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

S Trap vs P Trap – Understanding drain configurations and functions, such as s-traps vs. p-traps, is relatively simple, but technical aspects of each drain type may be a little more difficult.

An s-trap gets its name from its general shape, which resembles a “S.” A p-trap looks like a sideways “P” as well. Both types of traps have the same goal in mind: to create a water barrier between your home and the sewer.

P-traps were created to solve the main problem with s-traps: they tend to syphon away too much waste water, leaving the drain “dry.” Unwanted gases can enter the home through a dry trap.

Some sewer gases, but not all, have a foul odour. Some are odourless, while others are hazardous. Smelly gases, such as methane, are inconvenient. Carbon monoxide, for example, is odourless but dangerous.

S-traps and p-traps both create a water barrier that keeps all gases out of the house. Read in depth about difference between S Trap vs P Trap.

What Are P-Traps and What Do They Do?

To eliminate the problem of syphoning, p-traps replaced s-traps, especially under sinks. There are two main features of a p-trap that prevent syphoning.

A vented pipe is the first option. A p-trap is a vent that is typically located inside a wall and vented to the outside air through the roof. Because negative pressure or the “sucking” action required for syphoning does not occur when air pressures inside the drain are balanced, syphoning becomes much less likely.

Second, a drain side extension is added to the trap. Gravity’s ability to “pull” water through the pipe is greatly reduced by the extended pipe. A waste arm extension is the name for that piece of pipe.

To eliminate the possibility of syphoning, a common calculation used to determine the required length of the waste arm extension is simple: the extension must be 2-1/2 times the diameter of the pipe.
However, the water in a p-trap can still evaporate. As a result, there is no “ideal” configuration that can completely eliminate the risk of a dry trap.

If you have a sink that is rarely used, evaporation can result in a dry trap. Regular plumbing maintenance, as well as running water for a few seconds down the drain once in a while, can help to eliminate this possibility.

What Is The Function Of An S-Trap?

S-traps were a common drain configuration in the early twentieth century. They were also commonplace. An “S” shaped drain (under a sink or tub) was installed, which typically diverted slightly and went down into a drain line under the floor.

An s-trap drain configuration may be sufficient for certain applications. However, there are times when too much water flows through an s-trap, leaving it nearly empty. An air gap is left in the trap, allowing gases to escape. This is referred to as a dry trap. S-traps were made illegal in new construction several decades ago due to the dangers of dry traps.

S Trap vs P Trap

S-trap to p-trap conversion kits are available, but custom configurations are frequently required. The plumbing departments of most big-box construction stores are fairly well-equipped, allowing you to “build your own.”

Make certain you purchase a high-quality Air Admittance Valve. All three AAV products – Oatey Sure-Vent, StudorVent, and Sioux Chief Turbo Vent – are of high quality.

You will need to create a sub-stack for the AAV to sit on.

To prevent overflow or backup, the top of the sub-stack should extend to an elevation above the sink drain.

You can easily reconfigure nearly any drain in a single afternoon once you have all of the necessary parts. The entire conversion is completed within the cabinet beneath the sink.

Simply follow the guidelines for converting an s-trap to a p-trap (waste arm extension length and venting requirements), and the rest is a matter of getting it done quickly and looking halfway decent.

It is important to remember that the goal of this article is to simply introduce you to the differences between s-traps and p-traps. It may not contain all of the information you require to convert an s-trap. Although, if an s-trap conversion is your goal, I believe I covered most, if not all, of the details you need to know.

Er. Md. Shahin Akhter

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Md Shahin Akhter is a Civil Engineer from MIT and has more than 5 years of experience in Civil Engineering and Construction Department. Md Shahin Akhter is Civil Engineer in Nagar Parishad (Municipal Council), Supaul. He is Webmaster and Founder at ProCivilEngineer.com Civil Engineering Website.