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

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 unvented or inadequately vented plumbing traps. S-traps are particularly prone to these problems as well as a source of poor fixture drainage.

Page top photo: a typical S-trap below a sink. This S-trap was also corroded and leaky.

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 defines S-traps used on some plumbing fixture drains, and explains why S-traps may be un-safe.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Definition of S-Traps

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 here in Carson Dunlop Associate's illustration. Carson Dunlop Associates is a Toronto home inspection, education, & report writing company.

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]

Separately,

at PLUMBING TRAP HISTORY we explain the difference between P-traps and older S-traps as well as drum traps and bell traps, and explains trap siphonage and the dangers that can result from dry plumbing traps.

 

Issues with S-Traps on Plumbing Fixtures & Drains

S-trap toilet design (C) InspectApedia.comUse 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.

S-traps are still common in many toilet designs like the simplified toilet trap we show here. But most modern toilets, even ones flushing down through an "S" trap, include design features to protect the water seal in the toilet.

So what's the trouble with S-traps on sinks, showers, bathubs and similar fixtures?

S-traps in those uses 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.

Watch out: Because sewer gas contains methane gas (CH4) there is a risk of an explosion hazard, pathogens entering the home, or even fatal asphyxiation.

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.

Also see our description of the invention of S-traps

at PLUMBING TRAP HISTORY.

Below we illustrate some plumbing drain connections that form an "S-trap" likely to lose its water seal.

Twisted S-trap below a bath sink (C) Daniel Friedman at InspectApedia.com ... S-trap below a sink  (C) InspectApedia.com DF

And below we illustrate two types of S-trap designs: the "full" S-trap and the 3/4 S-trap.

Two type of S-traps - both illegal (C) InspectApdia.com

Reader comment: Iggy 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.

Typical S-trap below a sink (C) Daniel Friedman InspectApedia.com

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) InspectApedia.com

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 invented...to 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

Iggy,

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 excerpt from Chapter 10 of the Uniform Plumbing Code section 1002.3, noting item 5 specifically prohibiting S-Traps:

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:

...

5. “S” traps, of uniform internal dimension.

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.

 




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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: is this a permitted type of plumbing trap?

Unsafe and improper "S" trap - actually a 360 degree double S trap is prohibited (C) InspectApedia.com Kahn

Saw this trap beneath the master bathroom basin. Wasn't sure if this is correct or not.
Please advise.

PS: It's on new construction

Moderator reply: S-traps and similar devices are ineffective, risk lost of trap seal, methane gas explosions, and are prohibited

Dov, as you can see above on this page, S-traps and similar plumbing traps are prohibited by the model building codes.

The concern is that the water in the trap easily siphons out, leaving the trap dry, permitting sewer gases into the building, risking even a methane gas explosion.

And of course the drain will perform poorly too.

This isn't just code or theory; a few times a year in most large cities there's a methane gas explosion or two. These methane gas explosions in plumbing systems and buildings can be severe, even bringing down a building. For example see

On 2020-03-13 - by (mod) - get rid of S-traps when you can

It's best to get rid of that S-trap as it will tend to siphon; a P-trap is preferred. I'm unclear on how the S-trap can also be a properly vented trap.

Yes if there is no opportunity to actually vent the sink properly your local plumbing inspector may approve a combination of P-trap and Air Admittance Valve or AAV.

You don't give country, city, building age, but most-likely by now any lead drains are very old and a leak risk; if I were renovating plumbing I'd replace that material.

On 2020-03-13 by Alexi

Hello,

I am trying to replace a vanity and a plumber who came over noted that I had an s trap running into a lead waste line.

I've checked all the other waste lines in the house and they are all also s traps running into lead waste lines. I guess this was standard practice for a suburban ranch home build in the 50's.

I recently moved in but have not had any problem with clogs, leaks or siphoning in any of the traps. Given that I am trying to remodel with the intent to sell in a few years, and that everything was in relative working order, my questions are:

1. Is it necessary to replace the s trap if the line is vented?

2. If it is necessary, would it be permissible and advisable to use an AAV on a p trap running into the pre-existing waste line?

3. Is it necessary to remove the lead waste line pipes, in which case I would then just opt to run a p trap from the wall.

Thanks for any advice.

On 2017-05-17 - by (mod) -

Ig

Criticism is not always much fun, but my editorial policy is that we ought to keep people's opposing views as long as the text is not downright dangerous to readers and as long as the text is not obscene.

Not to exactly change sides in the debate, but I'm sorry to say I agree with and am disappointed in the appellation "home inspection industry" rather than "home inspection profession". The distinction is lost on some but not everyone. We have different expectations of a professional (competence, ethics, education, performance to standards) than we might of someone who views home inspections as "just a business".

It was in just such a group of business men in a home inspection (previously) professional association who told its national ethics enforcement committee not to enforce the society's ethical rules in a conflict of interest case because, as the group said, "this is business, ethics doesn't apply".

Ethics always applies. You can choose to follow ethical standards or not, but you can't decree ethics to be inapplicable to professional conduct.

I *think* that your claim that S-traps siphon less badly or less-often than an Un-vented P-trap may be based on the theory that the S trap may have a deeper water reservoir, but I've not been able to find authoritative data on that.

The citations that you didn't much like are intended to document that there is a very long history of association with sewer gas leaks into buildings with disease, and that the development of plumbing traps and venting were a part of efforts to correct that. The later citations are legal or "code" wherein all S-traps are declared "not to code" - at least in new construction.

You are welcome to post your horizontal drywall views over at

inspectapedia.com/BestPractices/Drywall_Installation_Procedures.php

On 2017-05-17 by Iggy

Fantastic and thank you for keeping my points and experience amongst numerous (hundreds) S-traps unedited! I really appreciate this getting out the PROPER information.

I do agree, as I stated, that ONLY P-traps WITH venting are an improvement. I'm sorry but, I don't feel your citations were even related to the debate and definitely weren't anything pertaining to ANY Trap's VENTING necessity.

The very same quick mock-up to demonstrate an S-trap's, possibly, OCCASIONAL siphoning are duplicated by P-traps,

BUT it's ALL OF THE TIME under ALL circumstances with a P-trap. Thus, my main and whole point that Inspectors MUST understand is that VENTING of ANY trap must be in place and functioning...as you eluded to in your last response in agreement with me.

I'm very glad that we agree on this VITAL point.

However, the Home Inspector Industry doesn't AT ALL look nor test for any venting of any trap and very simply assumes any and all S-traps are the SOLE problem. I say this because I've had many run-ins with family, friends and new neighbors that bought an S-trap house or building and followed the Home Inspector's warning and suggestion to "just switch out to P-traps".

Well, they sure did obey and got sewer gas and gurgling EVERY TIME they used a sink or washer outside of the Main Stack's venting zone. I didn't recommend returning to the S-trap, as I do wholly agree with the P-trap improvement (if done properly), and helped them install an AAV or Exterior Wall Vent.

TRUE disaster averted and I've done my part to protect Home Inspector's from being, rightfully, sued. A suggestion to "have a Plumber change you out to P-traps" is NO safeguard and just another lawsuit pocket to pick from.

Thank you again for getting this GROSS misunderstanding explained and presented to Home Inspectors, Building Inspectors and, yes, even very many Plumbers. Now, how about the absolute Deathtrap of Horizontally Installed (exceedingly wrong in every way) Drywall?

However, I don't know of a Non-Damaging test that a Home Inspector could perform to verify the MAJOR defect allowed by all Building Inspectors...other than finding the telltale humps in longer walls from the absurd butt-joints.


...

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