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Plumbing Fixture & Trap Defects
a source of sewer gas smells and septic tank odors
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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
This plumbing traps (interceptors) article describes plumbing traps (interceptors) and 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.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Problems With Plumbing Fixtures or Fixture Traps (Interceptors) Can Release Sewer Gas Odors Indoors
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'. 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.
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). 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.
Technical note on sewer gas smells: because the "sulphur smell" that some people may associate with dangerous sewer gases can have other sources having nothing to do with building plumbing systems, readers should also see CHINESE DRYWALL HAZARDS and ODORS, SULPHUR SMELL SOURCES and ODORS, SEPTIC or SEWER and finally, ODORS & SMELLS DIAGNOSIS & CURE where we describe how to track odors to their source or cause.
What's the difference between Plumbing Traps vs Plumbing Interceptors?
Note: recent changes in plumbing codes including ASME Standards, the Uniform Plumbing Code (UPC), Canadian Standards Association (CSA) and the International Plumbing Code (IPC) have dropped the term plumbing trap, and substitute a more inclusive word, "interceptor".
In some building codes and plumbing texts you will see increased use of the term interceptor where you may be more familiar with the term plumbing trap. In particular, grease traps are referred to now as hydromechanical grease interceptors or gravity grease interceptors.
Common Plumbing Trap or Interceptor Defects
Here are some plumbing trap (interceptor) 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:
Leaky, Missing, or Improperly Mounted/Located Plumbing Traps / Interceptors
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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. |
Definition & Issues with S-Traps on Plumbing Fixtures & Drains
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 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.
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Garbage Disposers (food grinders) and Plumbing Traps / Interceptors
Don't forget to check for smells at your garbage disposer drain too. And in buildings where a grease trap or grease interceptor is installed, the garbage disposer (food grinder) drainage is required to bypass the grease trap. This requirement is notwithstanding that garbage disposers (and the pre-rinse function of some dishwashers) is a substantial source of fats, oils, grease (FOG) that clog plumbing drain systems.
If that
is the odor source you may be able to remove the odor by cleaning liquids or even simple vinegar.
Dry Plumbing Traps / Interceptors
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 Definition of Dry Plumbing Traps: a dry trap is a plumbing trap or interceptor that has lost its water seal, thus permitting sewer gases to escape back up building drain piping and out into the building through the fixture - an unsafe, unsanitary, and potentially dangerous condition.
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.
Watch out: Because sewer gas contains
methane gas (CH4) there is a risk of an explosion hazard or even fatal asphyxiation.
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 from a laundry tub. Other owners simply pour some mineral oil into unused traps.
Self-Sealing Traps for Floor Drains
Self-sealing plumbing traps (interceptors) 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 Plumbing Traps / Interceptors
 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.)
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Loose Toilets Can Produce Sewer Gas Odors Indoors
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 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.
We provide a detailed list of sewer and sulphur gas odor sources at Sources of Sulphur Odors in buildings.
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Frequently Asked Questions (FAQs) about plumbing drain traps & traps on plumbing fixtures, sinks, tubs, showers, etc.
Question: can a dried out floor drain trap cause a sewage backup?
Can a dried out floor drain trap cause a sewer line backup? - Daryl 12/27/12
Reply: directly, no, indirectly ... maybe
Can a dry trap cause a sewer line backup?
No Daryl, a dried out floor drain trap doesn't cause a sewer line backup, at least not directly;
A dried-floor trap is simply an empty U-shaped pipe in the drain system. For a sewer line to back up we expect to find either that the drain system has become blocked downstream from the point at which the backup manifests itself, or the entire sewer system is flooded, such as during a storm, hurricane, or area flooding.
But in a more subtle way a dried floor drain trap *might* contribute to a downstream drain blockage if it served as an entry point for animals who nested in the dry drain, or if there is no screen over the floor drain and some fool sweeps dirt and debris into the drain system. Both of those are indirect contributions to a sewer line backup.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- 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.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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