Plumbing trap schematic (C) Carson Dunlop Associates Plumbing S-Trap Codes & Hazards
S-trap siphonage risks sewer gas entry

  • PLUMBING TRAPS & INTERCEPTORS - CONTENTS: How to inspect plumbing fixtures, loose toilets, & plumbing traps in order to track down building odors and sewer or septic gas smells, Definition of plumbing trap, definition of plumbing drain interceptor.
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Plumbing drain S-traps:

This article explains why an S-trap on a plumbing fixture is considered obsolete and why such traps are prohibited by modern plumbing codes.

We include research and history of health hazards, even deaths and explosions traced to sewer gas leaks into buildings that include leaks up through un-vented or inadequately vented plumbing traps. S-traps are particularly prone to these problems as well as a source of poor fixture drainage.

This article series describes the common problems that occur at plumbing traps: odors, leaks, noises, and we discuss plumbing trap types, requirements, locations, connections, installation, repair and replacement.

This article distinguishes between P-traps and older S-traps and explains trap siphonage and the dangers that can result from dry plumbing traps.

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Definition of S-Traps & Issues with S-Traps on Plumbing Fixtures & Drains

Sketch of illegal plumbing trap types (C) Carson Dunlop AssociatesImproper plumbing drain traps can release sewer gas odors indoors. That hazard is increased where an S-trap is installed. An illegal and troublesome S-trap is shown in the upper left of Carson Dunlop Associate's illustration. Carson Dunlop Associates is a Toronto home inspection, education, & report writing company.

Here we explain the following:

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.

Our page top photo shows a double S-trap below a sink. The downwards bend after the initial trap bend tells us that this trap cannot be properly vented.

[Click to enlarge any image]

Use of "S" Traps is both illegal and obsolete as plumbing fixture traps where a "P" trap is required. While not every S-trap will cause trouble, enough of them do that they are prohibited in new construction.

S-traps are often installed in older buildings where there is no venting provided for that plumbing fixture. So what's the trouble?

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.

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.

Reader comment: defends S-traps as harmless

Plumbing trap schematic (C) Carson Dunlop AssociatesOPINION from a reader: 2017/05/15 Iggy said:

Born and raised amongst S-traps and still have them decades later today. Never-ever had a problem whether they were within 5-feet of the stack or completely unvented 40-feet away from the stack. Sewer Gas DOES NOT search and lurk beyond the Main Stack!

Like water, Sewer Gas follows the path or least resistance! S-traps are EXTREMELY over-exaggerated, Hack Home Inspectors wear them like a badge upon discovery...too bad they don't have any other badges to notice, quite laughable. - Iggy

Reply: S-traps on plumbing fixtures are unsafe, have a long history of association with disease, function poorly, and are illegal in new construction

Glad to hear that your S-traps haven't given you trouble, Iggy though your case is the exception, not the rule.

Regarding your observation that "Sewer Gas DOES NOT search and lurk beyond the Main Stack! " I think you have not considered that an S-trap, by its very definition, location, design, is not vented through a main stack. Instead its connection downwards, without a horizontal section past the trap, is what permits siphonage and back-venting of sewer gases from the drain out through the fixture.

Regarding your observation that "Like water, Sewer Gas follows the path or least resistance!" - not quite. Water in a drain system wants to move down, impelled by gravity. But sewer gases, high in methane, and lighter than air, want to move "up" in a vent piping system.

In a properly designed trap and vent system those gases are vented above a roof. But in a system with incomplete venting or missing venting such as at an S-trap, sewer gases are perfectly happy moving up and out through a siphoned-dry sink or tub trap.

Unfortunately for them, S-traps are easily siphoned of their water seal and thus can vent sewer gas back into a building, and as the use of an S-trap means an un-vented fixture, often the fixture will not drain well either, as wastewater flowing down a poorly-vented or un-vented drain line creates a "sucking" vacuum behind it as it moves down the drain: thus slowing drainage and drawing water out of un-vented traps.

This problem has been well known for more than 100 years, as you can see in articles and research we cite here. As a result of widely-recognized health hazards from pathogens re-entering homes in sewer gas and the occasional dramatic explosion of sewer gas that sometimes levelled buildings, in the U.S. beginning in the 1880's authorities began developing and enforcing plumbing and sanitation codes that addressed, among other things, sanitary drainage requirements.

In an era of fake news and "alternative facts" presented as if they weighed as much as true facts, it's more important than ever that we respect authoritative, unbiased research on problems that confront us.

Lancet Paris France issue 121 1883 citing typhoid epidemic  (C)

Reader follow-up comment: disagrees that S-traps are a problem

Iggy replied:

Yikes! Talk about nonsense claimed to be research, fact or even relevant. A guy that built places with S-traps. Another guy that simply noted there's bacteria floating all around us. Some random guy that isn't even found on the internet. And the sole reason that S-traps were just remove stink.

I have no problem with change and embrace any improvement, but S-traps have NEVER been the terror laughably claimed by inspectors. P-traps, ONLY WITH VENTING, are an improvement! P-traps, WITHOUT A VENT, are VERY MUCH worse than any S-trap! Calling out an S-trap and not a lack of venting should be criminal!

