A Primer on "S" Traps

Home Inspector with Structure Tech Home Inspections

A common plumbing defect found in many older houses is an "S" trap.  An "S" trap is named so because it looks like an "S" on its side - it basically consists of a normal trap, and then another trap installed right up against it in an upside-down fashion, as shown below.  These traps are prohibited by the Minnesota State Plumbing Code, but that doesn't mean you can't buy them at any home improvement store.


The most common places you'll find  "S" traps are at plumbing fixtures that aren't vented, such as an illegal basement bathroom sink, a DIY basement bar sink, or a kitchen sink where the old steel drain in the wall was so corroded and blocked that someone installed a new drain, but never bothered to vent it.

The purpose of a trap is to prevent sewer gases, and possibly vermin, from coming in to the home.  When water sits in the trap, sewer gases stay out.  Have you ever noticed some nasty odors in a bathroom that you never use?  It's probably because the water in the trap has completely evaporated, allowing sewer gases to come in to the home.  I've actually found this condition on a number of larger occupied houses.  The easiest fix for this is to periodically run a little water in the plumbing fixtures.   As a longer-term solution, you could also pour RV antifreeze in to the drains; that stuff won't evaporate.

Back to "S" traps - The reason "S" traps aren't allowed is because they have the potential to suck, or 'siphon', water out of the trap as the water flows down the drain.  On a properly installed "P" trap, there is a vent at the same place the drain turns downwards, which breaks the siphon.  In other words, the vent prevents water from getting sucked out of the trap.  How much water can get sucked out of the trap?  Believe it or not, enough water to break the water seal at the trap and let sewer gases come in to the house.

To show how this works, I set up a little rig in my back yard to show how much water can get siphoned out of an "S" trap.  I forgot to take a close-up before photo, so I marked up this photo below with blue to show where there was standing water in the trap.  The top of the water level is called the 'weir' of the trap.

S-trap experiment

As I let the water in the 5-gallon bucket drain out, most of the water in the trap ended up getting siphoned out.  I added some food coloring to the water and held my flashlight up against the back of the trap to show this.

Water siphoned out of S-trap

To see how this happens, here's a quick video I took during a home inspection.  Listen to the gurgling noise at the very end of the water draining - this gurgling sound is a dead giveaway that a plumbing fixture isn't properly vented.

There's usually no simple way to properly correct an "S" trap.  It's usually an involved, expensive repair.  On the other hand, when water is flowing in to a sink from the faucet, there's usually not enough water draining at one time to create this siphon action, so "S" traps typically don't cause any big problems.  Problems occur when the sink is filled with water and then drained.  Even then, the simple way of dealing with this is to just run a little extra water down the drain after the water has drained out.  This will re-fill, or 'prime', the trap after the water has been siphoned out... this is the same way toilets work.

In other words, "S" traps are wrong, but they're not that big of a deal.


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Charles Buell
Charles Buell Inspections Inc. - Seattle, WA
Seattle Home Inspector

Reuben, this is a very cool visual of how they S-traps aren't such a good idea.  Around here, S-traps can easily be fixed with air admittance valves.  I remember seeing a lot of S-traps in older homes in the Northeast that were plumbed long after they were built and there was not access to walls to have p-traps.

Jun 05, 2012 10:04 AM #1
Donald Hester
NCW Home Inspections, LLC - Wenatchee, WA
NCW Home Inspections, LLC

Reuben, I see them fairly frequently here. And we are allowed to use an AAV like Charlie says. Pretty common practice around here. I did one home where they installed all AAV in the attic instead of venting through the roof.

Jun 05, 2012 11:23 AM #2
Donald Hester
NCW Home Inspections, LLC - Wenatchee, WA
NCW Home Inspections, LLC

Reuben, Oh yeah you got to love the accordion drain line also ; 

I have tried to find anywhere that actually states that those flex/accordion drains are an approved drain line. If anyone has anything on that I would sure love to get my hands on it.

Jun 05, 2012 11:26 AM #3
Richie Alan Naggar
people first...then business Ran Right Realty - Riverside, CA
agent & author

In the world of S traps, that's a fine looking full length picture there....I leave your post a little smarter than when I found it...thank you R


Jun 05, 2012 11:32 AM #4
Diane Daley
Caron's Gateway Real Estate - Northumberland, NH

I take mine s traps apart at least once a year for a routine cleaning to prevent problems. Good post

Jun 05, 2012 12:07 PM #5
Chris Smith
Re/Max Chay Realty Inc., Brokerage - New Tecumseth, ON
South Simcoe, Caledon, King, Orangeville Real Esta

Reuben, so simple, and yet, something I only learned after reading your post.  Thanks

Jun 05, 2012 04:15 PM #6
Paul S. Henderson, REALTOR®, Broker, Tacoma Washington
RE/MAX Professionals. - Tacoma, WA
Tacoma Washington Agent/Broker & area authority!

