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Plumbing trap schematic (C) Carson Dunlop

Plumbing Fixture & Trap Defects - a Guide to Sewer Gas Smells and Septic Tank Odors
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How to inspect plumbing fixtures, loose toilets, & plumbing traps in order to track down building odors and sewer or septic gas smells
How to prevent or cure sewer gas odors from septic systems, building plumbing, & other causes
Sewer gas smell diagnosis - plumbing checklist
Septic gas smell diagnosis - septic system checklist
Other causes of sewage odors, septic odors, sewer gas, rotten egg, or other indoor gas odors

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This page describes how to diagnose, find, and cure odors in buildings including septic or sewage 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. What makes the smell in sewer gas? Sewer gases are more than an obnoxious odor. The page top schematic of a typical plumbing trap is courtesy of Carson Dunlop'.

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). Depending on the sewer gas source and other factors such as humidity and building and weather conditions, mold spores may also be present in sewer gases. Also see Wet Weather or Cold Weather Septic Odors or Sewage Odor Diagnosis & Repair Guide for additional odor tracing and cure advice for odors occurring during wet or cold weather.

Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References." © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Problems With Plumbing Fixtures or Fixture Traps Can Release Sewer Gas Odors Indoors

Here are some plumbing trap defects to check. Any of these can produce drainage problems and in turn may leak or cause sewer gas or septic odors to be produced in or even outside of a building:

Inspect the building plumbing traps for leaks, defects or improper or missing traps: The photo shows a plumbing "trap" that is guaranteed to produce odors: it's not a plumbing trap at all, but rather this plumbing drain uses a car radiator hose.

This was a great idea for an emergency "Sunday night" plumbing repair, but not something to keep in a home.

Note that there is no bend in the trap to hold a water seal - sewer gases will pass readily out of this sink drain into the building.

This home made drain pipe also leaks, as you can see by the black mold on paper located below the sink.

Use of "S" Traps or other illegal and obsolete plumbing fixture traps where a "P" trap is required. S-traps are often installed in older buildings where there is no venting provided for that plumbing fixture.

S-traps easily lose the water from the plumbing trap, especially if the S-trapped fixture is near a toilet or other large plumbing fixture.Cs sketch at left shows several types of illegal plumbing traps including the "S" trap.

When the larger fixture is draining, the sudden and large volume of water rushing down the drain creates a vacuum in the drain line that can siphon water out of the nearby plumbing traps.

When a plumbing trap has lost its water seal, sewer gases pass readily back into the building.

Look below the sinks for antiquated or un-vented drains - if you see an "S" trap rather than a modern "P" shaped plumbing drain trap, the fixture is almost certainly not properly vented.

Don't forget to check for smells at your garbage disposer drain too.

If that is the odor source you may be able to remove the odor by cleaning liquids or even simple vinegar.

Dry Plumbing Traps: when a plumbing trap remains un-used for a long time the water can simply evaporate from the trap, permitting sewer gases to back up into the building.

Some places where we often find dry plumbing traps include:

Un-used basement plumbing fixtures like laundry sinks or basement bathrooms
Basement floor drains

There are various ways to deal with dry plumbing traps to stop smelly and dangerous sewer gas backups into the building.

Carson Dunlop's sketch at left shows how a trap primer system can be installed froma laundry tub. Other owners simply pour some mineral oil into unused traps.

Self-sealing traps containing check valves are available and are suited for floor drains. If water on the floor needs to escape it can flow out of the trapped floor drain, but the trap contains a mechanical seal which prevents sewer gas backups even if the floor drain trap is dry.

Clogged or blocked plumbing traps can cause leakage, and organic debris in plumbing traps may itself be a source of odors which people mistake for sewer gas backups.

If your drains are slow at only certain fixtures one of the first things to check is for clogging of the individual fixture traps.

As Carson Dunlop''s sketch shows, some fixtures such as laundry sinks are required to have traps which include a cleanout plug, making cleaning of the trap easier.

Otherwise you'll have to remove the entire plumbing trap to clean it. (Remember to put a bucket under a trap before trying to remove it, and remember not to try working on plumbing traps on Sunday night when you can't dash out to the building supply store to replace parts you've broken.)

Loose Toilets Can Produce Sewer Gas Odors Indoors

Check your toilets for leaks at the toilet base. The wax ring used to seal the toilet base to the waste pipe at the floor may be deteriorated or leaky, especially if the toilet is loose or was previously loose.

A wobbly toilet compresses the wax ring seal, leading to leaks and sewer gas odors in the bathroom.

In the photograph shown here, stains around the toilet base suggested that this toilet had been leaking at its base - a condition both unsanitary and smelly.

How to check for a loose toilet: Straddle the toilet and gently pinch it between your knees. Then gently push on each side of the toilet to see if it moves.

If the toilet moves it may be leaking into the floor (and ceiling below) - an unsanitary condition. The toilet needs to be removed, any damaged floor repaired, and then the toilet is reinstalled using a new wax toilet sealing ring before bolting it securely to the floor.

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MVOCs & MOLDY MUSTY ODORS
ODORS & SMELLS DIAGNOSIS & CURE
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  First Steps for Sewer Gas Odors
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  Plumbing Vent Definitions, Types
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  Site, Weather, or Failing Neighbors
  Tests for Indoor Sewer Gas
  Tracking Odors to Source
SEWER GAS ODORS in COLD WEATHER
Toxic Gas Test Procedures

Thanks to Slade Franklin for the reminder that a leaky wax ring at a toilet can lead to septic odors in bathrooms. 11/2007
Thanks to J.V. (privacy protected) for the reminder to make a detailed inspection of the plumbing vent system when sewer gas odors are present. 07/2008
Thanks to Roger Hankey & Cheryll Brown, www.hankeyandbrown.com, ASHI home inspectors in Minnesota, for the deteriorated transite pipe gas flue vent photograph and comments. Mr. Hankey is a past chairman of the ASHI Technical Committee, serves as co-chairman of ASHI legislative committee, and has served in other ASHI professional and leadership roles. 7/2007.
Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

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More Information on Building Diagnostic Inspections and Repairs

Cold Weather Septic Odors or Sewage Odor Diagnosis for additional odor tracing and cure advice focused on sewer gas odor tracking in cold or wet weather.
Drain Noises: may indicate defective or clogged plumbing: how to diagnose and cure drain sounds

The Septic System Information Website home page for this topic
Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
The Home Buyer's Guide to Septic Systems
Septic Tank Pumping Guide: When, Why, How to pump the septic tank
Home & Outdoor Living Water Requirements
Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.
What is a Septic System An Engineer's View - Types of treatment tanks, adsorption systems, pumps, and other special equipment are discussed in some further detail in this text
Components of a Septic System- the Basic Parts of a Conventional Septic Tank and Leachfield, a chapter in the Home Buyers Guide to Septic Systems
A Toxic Gas Testing Sampling Plan for Residential Indoor Air Investigations

Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)

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