S Trap vs P Trap – What’s The Difference?

s-trap vs p-trap diagram

Understanding drain configurations and functions, like those of s-traps vs p-traps, is fairly simple, while understanding technical aspects of each drain type might be just a little more complicated.

A licensed plumber often needs to understand the physics of plumbing for troubleshooting and remedying specific plumbing issues. Understanding the main differences between s-traps and p-traps is a matter of how they appear and physics. We’ll talk about both in this article.

S Trap vs P Trap: The Main Purpose of All Drain Traps

An s-trap is so-called because of its general shape, which looks like an “S”. Likewise, a p-trap resembles a sideways “P”. Both types of traps are intended to do the same thing, which is to provide a water barrier between the inside of your home and the sewer.

P-traps were designed to eliminate the main issue with s-traps, which is that they tend to siphon away too much waste water, leaving the drain “dry”. A dry trap allows unwanted gases to enter the home.

Some sewer gases smell bad, but not all of them. Some are odorless and some are dangerous. Smelly gases, like methane, are annoying. Some odorless ones, like carbon monoxide, can be dangerous.

Both s-traps and p-traps provide a water barrier that keeps all gases from entering the home.

What Does An S-Trap Do?

During the early 20th century, s-traps were a standard drain configuration. And, they were commonplace. An “S” shaped drain was installed (under a sink or tub) which typically diverted slightly and went down into a drain line under the floor.

Depending on specific applications, an s-trap drain configuration can work just fine. Sometimes though, too much water flows through an s-trap, leaving the trap nearly empty. An air gap is left, allowing gases to escape through the trap. This is called a dry trap. Because of the dangers of dry traps, s-traps became illegal in new construction several decades ago.

dry s-trap and functioning s-trap

S-traps No Longer Meet Code and Are Illegal in New Construction

The main reason s-traps are now illegal is that when a large amount of water, like a full sink, is drained, a siphoning action can happen, which often pulls the water through the pipe, leaving the bottom of the trap dry.

A Dry Trap Can Be More Than Just Smelly

When a trap is dry, gas and odors can travel through the drain pipe and enter your home. Gases like methane, hydrogen sulfide, and ammonia are present in sewers. Because these gases can be both smelly and dangerous, a barrier like a drain trap is necessary.

What may seem even worse, without some sort of barrier, vermin can enter your home through your drain as well. It might be a long way in, but they can get in. Not good! You definitely want and need a properly working drain trap.

A Simple Remedy For a Dry S-Trap (or P-Trap)

Of course, a simple remedy for a dry drain trap is to slowly pour water down the drain to ensure a seal. Actually, through evaporation, any trap can go dry over time. A rarely-used sink can develop a dry trap simply because of evaporation.

Most toilet drains have s-traps. Anyone can hear and see how water continues to run for several seconds and refills the toilet bowl after flushing. The exact reason for the water to continue to run is to ensure a full trap. The toilet bowl full of water that you see is actually half of the s-trap. You probably already knew that.

It’s easy to see how a toilet drain utilizes an s-trap configuration. They work well because the water continues to run into the toilet bowl while the tank is filling up.

s-trap drain on a toilet

P-Traps - What Does a P-Trap Do?

P-traps replaced the use of s-traps, particularly under sinks, to eliminate the siphoning problem. A p-trap has two main features that stop siphoning.

The first is a vented pipe. A p-trap is vented typically inside a wall and through the roof to the outside air. When air pressures are balanced inside the drain, siphoning becomes far less likely since negative pressure or the “sucking” action needed for siphoning doesn’t occur.

Second, an extension is added to the drain side of the trap. The extended pipe dramatically reduces the possibility of gravity “pulling” water through the pipe. That piece of pipe is called a waste arm extension.

A common calculation used to determine the required length of the waste arm extension is a simple one; the length of the extension needs to be 2-1/2 times the diameter of the pipe in order to eliminate the possibility of siphoning. For example, a 1-1/2″ drain pipe needs a waste arm extension no less than 3-3/4″ long to ensure a proper p-trap configuration.

The water inside a p-trap can still evaporate though. So, there is still no “perfect” configuration that can totally eliminate the possibility of a dry trap.

If you have a sink that is rarely used, a dry trap can happen simply due to evaporation. Regular plumbing maintenance and occasionally running water for a few seconds into the drain can eliminate that possibility.

Modern Drain Pipe Venting

In modern plumbing, a vent “stack” is used to eliminate negative pressure inside drains. A vent pipe typically runs vertically inside a wall, through the attic and roof to the outside of the house. A single vent pipe can serve more than one drain.

When a vent stack isn’t present, tearing into a wall and possibly having to drill large holes through beams, headers and the roof isn’t very practical and sometimes not even possible.

Converting an S-Trap to a P-Trap

The simplest fix for a problematic s-trap is to slowly run enough water after using the sink to make sure the trap is full. Sometimes though, an s-trap to p-trap conversion may become necessary. As we talked about earlier in this article, we need to change two aspects of the drain. We need to add a waste arm extension and vent the pipe.

typical s-trap conversion diagram

Air Admittance Valves (AAV's)

Fortunately, there is an alternative to a fully installed vent stack. Air Admittance Valves, or AAV’s, can be used to equalize pressure inside the drain. An Air Admittance Valve is a one-way air valve that allows air to enter the waste side of the drain pipe while preventing sewer gasses from getting out.

Some locales don’t allow AAV installations and you should check your local code before installing an AAV.

S-Trap Vs P-Trap Conversion Kits and Alternatives

There are s-trap to p-trap conversion kits available, but all too often, custom configurations are necessary. Most big-box construction stores have fairly well-equipped plumbing departments which allow you to “build your own”.

Make sure you get a good quality Air Admittance Valve. Oatey Sure-Vent, StudorVent, and Sioux Chief Turbo Vent – are all good quality AAV products.

You’ll need to build a sub-stack, which the AAV sits atop.  See the illustration above.

The top of the sub-stack needs to extend to an elevation above the sink drain to prevent overflow or back up. 

Once you have all the parts ready to go, you can easily reconfigure nearly any drain in a short afternoon. The entire conversion assembly is done inside the cabinet under the sink.

Simply follow the requirements for a proper s-trap vs p-trap conversion (waste arm extension length and venting requirements) and the rest is a matter of getting it done efficiently and making it look halfway decent.

It’s important to note that this article’s intention is simply to introduce you to the differences in s-traps vs p-traps. It does not necessarily cover everything you need to know about converting an s-trap. Although, I think I covered most, if not all, the details you need to know for an s-trap conversion if that’s your intention.

Also, I recently wrote an article on general home plumbing maintenance.  If you own a home and want to know how to avoid many plumbing emergencies, you may want to read it.

Phil Vandermeer

Philip Vandermeer is a husband, father, retired contractor, and handyman who enjoys playing the guitar and building things with wood. In the past 2 decades, he has flipped more than a half-dozen profitable house rehabs.

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