S-Trap Vs. P-Trap: What Is The Difference

In this blog post, you’ll read:Fixture traps maintain your home by preventing problems with drains connecting to the sewer or a septic system. A trap is a length of pipe bent in the middle and ends in a "P" or "S" shape. The curve has a pool of water that stops sewer gases from moving via the pipe.

Table of Contents

Fixture traps maintain your home by preventing problems with drains connecting to the sewer or a septic system. A trap is a length of pipe bent in the middle and ends in a “P” or “S” shape. The curve has a pool of water that stops sewer gases from moving via the pipe.

What is a P-trap?

Similar to an S-trap, a P-trap is a U-shaped configuration of pipes. The pipe accumulates water in the U-shaped section. Yet, it prevents sewer gases from reaching the structure. The addition of a vented pipe stops siphoning, and the extension of the waste arm reduces discharge suction.

What is the Function of P-trap?

Sewer siphon P trap
Sewer siphon P trap

P-traps work by using a water seal in their curved shape to prevent sewer gases from entering the residence while allowing the wastewater to pass through. Solids continue to degrade in the trap over time. New water flushes the conduit when the trap is used to remove debris. As with any plumbing system component, P-traps can deteriorate over time and need maintenance.

What is S-traps

S-traps are a type of trap used in sinks and other plumbing fixtures to prevent the backflow of sewage gases. The S-shaped pipe configuration captures wastewater before it enters the sewer line, creating a water seal that inhibits gases from ascending through the drain.

pvc s trap
pvc s trap

P-Trap Vs. S-Trap: Differences by Design

  • S-traps Design

S-traps have a sharp bend that makes a water seal to keep gases from reaching through. However, they tend to leak and get clogged. P-traps have a deeper, more gradual bend, which makes a better water seal and stops siphoning.

  • P-traps Design

P-traps also have vents in them that let air move through. This stops problems with air pressure and makes drainage better. Modern plumbing systems are better and more reliable when they use P-traps.

P-Trap Vs. S-Trap: Differences by Installation and Location

S-trap and P-trap drain pipe installations are similar, but significant differences exist. S-traps are typically more straightforward and quicker to install due to their simple design, but a vent is required to prevent siphoning. P-traps need a deeper trap arm and discharge, requiring additional fitting. Nonetheless, they offer superior protection against sewer gases and air pressure issues.

Typical locations for S-trap and P-trap drain pipelines are beneath sinks, showers, and other plumbing fixtures. P-traps require more vertical space than S-traps due to their deeper curves, but they are more effective at preventing sewer emissions of gases and fit with modern plumbing codes. S-traps are generally only suitable for low-flow fixtures, and numerous states prohibit their use in newly constructed buildings.

P-Trap Vs. S-Trap: Differences by Working

The main difference between S-trap and P-trap drain pipes is how effectively they stop sewer gas from entering the room. S-traps stop gas using a water seal made by the sharp bend in the pipe, but this design can lead to siphoning and dry traps, which can cause odors and other plumbing issues.

P-traps have a deeper, more gradual bend, making a more prominent water seal and stopping siphoning. This makes them a better way to protect against sewer gas. P-traps also need a way out to let air move and keep air pressure from getting too high. Overall, P-traps work better and are more reliable in modern water systems, while S-traps are only helpful in certain situations and have been prohibited in numerous jurisdictions.

P-Trap Vs. S-Trap: Differences by Black Flow Prevention

The purpose of S-traps and P-traps in drain pipes is to prevent the backflow of wastewater and sewer gases into the room. P-traps are superior to S-traps at preventing backflow because their more profound, gradual bend produces a better water seal. P-traps usually include openings for airflow to reduce pressure and stop any potential for backflow.

Backflow and sewage releases can be problematic with S-traps since they are more susceptible to siphoning and dry traps. P-traps are the most reliable and effective solution when stopping sewage gas smells and backflow.

