A Homeowner’s Guide to Plumbing Traps: Types, Functions & Care

Plumbing traps, such as P-traps, S-traps, and bottle traps, are curved sections of pipe that hold a small amount of water to create a seal against sewer gases. While P-traps are standard in modern plumbing, S-traps are largely phased out due to a tendency to siphon dry. Other types include gully traps (for collecting waste outside), floor traps (for indoor floor drains), and grease traps (for separating grease from wastewater).

Plumbing traps are one of the most important parts of your home’s drainage system, even though most homeowners rarely think about them. These curved pipe sections, located beneath sinks, tubs, and drains, help prevent sewer gases from entering your home while allowing wastewater to flow safely away.

Understanding how plumbing traps work can help homeowners recognize early signs of drainage issues and maintain healthier plumbing systems.

Plumbing Trap Types 

Several types of traps are common in residential plumbing systems.

P-traps

Most modern homes are equipped with simple P-traps. These traps are U-shaped pipes located underneath home drains that remain filled with water at all times. The water in these traps blocks sewer gases before they can enter your home through your fixture drains. While debris can accumulate in this type of plumbing trap, a well-designed trap should prevent excessive accumulation.

Drum Traps

Some older homes are equipped with drum traps. Drum traps are large, cylindrical devices that hold much more water than P-traps. While these traps do block sewer gases well, they are not installed in new homes built today for several reasons.

First, these traps are not self-scouring, which means that solid matter can build up inside them very easily, leading to frequent clogging. In addition, drum traps are typically installed under a home’s flooring, which can make them difficult to access when they become clogged or develop other problems that need repair.

S-traps

Another plumbing trap type found in some older homes is the S-trap. An S-trap is a simple pipe shaped like the letter S. Unfortunately, this trap type is not as reliable as the P-trap because the water inside of an S-trap can occasionally get siphoned, or sucked, out of it when large amounts of water enter the drain it is connected to. This will allow sewer gases to enter the home until the trap is refilled.

Sewer Traps

A sewer trap (also called a house trap or main trap) is different from the fixture traps under your sinks and tubs. Instead of sitting beneath an individual fixture, it’s installed in the main sewer line where it exits your home, before connecting to the municipal sewer or septic system. Shaped like a U-bend, it holds water to create a seal that blocks sewer gases from traveling back up through the entire drainage system and into your house.

Sewer traps were standard in older homes, especially those built before mid-century plumbing codes required individual fixture traps on every sink, tub, and toilet. Many homes in Tacoma’s older neighborhoods still have one, often with two cleanout openings on either side for clearing blockages.

Are sewer traps still used today?

Most modern plumbing codes no longer require sewer traps, and many plumbers recommend removing or bypassing them during a sewer line replacement or major renovation. Because they narrow the pipe and create a low point where debris, grease, and roots can collect, sewer traps are actually one of the most common causes of main line backups in older homes. The same water seal that blocks gases can also trap solids and slow drainage over time.

Signs your sewer trap may be causing problems:

• Frequent backups or slow drainage across multiple fixtures at once (not just one sink or toilet)
• Gurgling sounds from drains after using the washing machine or dishwasher
• Sewer odors near the trap’s cleanout access, usually in a basement or crawl space

If your home has an older sewer trap and you’re dealing with recurring backups, a sewer camera inspection can confirm whether the trap itself is the bottleneck and whether it makes sense to remove it as part of a broader repair.

Common Trap Problems

Even if your home is equipped with modern, reliable P-traps, these traps can develop problems. In Tacoma, WA, older homes throughout Pierce County may still contain aging or outdated plumbing trap configurations that are more prone to clogging, slow drainage, and sewer odors over time.

Dry Trap

When no water flows through a drain for an extended period of time, water in the trap can evaporate. If your traps dry out, sewer gases can enter the home through the attached drain. Traps that dry out most often include basement floor drain traps, traps in rarely used drains, traps in homes left unoccupied for extended periods of time, and traps with leaky seals that allow water to seep out of them.

To prevent trap dry-out in an occupied home, run water through the home drains that are rarely used periodically.

If you plan to leave your home unoccupied for a month or longer, fill the traps with clean mineral oil before leaving. This mineral oil will not evaporate, as water inside the trap can, and will continue to block sewer gases until you return home and begin running water through the drains again.

If your basement floor drain traps dry out frequently, have a licensed plumber replace your traditional traps with self-sealing interceptors. The seals on these traps continue to block sewer gas entry into your home after the traps dry out.

Finally, keep in mind that traps with leaky seals need repair or replacement to block the entry of sewer gases into the home.

Clogged Trap

A partial trap clog can cause a slow drain, while a full trap clog completely blocks the flow of water down a drain. Since it can be difficult for homeowners to determine whether a drain trap is clogged or the clog is located in another area of the plumbing system, contact a licensed plumber who can locate the clog and remove it without damaging your pipes in the process.

While understanding plumbing traps can help homeowners identify common drainage issues, some plumbing problems require professional inspection and repair.

Spartan Plumbing proudly serves Tacoma, WA, and surrounding Pierce County communities with expert plumbing repair, drain cleaning, and general plumbing services.

Every homeowner should understand what plumbing traps are, how they work, and the problems they can create. Contact the licensed plumbers for all of your drain trap needs today.