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Common Causes of Drain Odors: Key Considerations for Clean Restrooms

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Common Causes of Drain Odors: Key Considerations for Clean Restrooms

Have you ever walked into a restroom only to be hit by a musty, foul odor that instantly makes you want to turn around and leave? It’s more common than you might think—and it’s more than just an unpleasant experience. 

Persistent restroom odors can reflect poorly on your business, drive away customers, and make employees uncomfortable. But the good news is: these odors are entirely preventable. With a bit of knowledge and the right maintenance steps, you can eliminate restroom drain odors for good.

In this comprehensive article, we’ll walk you through everything you need to know about the most common causes of restroom drain odors and, more importantly, how to get rid of them and prevent them from coming back.

How to Eliminate Drain Odors in Restrooms

Restroom drain odors are typically caused by issues within the drain system itself. The source is usually organic buildup, stagnant water, dry P-traps, or even a problem with your building’s ventilation system.

Let’s go over exactly how to eliminate them. This isn’t just about cleaning a drain once—this is a process of correction, disinfection, and ongoing maintenance.

Here’s how to do it.

Materials Needed

Tools and Equipment Needed

Ensure Safety First

Always begin any cleaning or maintenance procedure by putting on the necessary personal protective equipment. For drain cleaning, safety goggles, disposable gloves, long sleeved clothing, and closed-toe shoes, and shoe covers should be worn to protect yourself.

Place “Restroom Closed” signs outside the restroom entrance to prevent anyone from entering during the cleaning process. Unauthorized entry could result in accidental exposure to cleaning chemicals, slips and falls and other hazards.

Now let’s dive into the most common causes of drain odors.

Common Causes of Drain Odors

Here are some common causes of drain odors to look out for:

  • P-Trap Has Dried Out
  • Biofilm and Organic Buildup
  • Debris in P-Trap
  • Water is Draining Slowly
  • Sewer Gas Leak
  • Poor Ventilation

Pro Tip: Need a preventative maintenance schedule for cleaning and maintaining your restroom drains? Check out our full guide.

Issue: P-Trap Has Dried Out

The P-trap is the curved section of pipe found beneath sinks, floor drains, and other plumbing fixtures. It’s designed to hold water, forming a physical barrier that prevents sewer gases from traveling up through the drain and into the building. 

If you’re dealing with a foul odor from a drain that doesn’t get used often (such as a floor drain in a restroom corner), chances are the P-trap has dried out. This is one of the most common causes of drain odors and can happen even when preventative measures are in place. Without water in the trap, sewer gases have a clear path into the bathroom.

Solution: Check and Refill the P-Trap

Materials, Tools, and Equipment Needed:

Pour a gallon of water down the drain to refill the P-trap.

Pro Tip: Use cold or lukewarm water to refill the P-trap. Avoid using boiling water because water above 175° F can melt the joints of PVC pipes. 

For toilet drains, simply flush the toilet to refill the P-trap.

To slow evaporation and extend the life of the water barrier, add two to three ounces of mineral oil on top of the water. The oil forms a thin layer that prevents the water from evaporating too quickly.

This is one of the simplest and most effective ways to immediately reduce odors from infrequently used drains.

Check for dry P-traps weekly as part of your routine maintenance.

Issue: Biofilm and Organic Buildup

Over time, bacteria, grime, and organic material can accumulate inside the drainpipe. This buildup, also known as biofilm, thrives in warm, damp environments like restroom drains. As the bacteria break down the organic matter, they produce gases and odors that can be very offensive. Without regular cleaning, biofilm can become a persistent problem.

Solution: Use an Enzymatic Cleaner

Materials, Tools, and Equipment Needed:

Enzymatic cleaners are your best friend in the fight against biofilm and organic buildup. These products contain live bacteria and enzymes that feed on the grime inside your pipes, breaking it down naturally and eliminating the source of the odor.

  1. Pour the enzymatic cleaner directly into the drain.
  2. Follow the product instructions for proper dosage and dwell time.
  3. Avoid using hot water or chemical disinfectants immediately after, as they can kill the beneficial bacteria.

