P Trap Vs. S Trap: What Is the Key Difference

In this blog post, you’ll read:This post of p trap vs s trap which also known as drain traps prevent the passage of sewer gases into a building or house. These capture a quantity of water within the drain line. Prevalent variations include traps and p traps. Both of these are vital for the inhabitants' health and hygiene. To maintain hygienic conditions in a house, installing plumbing traps of the suitable type is critical.

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This post of p trap vs s trap, also known as drain traps, prevents the passage of sewer gases into a building or house. These capture a quantity of water within the drain line. Prevalent variations include traps and p traps.

Both of these are vital for the inhabitants’ health and hygiene. To maintain hygienic conditions in a house, installing plumbing traps of the suitable type is critical.

This article explains the similarities and distinctions between s trap vs p trap plumbing.

What Is P Trap? 

PVC p trap works in basin waste
PVC p trap works in basin waste

Do you know why it is called a p trap? A P trap is a type of plumbing trap having a U-bend. It contains a long and short arm; despite its “U-bend” shape, the bend resembles the letter “J.”

The sink’s tailpiece connects to the long arm, while the short arm attaches to a horizontal arm fixed to the wall. Proper trap configuration removes the chance of obstruction, backflow, and foul odors. All appliance and fixture designs permit P-traps.

What Is S Trap? 

S trap pvc
S trap pvc

Plumbing s trap are the most basic type of trap. Established in 1775, this is the first trap ever designed.

The name S traps comes from its shape, resembling the letter S. The U-shaped section of the trap connects to the tailpiece. However, the reversed U-shaped section connects the trap to a floor drain.

This type of trap functioned because of the reduced water pressure. Higher water flow can create a vacuum, causing the trap to drain.

A dry trap cannot seal shut and prevent wastewater releases from entering a house. People consider them ineffective and prohibit their use in modern plumbing systems.

With one exception, most toilets feature built S-traps that connect to the floor drain rather than a wall extension.

Difference Between P Trap Vs. S Trap: Key Differences

Now let’s differentiate s-trap vs p-trap by elaborating on their key points

Code Compliance

Adhering to legal codes is a main consideration when installing a plumbing system.

S traps are prohibited throughout the United States by the International Plumbing Code. They have been banned since 2006 because of the siphonage hazard. It could cause the water seal to escape the conduit.

With this, S traps have not disappeared. Older residences still contain them, especially those with unvented plumbing systems.

The traps on these drainage networks’ floor are linked to the primary drain pipeline. Thus, this setup would make it impossible to install a P-trap.

Home inspections do not permit s traps because regulations prohibit them. If the house fails the inspection and you intend to sell, you must upgrade the plumbing system.

P Trap Vs S Trap: Installation

It is much easier to install a P trap, and it doesn’t cost much, either. Maintaining all connections and preventing leakage, however, requires proper installation.

Thus, it is preferable to hire qualified engineers to complete the task. With their experience, they can install plumbing in a way that ensures its durability.

When installing a chromium P trap, you need a pipe wrench to connect pipes securely. This results in a more complex installation process.

Most plumbing codes no longer permit the installation of S-traps because they do not follow them.

Siphonage Risk

Siphonage risk
Siphonage risk

A common issue with S-traps is siphonage. The flow rate of the effluent through the system is the primary cause.

Up to 80 PSI of water pressure is typical in modern homes. Because of advanced pumping systems, the water pressure was higher than in 1775. Many homes didn’t have running water back then.

S-traps experience siphonage due to a lack of design to handle high pressure and volume.

P-traps exhibit resilience towards higher water pressure and volume. It has a flow-restraining horizontal arm.


P and S traps are identical in size and serve similar purposes. However, in more complex environments, both varieties increase in size.

Kitchen sink plumbing traps are usually 1.5 inches wide, while bathroom sink traps are 1.25 inches wide. They are identical in dimension and intended for a similar location.

P Trap Vs S Trap: Configuration

P trap vs s trap configuration
P trap vs. s trap configuration

The S trap connects the sewer line beneath the floor to the underground area beneath the sink or toilet. It keeps water in the pipes to stop sewer gases from entering the house through the shower or sink drains.

Traps in toilets ensure the bowl is filled to a specified level in contrast.

An S-trap drain has a drawback: it can siphon empty if water drains away.

A P trap adheres to a complex installation procedure and maintains a secure water seal to prevent easy siphon drying.

