Types of Sink Drain Pipes

Most sinks drain into P-shaped drain pipes.

Sink drain pipes come in several different shapes. Modern homes contain drain pipes in a P shape, called "P-traps." These are the only code-legal sink drain traps in the United States, but older homes may contain obsolete trap designs such as S-traps and drum traps. These traps do not vent correctly and may create sewer smells in a house.

P-Traps

This type of sink drain appears in most modern construction. P-traps allow for better venting than S or drum traps and prevent sewer gases from moving backwards through the drain and out through the sink. P-traps consist of a U-shaped bend and an arm and connect to drain pipes in the wall.

S-Traps

S-traps connect directly to drain pipes at the floor level and cannot vent correctly. These pipes are illegal for remodels and new construction. S-traps notoriously allow sewer gas to pass back into a building if they are located too close to a bathtub, toilet or other large fixture.

Drum Traps

Drum traps have a juglike shape and are usually found under the bathroom floor. The top, capped with a brass hexagonal cleanout plug, may protrude through the floor. Most drum traps work for both the sink and bathtub drain pipes and empty into toilet piping. According to the Ultimate Guide to Plumbing, drum traps are most common in homes built before the 1940s.

References

• Inspectapedia: Plumbing Fixture & Trap Defects -- a Guide to Sewer Gas Smells and Septic Tank Odors
• "Ultimate Guide to Plumbing: Complete Projects for the Home"; Merle Henkenius; 2006