienced plumbing power snake handler can tell if the drain line is broken or damaged or clogged by tree roots. If this is the case an excavator and drain line repairs are needed.


Slow Drains as a Cause of Septic or Sewer Gas Odors

If building drains are not blocked but drainage is slow, sluggish, smelly, or backing up into the building, an inspection of the septic tank can indicate (by abnormally high level) that the tank inlet baffle or outlet baffle are blocked (or missing entirely), or that the tank outlet to drainfield is backing up, or blocked, or the drainfield not working. Also see CLOGGED DRAIN DIAGNOSIS & REPAIR

Pumping out or cleaning the septic tank to address an "odor problem" probably won't help: while periodic pumpout of a septic tank is important maintenance to protect the drainfield, will never "fix" one of these problems. The cause of blockage or failure needs to be identified and repaired.

We can think of a case, however, where pumping and inspecting the septic tank baffles might help diagnose a sewer gas odor outdoors or in the building. If the septic tank inlet or outlet baffle is partially blocked by the floating scum layer (the tank is past due for cleaning) sewer gases may back up into the building.


for a guide to inspecting septic tank baffles, see SEPTIC TANK BAFFLES.


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SEWER GAS ODOR REMEDIES at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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