Venting is the ONLY "defect", patently not the type of trap! That these guys and gals walk into a 100 or 50-year old house and fail the S-traps unbelievable fear-mongering. I'm the exception? You do realize we're talking about a likely still 100-million buildings, not just homes, that are still HAPPILY S-trapped out there.

Reply from moderator: S-traps are improper, unsafe, and are prohibited by plumbing codes


Sorry, Iggy but it just won't suffice for you to call plumbing experts from the past 100+ years a bunch of dopes.

A danger in failing to distinguish between our opinions and sound expertise is that we may end up asserting that nonsense is "true" - that doesn't change it from nonsense to truth. Opinion based on experience is valuable but it is a poor substitute for understanding, fact, or science.

I can't tell from your comments if you're just not writing precisely or if you actually don't understand plumbing traps. Contrary to your assertion, plumbing traps do not "remove stink" - they don't "remove" anything.

The function of a plumbing trap and the water seal it must contain is explained here. The very old plumbing articles we cited illustrate that more than 100 years ago serious disease and death were traced to the back-venting of sewer gases into buildings. The building code citations that prohibit S-traps represent the experience and judgment of plumbing experts charged with writing plumbing codes in countries around the world.

I do agree with you that venting is critical for proper, effective plumbing drain function.

But you are quite mistaken in your s-trap-philia. S-traps are prohibited as new construction under every plumbing code, though codes may permit the repair or replacement of existing S-traps as long as the plumber doesn't observe a problem at that particular fixture.

It's indeed the case, as you point out, that not every S-trap causes trouble and it is indeed the case that there are plenty of S-traps still in service. If all of them did cause trouble, even hard-core S-trap-philiacs like you would have changed their minds long ago.

The difficulty is that a building code committee or a plumbing inspector cannot possibly predict which S-trapped fixtures will be safe and which will give trouble risking poor drainage, sewer gas venting, infections or deaths from the biohazards, and an occasional sewer gas explosion. For all readers:

The S-shaped or S-bend plumbing trap, was invented by Alexander Cummings in 1775. To be legal, a plumbing trap must run horizontally on its outflow side. That's the feature that is missing from an S-trap.

Plumbing fixture drain traps protect your home from sanitation issues involving drains that tie into the sewer or a home septic system by providing a water seal that in turn prevents sewer gases from venting back into the building from the fixture drain.

If the horizontal run on the outflow side of a drain trap is omitted, water flowing down the drain of an S-trap can siphon water out of the trap at the end of the drain cycle, thus leaving a trap dry and ready to vent sewer gases back into the building.

Water can also be drawn out of any plumbing trap, S-trap or P-trap, by nearby fixture drains if the building venting system is missing or inadequate.

Model plumbing code prohibition of S-traps is explicit, clear, unambiguous. See this example from Chapter 10 of the Uniform Plumbing Code section 1002.3, noting item 5 specifically prohibiting S-Traps:

1002.3 Prohibited traps.

The following types of traps are prohibited:

1. Traps that depend on moving parts to maintain the seal.
2. Bell traps.
3. Crown-vented traps.
4. Traps not integral with a fixture and that depend on interior partitions for the seal, except those traps constructed of an approved material that is resistant to corrosion and degradation.
5. “S” traps.
6. Drum traps. Exception: Drum traps used as solids interceptors and drum traps serving chemical waste systems shall not be prohibited.

and from the 2009 National Standard Plumbing Code 5.3.5 prohibiting S-Traps

5.3.5 Prohibited Traps

a. The following types of traps shall be prohibited:

1. Traps that depend upon moving parts to maintain their seal.
2. Bell traps.
3. Crown vented traps.
4. Separate fixture traps that depend on interior partitions for their seal, except if made from plastic, glass or other corrosion resistant materials.
5. “S” traps, of uniform internal dimension.
6. Drum traps.

b. Hair interceptors, precious metal interceptors and similar appurtenances shall be permitted as required.

S-traps on plumbing fixtures are also discussed in the article where your comments were first-posted: PLUMBING TRAPS & INTERCEPTORS
and in the article's REFERENCES section we include additional links to authoritative sources and model building codes pertaining to plumbing traps.

Thank you for arguing, Iggy. Spirited debate can, properly-conducted, help us reach clear, authoritative answers to important questions and can help us move beyond arm-waving and bullying or bluffing into useful and accurate information.


Continue reading at PLUMBING TRAPS & INTERCEPTORS - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BACKDRAFTING & SEWER / SEPTIC ODORS as back-drafting can also draw sewer gases from dry plumbing traps

Or see DRAIN PIPING & SEWER ODORS full discussion of odors from building drains & drain piping.

Or see HYDROGEN SULFIDE GAS where we discuss the effects of exposure to H2S and H2S exposure limits.


Or see PLUMBING DRAIN NOISES how errors cause trap siphonage, odors, and noises

Or see PLUMBING SYSTEM ODORS - home for guidance in tracking down plumbing smells




Or see this

Article Series Contents

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