Finally, the function of both S trap and P trap drain pipes is to prevent sewage gas from entering the home. On the other hand, P traps have a better seal and are less of a challenge to install and maintain than S traps. When doing plumbing work on a new home or remodeling an old one, use P-trap drain pipes.

P-Trap Vs. S-Trap: Differences by Efficiency

P-traps are reliable and efficient in preventing water from leaking out. The performance of four-row and five-row cylinders is nearly the same. It relates to how much water is accessible to them.

In terms of size, this is not the situation. A larger cylinder costs more to produce than a smaller one of the same size. Concerning p-traps, larger diameter pipes are better for collecting water.

Drying out reduces the efficiency of S-traps. That way, gas may freely pass. They are also far more challenging to keep clean. It may be costly to replace the complete trapping system. S-traps can become an ideal environment for unpleasant odor-causing bacteria. P traps are more dependable than S-traps when holding water traps in place.

P-Trap Vs. S-Trap: Differences by Use of a Vent

Thomas Crapper established the U-bend (another term for the P crack) in 1880, over a century after the invention of the S trap in 1775. At this time, there was less concern for health and safety, and the effects of these toxic gases were not completely understood. The use of a vent distinguishes the P trap from the S trap in a significant way.

The P trap lets the waste arm extension reduce the siphoning effect and improve the trap’s reliability. That alone was inadequate.

As a result, it has installed a vent that pulls outside air into the drain as the water drains away. This lowered (or equalized) the pressure within the system, significantly lowering the chance that extra pressure would suck out the essential trap water.

P-Trap Vs. S-Trap: Differences by Plumbing Code Standards

S traps have been gradually phased out of new construction due to their inherent defects in design and risk of drying out, which can cause harmful gases to back up into the new building. In several countries, their installation is prohibited. The P trap is code-compliant and significantly safer than the S-trap.

Straps can still be purchased and are not illegal, but installing one is against the plumbing code; if found, you must replace it with a P-trap or use a mod kid to change it to a P-trap.

P-Trap Vs. S-Trap: Differences by Size

At first glance, it might look like the S trap has a small footprint in your bathroom due to its tight snake-like structure, and a P-trap has that long waste arm coming out from it.

But in fact, it’s the opposite way. Because the S trap requires conserving water in the sink to work, the piping is made larger to accommodate this.

The P-trap doesn’t require this and can get away with holding far less water and, as such, apply thinner piping, exiting a tiny footprint than that of the S trap.

What Are the Risks Associated with P-Traps and S-Traps?

Problems are associated with each trap, including methane gas leaks and other challenges if the pipelines are not installed correctly. This is why it’s so crucial to ensure that the pipes are correct; otherwise, you’ll have significant problems.

P-Trap Problems

When P-traps are improperly installed, they can become damaged and release harmful sewer odors into the home. Incorrectly installed P-traps present several complications.

Dry P-traps can occur if the trap has lost its water seal and waste gas accumulates through the conduit. Slow-moving drains are the source of clogged P-traps. P-traps that are leaking and emit obnoxious odors if left untreated.

S-Trap Problems

An S-trap resembles the letter “S” and was frequently utilized in old plumbing systems. Due to their potential drawbacks, S-traps are now less frequently utilized:

Loss of Water Seal:

S-traps are more prone to losing their water barrier, mainly when the fixture’s outflow is below its inlet. This may be the result of siphoning or backpressure.

Solution: Replace S-traps with P-traps as often as feasible. Consider installing an AAV or a mechanical vent to mitigate the loss of the water seal if replacement is not possible.

Airlock and Gurgling:

S-traps can create airlock issues and cause gurgling noises as the air becomes confined in the trap and impedes drainage.

Solution: Installing a vent conduit that allows for proper air circulation and prevents airlocks can mitigate gurgling and drainage issues.

P-Trap Vs S –Trap: Material

Both kinds of traps serve the same purpose despite their different construction materials. Different homeowners will use different materials for their traps. There are pros and cons to using each material as a p-trap, but ultimately, they all serve the same purpose. One of the following components most likely makes up the s or p-trap in your bathroom.

