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The P-trap, the U-shaped bend in the pipe under your basin, sink, or shower, is an essential barrier against harmful and unpleasant sewer gases. It works by holding a small amount of water, creating a seal that prevents odours from entering your home.
However, when this trap runs dry or becomes clogged with organic debris, it can produce an unpleasant smell that quickly permeates your entire room. Fortunately, fixing and maintaining a smelly P-trap is often a simple, DIY task.
Here, The Bathroom Showroom offers a quick guide to help you identify, fix, and maintain your bathroom's P-trap with ease.

Tools You’ll Need

  • Bucket (for manual cleaning)
  • Wrench/Pliers (for manual cleaning)
  • White vinegar
  • Baking soda
  • Mineral oil (optional, for maintenance)
  • Hot tap water
  • Old toothbrush or bottle brush (for manual cleaning)

Step-by-Step Guide to Fixing a Smelly P-Trap

A smelly P-trap is typically caused by either a broken water seal (dry trap) or by organic material decaying inside the pipe. Here are three of the most common P-trap problems and advice on how to fix them:

Scenario 1: Sewer Gas Smell - Fixing a Dry P-Trap

This is the easiest fix and is common in basins, sinks, or showers that haven't been used in weeks or months.
Step 1: Diagnose the Problem
  • If the drain is seldom used and the smell is strongly like sewer gas or rotten eggs, the water seal has likely evaporated.
Step 2: Refill the Trap
  • Turn on the hot water tap for the affected basin, sink or shower.
  • Let the water run for 1 to 2 minutes. This is usually enough to fully restore the water level and re-establish the seal. (Alternatively, pour about 1/2 gallon of water directly down the drain.)
Step 3: Check for Odour Elimination
  • Wait 30 minutes, then return to the bathroom. If the odour is gone, the problem was a dry trap.
Step 4: Add Evaporation Barrier (Maintenance)
  • For drains you don't use often, pour 1 tablespoon of mineral oil (or unscented baby oil) down the drain. The oil floats on the water and significantly slows down evaporation, extending the seal's life.

Scenario 2: Musty/Decaying Smell - Cleaning a Clogged/Biofilm P-Trap

If the drain is used regularly but still smells musty or sour, there is likely a buildup of biofilm (hair, soap scum, grease) inside the trap.
Step 1: Clear Visible Debris
  • Remove the drain cover.
  • Use a plastic plumbing snake or a bent coat hanger to gently reach into the drain and pull out any visible hair, soap clumps, or sludge near the opening.
Step 2: Apply Baking Soda
  • Pour ½ cup to 1 cup of baking soda directly down the drain.
Step 3: Add Vinegar
  • Slowly pour 1 cup of white vinegar down the drain.
  • Wait: The mixture will fizz vigorously. Let it sit for 15–30 minutes. The chemical reaction helps break down the organic grease and scum clinging to the pipe walls.
Step 4: Flush with Hot Water
  • After the resting time, flush the drain with hot tap water for several minutes to wash away the loosened debris and neutralise the smell. Do not use boiling water if you have PVC pipes, as extreme heat can damage them.

Scenario 3: The Deep Clean - Manual Cleaning for Stubborn Clogs

If the natural cleaners and flushing do not resolve the smell or if the drain is slow, the trap may need to be physically cleaned. This applies mainly to under-sink P-traps.
Step 1: Prepare the Area
  • Turn off the water supply to the area (if possible).
  • Place a bucket directly under the P-trap to catch residual water and debris.
Step 2: Disconnect the Trap
  • Using a wrench or pliers, carefully loosen the slip nuts (or coupling nuts) located on both ends of the P-trap (the U-shaped bend).
  • Unscrew the nuts completely and gently remove the U-shaped pipe.
Step 3: Clean the Pipe
  • Empty the contents of the P-trap into the bucket.
  • Use an old toothbrush, a wire brush, or a flexible bottle brush with dish soap and hot water to thoroughly scrub the entire inner surface of the pipe until all slime and debris are removed.
Step 4: Reassemble and Test
  • Carefully reconnect the P-trap, ensuring the seals (washers) are correctly seated. Tighten the slip nuts firmly, but do not overtighten (which can crack plastic).
  • Run the water for a minute to check for leaks and confirm the seal is holding.