I have to agree with Richie Nader, I am much smarter after reading this post. Who would have thunk it. I've taken S traps for granted... 

Jun 05, 2012 05:15 PM #7
Cindy Jones
Integrity Real Estate Group - Woodbridge, VA
Pentagon, Fort Belvoir & Quantico Real Estate News

Well I just learned something new.  I'll make sure to go run the water in the guest bath more often to keep the smells at bay.

Jun 05, 2012 05:24 PM #8
Joan Whitebook
BHG The Masiello Group - Nashua, NH
Consumer Focused Real Estate Services

This is very interesting.. Glad to know there is an inexpensive solution to this problem.

Jun 05, 2012 07:43 PM #9
Debbie Cook
Long & Foster Real Estate, Inc - Silver Spring, MD
Silver Spring and Takoma Park Maryland Real Estate

Made me want to check by pipes and go run the water in the bath upstairs pronto!  Thanks for an interesting post about something that is usually not interesting - who would have thunk that plumbing pipes  and how they work could keep my attention.

Jun 05, 2012 07:59 PM #10
Athina Boukas
Athina Boukas, Broker/Owner (Greensboro, NC) - Greensboro, NC

Who would have thought that S-traps would be such an interesting feature! Actually, I had a main bath's plumbing re-done on a foreclosure home because the previous owner did a botched job installing this "extra bath".  Like you mentioned, water gurgled in the drain...and yes, there was no vent!

Jun 06, 2012 06:11 AM #11
Evelyn Kennedy
Alain Pinel Realtors - Alameda, CA
Alameda, Real Estate, Alameda, CA


That video is very cool. Thanks for the information about "s" traps.  I am learning more about plumbing things than I really want to know.  However it is nice to be able to talk about plumbing stuff like I know what I am talking about.

Jun 06, 2012 08:35 AM #12
Robert Butler
Aspect Inspection - Montreal West Island, QC
Montreal Home Inspector | Aspect Inspection

Nice job on the post and video Reubin. I imagine many viewers would benifit fron a diagram of the correct set-up including the venting.

We don't see that whte stuff around here much, and I would not have though to use the flashlight like that.

Jun 06, 2012 09:16 AM #13
Jay Markanich
Jay Markanich Real Estate Inspections, LLC - Bristow, VA
Home Inspector - servicing all Northern Virginia

Great primer Reubs.  They are dead give aways for unprofessional work.  About the only thing a plumber can do to help it is to put in a P trap with an air admittance valve.

That trap sounds a little like me after dinner.  Too much info?

I used to know a guy named Jason Weir, but I don't remember him as being at the top of any trap.  Could be my memory is bad though...

Jun 06, 2012 02:54 PM #14
Kevin O'Rourke - Keller Williams Miami Beach Realtor
Keller Williams Miami Beach Realty - Miami Beach, FL
CDPE Miami Short Sale Agent 305-520-9436
Thanks Rueben, I now feel like I know the pros and cons of S traps! You really explained it all very clearly. Thank you.
Jun 06, 2012 07:29 PM #15
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Minneapolis Home Inspections

Charles - air admittance valves are a good alternative.  We seem to have a strong plumbers union here in Minnesota, so those valves aren't allowed :(

Donald - those accordian drains aren't allowed anywhere, as far as I know.  Not to say I don't see them all the time...

Richie - thank you, sir :)

Diane - smart.

Chris - you bet, thanks for reading.


Jun 07, 2012 06:26 PM #16
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Minneapolis Home Inspections

Cindy - or just pick up some RV antifreeze.  It's safe.

Joan - yeah, it's not as big of a deal as some people make it to be.

Debbie - did you hear any gurgling?

Athina - that gurgling sure is annoying, isn't it?

Evelyn - I love doing experiments like this too :)

Jun 07, 2012 06:28 PM #17
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Minneapolis Home Inspections

Robert - good idea.  I might have to do a follow up post on proper and improper traps.

Jay - I installed one of those air admittance valves in my mother-in-law's house, and it works like a charm.  No more gurgling.  I'm sure they named the top of the water level after Jason. 

Kevin - thanks.

Jun 07, 2012 06:31 PM #18
Jay Markanich
Jay Markanich Real Estate Inspections, LLC - Bristow, VA
Home Inspector - servicing all Northern Virginia

Our kitchen sink is at the back of the house and while connected to the house vent system, it is far away.  I think the plumbers put the AAV on there just to make sure there was no gurgle.

No S trap though...  and, happily, Jason isn't under there.

Jun 08, 2012 03:14 AM #19
James Quarello
JRV Home Inspection Services, LLC - Wallingford, CT
Connecticut Home Inspector

Good demo of how an S trap works. Many times buyers don't understand how this can be a problem. 

Jun 08, 2012 05:05 AM #20
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Reuben Saltzman

Minneapolis Home Inspections

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