For best results, use enzymatic cleaners on a weekly basis as part of your routine maintenance.

Issue: Debris in P-Trap

If enough biofilm builds up, it can start to restrict waterflow. The curve of the trap can also catch debris like hair, dirt, and other small objects, preventing them from entering the main plumbing system and causing major clogs.

Solution: Flush the Drain with Hot Water and Drain Cleaner

Materials, Tools, and Equipment Needed:

Sometimes your drain needs a deeper cleaning. Flushing with a combination of hot water and a safe commercial drain cleaner can help dissolve grease, soap scum, and organic waste that feeds bacteria.

Make sure your selected drain cleaner is compatible with your plumbing before using.

Follow these steps:
  1. Heat 1–2 gallons of water to 140° F. Avoid boiling the water as this can damage PVC piping.
  2. Add a commercial drain cleaning solution (again, make sure it’s compatible with your plumbing).
  3. Slowly pour the mixture down the drain.
  4. Let it sit for 15–20 minutes.
  5. Flush with clean hot water to rinse away loosened debris.

Repeat this process monthly or more often as needed to prevent buildup.

Issue: Water is Draining Slowly

Slow-moving water is a breeding ground for odor-causing bacteria. When drains are partially clogged, water and organic material don’t flush away efficiently. Hair, soap scum, and other debris can accumulate in the P-trap or drain line, feeding bacteria and worsening the smell.

Solution: Inspect for Clogs and Slow Drainage

Materials, Tools, and Equipment Needed:

If you notice water pooling or draining slowly, it’s time to inspect the drain for clogs. Even small blockages can lead to major odor problems over time.

Use these tools and equipment to remove the clog:

Check monthly or more often as needed to prevent clogs before they can happen.

If the problem persists or the drain is completely blocked, you may need to call a professional plumber for an inspection and deep cleaning.

Issue: Sewer Gas Leak

Cracks in pipes or failing seals around the P-trap can allow sewer gas to leak into the restroom.

This gas contains methane and hydrogen sulfide—both of which smell terrible and can be hazardous to your health.

Solution: Call a Professional

If odors persist even after cleaning, it may be a structural or plumbing issue. Contact a licensed plumber to inspect the system and replace broken seals or damaged pipes.

Issue: Poor Ventilation

Even if your drains are in perfect shape, bad ventilation can make odors linger in the restroom. Without consistent airflow, the bathroom can trap and amplify smells, turning minor issues into major complaints.

Solution: Improve Ventilation

Proper ventilation is critical for odor control, especially in high-traffic or humid environments.

Ventilation Tips:
  • Ensure exhaust fans are functioning and running consistently.
  • Check that air vents are unobstructed.
  • Consider upgrading to a higher-capacity exhaust system if necessary.

Improved airflow not only eliminates odors but also reduces humidity, which can help slow down bacterial growth in drains.

We recommend a monthly check to make sure fans and vents are working properly and clean.

Preventative Maintenance

A preventative maintenance schedule is your best defense against recurring restroom odors. When done consistently, it not only keeps the space fresh but also reduces plumbing issues and extends the life of your fixtures.

Need a preventative maintenance schedule for cleaning and maintaining your restroom drains? Check out our full guide.

Final Thoughts 

A fresh-smelling restroom isn’t a luxury—it’s a necessity. Whether you manage a commercial facility, operate a busy restaurant, or maintain public restrooms, tackling drain odors proactively shows professionalism and care for those who use your space.

By understanding the causes of drain odors and following a routine maintenance plan—including filling P-traps, using enzymatic cleaners, flushing with hot water, checking for clogs, and ensuring good ventilation—you can eliminate odors for good.

Say goodbye to embarrassing smells, negative reviews, and complaints. With a little effort and consistency, your restroom can always offer a clean, fresh, and welcoming experience.

Need help implementing a maintenance plan or choosing the right cleaning products?

Reach out to our team of Imperial Dade experts for more restroom cleaning and facility maintenance tips!

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