P-Trap Vs. S-Trap: Applications

The trap for the kitchen sink has a drying issue, so it cannot be used in the kitchen sink. Sink p traps prevent harmful substances from entering the kitchen sink. You can recover weighty objects deposited into the P trap by turning off the water.

In the case of toilets, a P-trap is once more favored because of its ability to seal water. When using an S trap, the distance between the floor and the sewage pipe should be 140-165 mm by modern standards.

1.5 inches in diameter is the optimal measurement for a bathtub trap and drain, no matter the bathtub size. Because of its precise dimensions, you can attach a P-trap to the pipe without extra tools.

Plumbing S Trap Vs. P Trap: Drain Connection 

The drain that these traps need is another crucial distinction.

P-traps are connectors that end at the wall. This leftover piece is linked to the main sewer line through a wye or elbow.

Water flow is slowed by the minor incline of the arm connecting the trap to the wall stub. Slow, consistent flow and venting pipes regulate the air pressure within the system.

By connecting S-traps to a floor drain. The water’s velocity is increased by gravity. The efficiency is reduced since these systems do not have venting.


P-traps are efficient and reliable at preventing water loss. Four-row and five-row cylinders exhibit identical performance. It relates to the quantity of water at their disposal.

We are not dealing with an issue of this size here. Producing a larger cylinder costs more than a smaller cylinder of equal dimensions. About p-traps, pipes with a more significant diameter are effective at gathering water.

As they dry out, S-traps become less effective. This would allow gas to pass readily. They are also more challenging to maintain. The whole trapping system may be expensive to replace.

S-traps have the potential to foster the growth of malodorous microorganisms. P traps exhibit more reliability in securing water traps compared to S traps.

P Vs S Trap: How to Convert S Trap to P Trap

Between s trap and p trap
Betweens trap and p trap.

Most jurisdictions prohibited shock traps for new plumbing installations decades ago. S-traps help gases move through the system by creating an air gap. This happens when too much water is drained from the trap.

P-traps, which succeeded S-traps, have a design comparable to the vent and discharge lines. To transform an S-trap into a P-trap, a waste arm extension and a vent pipe within the AAV are required to connect the trap to the discharge line and vent pipe. A P-trap transformation involves pipework and an extension of the waste arm.

  • You must connect a discharge conduit to the P-trap to impede siphoning. You can use an air admittance valve(AAV) without an adjacent exhaust stack. An AAV is a unidirectional valve that permits the entry of wastewater gas but inhibits air exit.
  • The next stage involves attaching an extension of the waste arm to the drainage pipe.
  • Set the AAV above the level of the fixture’s drain at the apex of a short vent pipe located near the fixture. Its length must be two to two and a half times the pipe’s diameter to prevent siphoning.
  • After installing the waste arm and vent pipe, connect the P-trap to the fixture’s discharge line and the waste arm connection. For instance, a 1 ½-inch pipe requires a minimum waste arm extension of 3 3/4 inches.

Why Is the Use of S-Traps Banned in New Construction? 

S-traps can let in smelly gas and lose water if siphoning occurs, allowing wastewater gas to enter buildings.

  • S-traps are susceptible to blockages and other plumbing complications because of their abrupt bends and lack of ventilation. It leads to air pressure issues and drainage obstructions.
  • Current plumbing regulations and standards do not conform to S-traps and need using P-traps or other approved traps.

Using S-traps in new buildings can cause legal problems and failed inspections, creating issues for contractors and property owners. To prevent these problems, we must avoid using S-traps. Failure to follow regulations can result in penalties and delays in the construction process. Using P-traps instead of S-traps is recommended to ensure compliance with building codes.

S Vs P Trap: Which One Is Better

Experts suggest using the P-trap instead of the S-trap because it traps water. Furthermore, the P-trap design is practical and less prone to seal deterioration or drying up. If you engage a professional plumber, the P-trap will always maintain its impermeable seal.

What Is the P Trap Alternative?

If space is limited or you prefer a different type of trap, however, a bottle trap, shallow trap, waterless valve, or S trap will suffice. Alternatives to P-traps include bottle traps, shallow traps, and waterless valves.

Wrap Up with S and P Trap

In this post of s trap vs p trap, both s and p traps are structurally identical, with one distinction in their function. Although they do the same function, S-traps frequently fail. Thus, residential settings forbid their use. Installing P-traps requires systems to adhere to building codes and complete an inspection.

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