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What is an S Trap?

typical s trap

An S trap is a type of drain trap that was used on the plumbing drain system on many older homes. It is called an S-trap because it looks like a letter “S” sitting on its side. The plumbing systems in most older homes used S-traps on the drain lines under sinks to keep sewer gases out of homes.



S traps are now prohibited by code throughout the country because they are not vented and, therefore, are not as safe as the more modern vented p-traps.

Although S-traps have been “outlawed,” as a home inspector I still regularly see them when I inspect older homes. I also occasionally see these on newer homes where some non-licensed remodeling or plumbing work has been done.

What is a Drain Trap?

drain trap on s trap


Before I explain more about S-traps, let me first answer the question, what is a plumbing drain trap? A drain trap is a part of the pipe that remains full of water in order to prevent sewer gases from entering the home.

If you look at the picture, you can see a “U”-shaped pipe which is filled with water. The water in this pipe acts as the “trap” that prevents the sewer gases (methane, hydrogen sulfide, ammonia, etc.) on the left side from traveling back through the pipe, up through the drain, and into the home. In other words, the sewer gases are trapped or blocked from flowing through the pipe and into the home. All of the plumbing fixtures in your home (tubs, showers, sinks, toilet, washing machine) should have a trap on the drain line to prevent sewer gases from getting into your house.

What is the Problem with “S” Traps?

s trap- unventedThe problem with S traps is that they are not vented. The absence of venting allows the water in the trap to be sucked out of the trap when a sink or tub that is full of water is drained. If you have a sink full of water and then open the stopper, you will have a large slug of water filling the drain pipe and flowing down through the drain pipe. This “slug” of water can siphon the water out of the trap, leaving it mostly empty and unable to block the sewer gases from coming into the home. (See the photo on the left.)

If you don’t use a stopper and are simply running water into a sink, tub, or shower and letting it drain as the water runs, the trap will not be siphoned empty because you don’t actually have a slug of water, but instead you have a much smaller amount of water flowing through the drain line. The problem of siphoning the trap only occurs when the tub or sink has been stoppered and then the stopper is removed allowing a large quantity to run down drain in a short period of time.

See our related post about the purpose of plumbing vents.

S traps vs. P traps

typical P trap

A typical P trap

S traps and P traps both do the same thing (block sewer gases from coming into the home), but there is one big difference between them. P traps are vented, so they cannot be sucked dry like “S” traps can. While an S trap looks like an “S” on its side, a P-trap looks like a “P” on its side. If you look at the two pictures below, you will see the difference between a P trap and an S trap.

The photos below show the difference between an S-trap and a P trap. On the drawing of the P trap, you can see that there is a vent that allows air to be drawn into the pipe behind the slug of water. This air will break the siphon, preventing the trap from being sucked dry. On the drawing of the S-trap, you can see that there is no vent pipe to break the siphoning action. Due to the advantages of a P trap, plumbing code was changed many years ago to require P traps to be installed and banning S traps from being installed.

s trap and p trap

How to Fix an S-Trap

What should you do if you have an S trap under one or more sinks in your home or in a home that you are considering purchasing? Well, you have several options. This list starts with the hardest and most expensive to the simplest and lowest-cost option.

  1. You could have the S-trap replaced with a P-trap by cutting into the wall to run the vent pipe up and out through the roof. In my opinion, unless you are doing some significant remodeling and opening up the wall anyway, this is totally unnecessary.
  2. You can install a vent pipe and run it up the exterior wall and up through the roof through the exterior of the home. This is simpler than #1 above, but still quite a project. air admittance valve (AAV)
  3. Install an air admittance valve (AAV). An air admittance valve serves the same purpose as a vent going through the roof does. They are often installed on the drainpipe under the sink. AAV’s are basically a check valve (allow flow in only one direction), so they allow air to be drawn in through the valve and into the pipe in order to break the siphon, but they prevent air (and sewer gases) that are in the pipe from exiting the pipe through the valve and getting into the home. It is normally a fairly simple and inexpensive process to install one of these in your home. The photo below shows a drain line where an s-trap was corrected by adding an air admissive valve.

corrected s trap

4. Do nothing. Yes, you read that right. If you do nothing to eliminate or correct the S trap, you can simply run water for a few seconds into a sink or tub to ensure that the trap is refilled with water anytime you drain a sink or tub that was full of water.

What to Do if You Smell Sewer Gases in Your Home

s-trapOn a related note. Have you ever noticed some nasty odors in a bathroom that you never or rarely use?  It’s probably because the water in the trap has evaporated, allowing sewer gases to come into the home.  I’ve seen this several times on some larger, occupied houses where part of the home is not being used.

The easiest fix for this is to periodically run a little water into the plumbing fixtures. A longer-term solution is to ensure that there is water in the trap and then to pour a little bit of vegetable oil into the drain. The oil will act as a barrier and will slow the water from evaporating. This is not a permanent fix, but it will cause the water to remain in the trap for a lot longer than it otherwise would have.

Should I Worry if I Have S-Traps in My Home?

In summary, although S-traps are against current plumbing codes, they are not really that big of a deal. I would never be afraid to purchase a house because it has S-traps. I would simply run a little water to make sure that there is always water in the trap. Also, if I ever decided to do some remodeling of the room where the S trap was located, I would take advantage of the opportunity to eliminate it at that time and bring things up to code.

While S traps are against current plumbing codes, they are not cause for immediate concern.

See our post on Air Admittance Valves

© 2020 Mike Morgan

This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the central Texas area since 2002. He can be reached at 325-998-4663 or at No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.

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