P-traps vs. S-traps: What’s the Difference?

Drain traps like P-traps and S-traps keep sewer gases from entering your home or building by trapping a small amount of water in the drain line. This water keeps sewer gases and pests from entering the plumbing system as well. There are several types of drain traps: floor drain traps, gully traps, bottle traps, and grease traps. The most common are S-traps and P-traps.

What is an S-trap?

S-traps are a type of plumbing trap that blocks sewer gases in sinks and other plumbing fixtures. The S-shaped pipe configuration traps wastewater before it drains to the sewer line, creating a water seal that prevents gases from flowing up through the drain. P-traps were common in the early 20th century, though they are no longer in use. When a large amount of water flows through the S-trap, it can create too much suction and pull water out of the trap and leave it dry. The dry trap creates an air gap, drawing sewer gases into the building.

S-traps are no longer allowed in modern plumbing code. Most, if not all, jurisdictions in the United States have banned the use of S-traps in new construction, and any S-traps in older buildings are to be replaced by P-traps.

What is a P-trap?

A P-trap is a U-shaped piping configuration, similar to an S-trap. The drain collects water in the U portion, creating a barrier that prevents sewer gases from entering the building. The addition of a vented pipe prevents siphoning and the waste arm extension helps reduce suction in the drain.

P-traps are approved for all new plumbing installations. They don’t totally eliminate the chance of a dry trap, but there is less chance of this occurring with a P-trap than an S-trap. If a fixture isn’t used often, you may need to run water through it to refill the trap.

How to convert an S-trap to a P-trap

Most jurisdictions banned the S-trap from new plumbing installations several decades ago. S-traps drain too much water which leaves an air gap in the trap thereby allowing gasses to flow through the system. P-traps, which replaced S-traps, have a similar configuration with the addition of a vent and drain line. Converting an S-trap to a P-trap involves installing a vent pipe within AAV and a waste arm extension that connects the trap to the vent pipe and drain line. There are two parts to a P-trap conversion: vent piping and a waste arm extension.

  • The P-trap must tie into a vent pipe to prevent siphoning. If there isn’t a vent stack nearby, you can use an air admittance valve, or AAV. An AAV is a one-way valve that vents air in but doesn’t let sewer gas out. Install the AAV at the top of a short vent pipe near the fixture and be sure to place it above the level of the fixture’s drain.
  • The second step is adding a waste arm extension to the drain line. Its length needs to be two to 2 ½ times the diameter of the pipe to eliminate siphoning. For example, a 1 ½ inch pipe needs at least a 3 ¾-inch waste arm extension.
  • Once the waste arm and vent pipe are installed, the P-trap is connected to the fixture’s drain line, then to the waste arm connection. The waste arm connects to the vent and drain line via a wye connection.

MT Copeland offers video-based online classes that give you a foundation in construction fundamentals with real-world applications, like plumbing basics. Classes include professionally produced videos taught by practicing craftspeople, and supplementary downloads like quizzes, blueprints, and other materials to help you master the skills.


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Mary-Anne Bowcott

Mary-Anne Bowcott is a residential and commercial plumber, certified gas fitter, and owner of Westcom Plumbing and Gas Ltd. She received the 2016 and 2018 Sooke Region Chamber of Commerce Trades and Skilled Labour Excellence Award and the British Columbia Construction Industry Leadership Award in 2018. She was named Canadian Construction Association’s Person of the Year in 2019. A champion of women in the building trades, Mary-Anne mentors tradeswomen to help promote positive change within the industry. When she’s not running her business or tackling plumbing and roofing projects, you can find her rock climbing, exploring the outdoors, surfing, or crushing a CrossFit workout.

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