Photograph of a home made septic tank in snow How to Separate Plumbing Fixtures from Plumbing Drain System as the Source of Sewage or Septic Odors

  • FIXTURE vs SYSTEM DRAIN BLOCKAGE - CONTENTS: How to separate Plumbing Fixtures from Plumbing Drain System as the Source of Sewage or Septic Odors. Causes and cures for sewer gas odors diagnosed by making a distinction between fixture-related odors and main drain or septic system related odors. How to find and cure bad smells in buildings by narrowing down the investigation
  • POST a QUESTION or READ FAQs about diagnosing sewer gas smells: how to distinguish odors from drains from odors due to sewage backup or septic failures

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Sewer or Septic Odor troubleshooting: here we explain how to sort out the causes of a methane or sewer gas smell by distinguishing between odors at plumbing fixtures compared with odors due to drain. waste, or vent piping troubles. The photograph at page top provides a lot of septic odor diagnostic information, as we will elaborate here.

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Diagnosing fixture versus plumbing system blockage problems for correcting septic or sewer gas odors

How to tell a local problem at one fixture from a system wide plumbing problem:

Toilet wax ring Toilet installation

  • If one fixture or one group of fixtures is not draining well, suspect a local problem with blockage or venting at an individual plumbing fixture or at one or two plumbing fixtures in the same room, sharing a common (blocked) drain, while plumbing fixtures in other areas of the building are performing normally.

  • Odors that occur only at one bathroom when no drains are slow might be due to a leaky toilet seal, or to trap or drain problems serving just that bath.

The photos just above show a toilet wax ring being replaced. Even if the toilet is not rocking on the floor - that is, it appears securely bolted down, if the wax ring is damaged sewer gases may leak out in that bathroom around that individual fixture.


  • If all building drains are slow or blocked throughout a building then we suspect a system waste line blockage or blocked septic system.

Leaky cast iron building drain proven by septic dye (C) D Friedman

  • If lower floor plumbing fixtures back up while upper floor fixtures appear to drain normally, the problem may still be a system blockage, such as a blocked main drain.

    The upper floor fixtures are simply draining into the sewer line with apparently no problem, but when the volume of wastewater in the sewer line is great enough the sewage backs up into the lowest fixtures in the building, often in a basement.
  • If odors occur all the time when the building is in use, (as opposed to only when a particular area, bathroom, or fixture is in use) the problem could be from one particular smelly fixture or leak (see photo at left and notice that pink septic dye proving a leak at a damaged main drain connection) whose odors travel in the building, but we also consider that there may be a main drain or vent leak or blockage.

Odors caused by slow drains or septic drainfields: Included here but not common is the odor causing effects of a slow drain or drainfield: sometimes there will be a partial clog, causing septic effluent to back up and then recede as the drain empties.

In this case there may be no visible actual sewage backup into the building. The same thing could occur if the backup was caused by a failing drain field - the system may back up when there is a large volume of wastewater but then slowly recede. Or it might just create enough back pressure to push gas through the traps but no actual effluent.

See Septic Inspection Testing for details.

How to diagnose partial drain blockages: If a partial drain blockage is the problem (watch out it could be due to a damaged pipe or tree roots in which case simply snaking the line is just a band-aid short-term fix), you might notice the problem more when the volume of wastewater surges, such as when doing laundry or emptying a bathtub.


An increase in the number of occupants in the building (and thus using the bathrooms) will also cause a surge in wastewater volume and may disclose these conditions.

Indoor Septic Odors with Outdoor Causes

Septic odors in the house could indicate that effluent is backing up into the house past the traps which normally intercept the odors, or entering at a building window or door which has had a plumbing vent terminated too close by. Such odors may or may not mean the septic system is failing. A backed up septic system does not always equal a failing septic system

See and also SEPTIC / SEWER ODOR SOURCE TABLE for details.

Outdoor Septic System or Sewer Gas Odor Sources and Causes

Septic Blockages & Sewer Odors in Cold or Wet Weather

Photograph of a suspicious wet area in deep snow on a residential property near the reported septic drainfield.A very common source of drainfield failure as well as an occasional source of septic tank back-flooding and occasional push of sewer gases (or sewage) back into a building is the failure to protect the septic tank and drainfield from surface runoff and ground water. An intercept drain may be needed. If this is the case septic problems (odors or backups) will be associated with heavy rainfall or wet weather.


If you are smelling sewage odors outside in cold weather but not inside be sure to check these items:

Check for wet areas that may indicate septic system effluent leaking to the surface of your own or nearby properties.

The photograph shows that even in deep winter snow, melted wet areas may be observed if they are active.

See Septic Inspection Testing for a list of diagnostic articles.

  • Is there a municipal sewer?: Make certain that your building and your neighbors are served by private septic systems as opposed to a municipal sewer. Gases from a municipal sewer are also explosive and can be present in large quantity.
  • Is it a sewer gas or a fuel gas leak?: Make certain that what you're smelling is sewer gas and not a fuel gas leak such as LP gas or natural gas from a gas appliance, fixture, or gas line. There are serious explosion hazards with gas fuels. If you suspect a fuel gas leak leave the building and call your fire department from a safe location.



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FIXTURE vs SYSTEM DRAIN BLOCKAGE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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