Sink or Tub Drain Smell Like a Sewer?

Advisory Report - December 2000

Sink or Tub Drain Smell Like a Sewer?
With the holidays just around the corner, you're likely to have company drop by your home, perhaps even a few surprise guests.  Will your friends and relatives be overcome with offensive odors emanating from your sink or drain?

The trap keeps sewer gas at bay by "trapping" water in the U-shaped portion of the pipe, forming a water seal.  The water blocks the movement of sewer gas and prevents it from flowing up through the trap, the sink and into the room.

The sewer-like odor or gurgling sound is most commonly experienced with S-traps (right), as opposed to P-traps.  S-traps, even though they are no longer permitted to be used in new construction, are prevalent in older homes.  The problem with an S-trap is that the water seal may be lost due to siphonage.  The action of the water discharging into the drainpipe removes the air from that pipe and thereby causes a negative pressure, or vacuum, in the drain line, resulting in siphonage of the water from the trap.  Once the water seal is lost, sewer gas can flow freely into the room.

Siphonage is unlikely when a P-trap (right) is used.  The horizontal portion of the trap is connected to a plumbing vent pipe, which extends through the roof, so there is always atmospheric pressure on the downward side of the drain.

Learn more about plumbing traps.