ABS (Acrylonitrile Butadiene Styrene): 

abs p trap
ABS P-trap

For instance, you can tell whether the p-trap under your sink is constructed of ABS since it is frequently black. Since these plastic pipes disintegrate quickly in direct sunlight, they are often used inside.

Polyvinyl Chloride (PVC):

s trap and p trap abs
S-trap and P-trap abs

When used for plumbing, PVC pipes, a kind of plastic, are typically white. PVC pipes are flexible and simple, making them a popular plumbing material. PVC is a typical material for P-traps.

Brass P-trap

brass p trap and accessories
Brass p trap and accessories

P-traps and other plumbing pipes are made of brass, which has a yellowish-gold color. To be sure, brass pipes aren’t as prevalent as plastic p-traps, but they do exist. Although brass is long-lasting, it’s less popular than galvanized or stainless steel in the plumbing industry.

Galvanized steel

Anti-corrosion properties are a characteristic of this pipe material. It is more expensive than plastic pipes but has a longer lifespan and greater resilience, making it a preferred choice for p-traps.

Polypropylene (PP)

PP p-traps are long-lasting plastic pipes that cost far less than their stainless steel counterparts. This material has the advantage of being lightweight but lacks the durability of polymers like PVC.

S-Trap: Why is it Illegal in New Construction?

  • S-traps are susceptible to siphoning, which can lead to the loss of water closure and the entrance of sewer gas into the structure.
  • S-traps lack ventilation, causing air pressure issues and impeding drainage.
  • Due to their sharp bends, S-traps are more likely to become obstructed with debris, causing blockages and other plumbing problems.
  • S-traps do not meet current plumbing regulations and standards, requiring P-traps or other approved traps.
  • Using S-traps in new construction could result in code violations, unsuccessful inspections, and legal liability for contractors and property owners.

How to Convert an S-Trap to A P-Trap?

Several decades ago, most jurisdictions prohibited using S-traps in new plumbing installations. S-traps discharge excessive water, leaving an air gap in the trap and allowing gas to pass through the system. P-traps, which replaced S-traps, are comparable to S-traps, with the addition of a vent and drain line. Installing a vent pipe within Auto Air Vent (AAV) and a waste arm extension that attaches the trap to the vent pipe and discharge line is required to convert an S-trap to a P-trap. A P-trap conversion comprises two components: exhaust piping and a waste arm extension.

  • The P-trap must be attached to a drain conduit to avoid siphoning. You may utilize an air admittance valve or AAV if there is no adjacent emission stack. An AAV is a one-way valve that permit air to enter but prevents wastewater and gas from escaping. Install the AAV at the top of a small vent pipe near the fixture, ensuring the position is above the drain level.
  • The second step is to add a waste arm extension to the drain line. To prevent siphoning, its length should be two to 2 ½ times the diameter of the pipe to remove the siphoning. For example, a 1 ½ inch tube requires a 3 ¾-inch waste arm extension.
  • When the waste arm and vent conduit have been installed, the P-trap is attached to the fixture’s drain line, followed by the waste arm connection. The waste arm connects to the drain line and vent using a wye connection.
convert an s trap to a p trap
Convert an s trap to a p trap.

P-Trap or S-Trap: Which One Is Better?

Experts recommend the P-trap over the S-trap due to its superior efficacy and consistency in maintaining the trapped water. In addition, the P-trap design is efficient and less susceptible to seal damage or drying out. If you hire a professional plumber, the P-trap will never lose its watertight seal.

What Problem We Solve in this Article?

In this article, we briefly explain the difference between p-trap and s-trap. We also describe the working mechanism of each trap and which one is better for your home, as the safety concern is most important. Moreover, you will also learn how to convert the s trap into a p trap, as the s trap is illegal, and experts do not recommend using it.

Ask For A Water Control Solution Now

We do not only provide basin waste, but we can provide solutions for water control.

Get In Touch Now!

*We respect your confidentiality and all information is protected.

× How can I help you?