What’s That Smell? Understanding Sewer Odors and P-Traps

A young child with curly hair is pinching their nose, likely reacting to something unpleasant or funny.

January 19, 2026

Have you ever noticed an unpleasant sewer smell coming from a sink, shower, or floor drain in your home or business? While it can be alarming, the cause is often simple—and preventable. One of the most common reasons for sewer odors is a dry or missing P-trap.

Diagram of a functioning P-trap showing water flow from a sink to the sewer, preventing gas and odors from entering.

What Is a P-Trap?

A P-trap is the curved section of pipe located beneath sinks, floor drains, showers and other plumbing fixtures. Its purpose is simple but important: it holds a small amount of water that acts as a barrier, preventing sewer gases from traveling back into your home or business.

Why Sewer Odors Happen

When a drain is not used often, the water in the P-trap can slowly evaporate. Once the trap dries out, sewer gases can escape into indoor spaces and cause unpleasant odors.

This can happen when:

The image illustrates a dry P-trap, showing how water flows from a sink to the sewer while preventing sewer gas odors.
  • A sink, shower, or floor drain isn’t used regularly, 
  • A home or building has been vacant for an extended period, 
  • Hot or dry weather causes faster evaporation.

Common Places to Check

Dry P-traps are most often found in:

  • Guest bathroom sinks or showers
  • Basement or garage floor drains
  • Laundry room drains
  • Utility sinks

Easy Ways to Prevent Sewer Odors

The good news is that preventing sewer odors is usually quick and easy:

  • Run water in all sinks, showers, and floor drains at least once a month.
  • Pour a cup or two of water into seldom-used drains to refill the P-trap.
  • Mineral oil can be used in seldom-used P-traps to prevent the water seal from drying out. The oil forms a layer on top of the water, significantly slowing evaporation and creating a long-lasting seal against odors. Mineral oil is preferred over cooking oils because it does not go rancid and cause its own foul odor.
    • To use mineral oil:
      1. Ensure the Trap is Wet: First, fill the fixture or floor drain with enough water to create the seal.
      2.  Add the Oil: Slowly pour about half a cup to a few ounces of mineral oil into the drain.
      3. Wait for the Oil to Settle: The oil will float on top of the water, forming a protective barrier against evaporation.

When to Call a Professional

Property owners are responsible for the plumbing inside their homes and businesses, including P-traps. If odors persist after refilling drains with water, there may be another issue such as a damaged trap, blocked vent, or plumbing leak. In these cases, a licensed plumber can help identify and resolve the problem.

What About Sewer Odors Outside?  

The South Davis Sewer District is responsible for maintaining the public sewer system. If you smell sewer odors outside or believe the odor issue you’re experiencing may be related to the public sewer system, please submit a report here.

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