How To Clean A Shower Trap

A quick and simple video showing you how easy it is to clean out your shower trap.

How To Clean a Sink / Basin P-Trap

A quick, effective video that shows you an easy way to locate and clean the P-Trap in your sink or basin.

Frequently Asked Questions (FAQs) on P-Trap Maintenance

1. What is a P-Trap, and why is it so important?
The P-trap is the U-shaped or curved section of pipe located directly under every sink, shower, tub, and floor drain.
Purpose: Its essential function is to hold a small amount of water, which creates a water seal that prevents sewer gases (which can contain harmful gases like hydrogen sulfide and methane) from backing up through the drain and into your home.
2. Why does my P-trap smell like sewage or rotten eggs?
The most common reason is a Dry P-trap. If a fixture isn't used for an extended period (usually 3–5 months), the water in the trap evaporates, breaking the water seal and allowing sewer gas to flow directly into the room.
Secondary Cause: Significant organic build-up (biofilm, hair, soap scum) inside the trap is decaying, producing foul, gassy odours.
3. How do I fix a dry P-trap?
The fix is usually simple: Pour water down the drain. For a sink or shower, run the water for 1 to 2 minutes, or pour about a half-gallon down the drain to restore the water seal.
Maintenance Tip: For infrequently used drains (such as guest bathrooms or basement floor drains), pour a cup of water down each drain at least once a month to prevent evaporation.
4. How can I keep the water from evaporating so quickly?
After refilling the trap with water, pour about 1 tablespoon of mineral oil (or unscented baby oil) down the drain. The oil is lighter than water and floats on top, creating a barrier that slows down water evaporation.
5. What should I use to clean a smelly P-trap?
Natural solutions are often the safest and most effective for odor-causing biofilm:
Pour 1 cup of baking soda down the drain.
Follow with 1 cup of white vinegar.
Let it fizz and sit for 10–30 minutes to break down the grime and neutralize odors.
Flush with hot tap water.
Caution: Avoid mixing different chemical drain cleaners, as this can create dangerous gases.
6. Do I ever need to manually clean the P-trap?
Yes, manual cleaning is necessary if the drain is slow or consistently smelly due to a severe hair or grease clog.
  • Place a bucket under the trap.
  • Carefully use a wrench or pliers to loosen the slip nuts holding the trap in place.
  • Remove the U-shaped pipe and manually clear the accumulated debris.
7. What if the P-trap is full, but the smell persists?
If the water seal is clearly present and the drain is clean, the issue is likely related to the larger plumbing system:
Vent Pipe Blockage: A clogged plumbing vent (usually on the roof) can cause negative pressure, allowing sewer gas to be forced into the home.
Leaky/Damaged Pipe: There could be a leak or crack in a drainpipe farther down the line, or a failed seal (such as a toilet wax ring).
Action: At this point, you should call a licensed plumber to professionally diagnose the venting or pipe damage.

P-Trap Maintenance: Preventing Sewer Gas and Knowing When to Call a Plumber

By following our helpful guidance, you can fix any issues with your shower, basin, or sink to ensure a fully functional space.
Regular P-trap maintenance, especially refilling the trap in rarely used areas and periodically flushing with water, is the best way to prevent sewer gases from invading your living space.
If you've cleaned the trap and refilled the water seal but the smell persists, the problem may lie deeper in your venting system, requiring a professional plumber's inspection.​
Why not check out our Help & Advice section for further guidance on tackling common DIY bathroom jobs? From further information on How to Clean a Smelly Shower Drain, to Why Does My Shower Drain Smell Like Eggs? we can help you promote a healthier space for everyone in the home to enjoy.
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With more than 290 showrooms and several decades of experience, The Bathroom Showroom can help you create your dream bathroom with ease. Book an appointment today to learn more about the fantastic services and the huge range of bathroom products we offer.