Plumbing Drain Trap Basics

by | Jan 6, 2023 | Repairs and Updates | 0 comments

Drain Trap Types

Your Barrie Inspections home inspection includes inspection of the plumbing system, including checking for improper drain traps, leaks, and poor connections. There are roughly 14 types of traps in existence, including the Q, P, S, Gully, floor, bottle, running, drum, as well as many others.  “P-Trap”s and “S-Trap”s are the most common seen in residential sink applications during inspections. Here are some Plumbing Drain Trap Basics.

Today we will discuss P and S Traps. Both of these drain traps work by using wastewater to create a barrier in your drainpipe, between the sink and the sewer piping. The barrier creates a seal that prevents unwanted sewer gases from coming back up through the drain and into your home or business. When a trap is dry from lack of use or siphoning, noxious sewer gas and odors area able travel through the drainpipe. Gases like methane, hydrogen sulfide, and ammonia are present in sewer lines. Because these gases can be both smelly and dangerous, a barrier like a drain trap is necessary.

Plumbing drain piping diagram from Barrie Inspections in Gonzales Texas shows the difference of an S-Trap vs P-Trap in Plumbing drain trap basics

What is an “S-Trap”?

This type of trap is generally shaped like the letter “S”, positioned sideways. The trap does not support the goal to effectively keep sewer gas out of the home. The “S-Trap” is prohibited under the Uniform Plumbing Code throughout the United States. This is because an “S-Trap” does not maintain a water seal between the fixture and the trap and as such, “S-Trap” can siphon or suck water out from the trap as wastewater flows through the piping. This will end up releasing the foul sewer odors/gases into the home.

What is a “P-Trap”?

A “P-Trap” has two main features that stop siphoning of wastewater;

The first is a vented pipe- A “P-Trap” is vented typically inside a wall and through the roof to the outside air, or to an Air Admittance Valve (AAV).  When air pressures are balanced inside the drain, siphoning becomes far less likely since negative pressure (sucking action) is needed for siphoning.

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

One Caveat on the “P-Trap” should be noted. It’s important to keep in mind that the water inside a “P-Trap” can still evaporate. There is no perfect configuration that can completely eliminate the possibility of a dry drain trap. If you have a sink that is rarely used, a dry drain trap can happen due to evaporation. Basic plumbing maintenance can help ensure this doesn’t happen. Just run water for a few seconds into the drain can eliminate this drying out.

Changing from an “S Trap” to a “P Trap”

There are several way to convert from an “S-Trap” to “P-Trap”, including conversion kits. Often custom configurations are necessary. Most big-box construction stores have well-equipped plumbing departments which allow you to obtain the necessary components.

Barrie Inspections Real Estate Inspections describes a plumbing drain pipe conversion from S-Trap to P-Trap in Victoria Texas

Using a good quality Air Admittance Valve (AAV).

  • 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. 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.

Installing a “P-Trap” to replace the “S-Trap”

  • You will need a complete “P-Trap” set and a 90deg elbow
  • Once you remove the old “S-Trap”, you can install the “P-Trap” into position on the tailpiece
  • Install the elbow on the waste arm, facing downward and adjust the piping in place
  • Tighten all connections to ensure no leaks are present.

Proper Drain Pipe Type

When replacing leaking/damaged piping, or when replacing an “S-trap”, be sure to use the proper piping type. Real Estate Inspectors routinely come across corrugated (accordion) pipe under sinks. These are prone to clogging, and are listed as a defect in the plumbing drain sections of reports. Do it right the first time and avoid taking the corrugated pipe route! Below is a product called “Snappy Trap”, that is acceptable for use because it has a smooth inner surface rather than a jagged surface. Professional plumbers will not generally use this product, but it is code compliant. If you choose to not use all smooth pipe and figure out complicated angles, use of this product is a good alternative.

Barrie Inspections recommends use of Snappy Trap instead of corrugated drain piping.

As always, Barrie Inspections recommends a licensed plumber handle all your plumbing needs where necessary. Some cities require licensed trades to pull permits to perform work, while others and more rural areas do not.

Barrie Inspections is ready to handle all your real estate inspection needs! We have the experience and expertise to give you the best value and the most detailed inspection! Call 361-298-0472, email James@BarrieInspections.com, or schedule on our website 24/7 HERE. Barrie Inspections is here to to help you! If you’re interested in seeing a sample of these reports from actual Barrie Inspections client inspections, please visit HERE.