Reuben's Home Inspection Blog


A Primer on "S" Traps

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.

Reuben Saltzman, Structure Tech Home Inspections


Comment balloon 20 commentsReuben Saltzman • June 05 2012 02:53AM


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.

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 7 years ago

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.

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) over 7 years ago

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.

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) over 7 years ago

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


Posted by Richie Alan Naggar, agent & author (people first...then business Ran Right Realty ) over 7 years ago

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

Posted by Diane Daley (Caron's Gateway Real Estate) over 7 years ago

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

Posted by Chris Smith, South Simcoe, Caledon, King, Orangeville Real Esta (Re/Max Chay Realty Inc., Brokerage) over 7 years ago

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... 

Posted by Paul S. Henderson, REALTOR®, CRS, South Puget Sound Washington Agent/Broker! (Fathom Realty Washington) over 7 years ago

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.

Posted by Cindy Jones, Pentagon, Fort Belvoir & Quantico Real Estate News (Integrity Real Estate Group) over 7 years ago

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

Posted by Joan Whitebook, Consumer Focused Real Estate Services (BHG The Masiello Group) over 7 years ago

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.

Posted by Debbie Cook, Silver Spring and Takoma Park Maryland Real Estate (Long & Foster Real Estate, Inc) over 7 years ago

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!

Posted by Athina Boukas, Certified Residential Specialist (CRS) (Virginia Capital Realty) over 7 years ago


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.

Posted by Evelyn Kennedy, Alameda, Real Estate, Alameda, CA (Alain Pinel Realtors) over 7 years ago

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.

Posted by Robert Butler, Montreal Home Inspector | Aspect Inspection (Aspect Inspection) over 7 years ago

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...

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 7 years ago
Thanks Rueben, I now feel like I know the pros and cons of S traps! You really explained it all very clearly. Thank you.
Posted by Kevin O'Rourke - Keller Williams Miami Beach Realtor, CDPE Miami Short Sale Agent 305-520-9436 (Keller Williams Miami Beach Realty) over 7 years ago

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.


Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

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 :)

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

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.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

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.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 7 years ago

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

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 7 years ago