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That nauseating smell of rotten eggs wafting up from your shower drain isn't just unpleasant; it's a critical signal. This odour is caused by hydrogen sulfide gas (H2S). While H2S is non-toxic in small amounts, it's a component of sewer gas. The presence of this sulfuric smell indicates a breakdown in your plumbing system's defenses or a specific bacterial issue.
Unlike a musty smell that suggests simple mildew, the strong rotten egg odour requires a targeted diagnostic approach, as the source could be inside the drain, in the sewer line, or even in your hot water system.
Here, The Bathroom Showroom offers a quick, informative piece on why your shower drain is emitting a rotten smell and how to solve the problem.

The Sulfur Stink: The Three Sources of Hydrogen Sulfide Odour

The smell is caused by H2S gas, produced either by anaerobic bacteria (bacteria that thrive without oxygen) or by the direct infiltration of sewer gas. Diagnosing the source depends on when you notice the smell:

1. The Constant Culprit: Plumbing System Failure (Dry P-Trap)

If the smell is present all the time, regardless of whether the water is running, the water barrier separating your home from the sewer line has failed.
  • The Problem: The P-Trap water seal has evaporated, allowing raw sewer gas to flow directly into your bathroom. This is common in drains that haven't been used in several weeks.
  • The Fix: Simply run the shower or pour a gallon of water down the drain to restore the water seal. For long-term prevention, add a tablespoon of mineral oil.

2. The Intermittent Culprit: Biofilm Breakdown

If the smell is noticeable primarily when you first turn on the water or when the drain is slow, the source is likely bacterial decay in the drainpipe.
  • The Problem: Anaerobic bacteria are thriving in the dark, low-oxygen environment of accumulated hair, soap scum, and oils (biofilm). As these bacteria consume the organic matter, their waste products include gas.
  • The Fix: You must physically remove the clog and chemically clean the biofilm. Use a drain snake to remove hair, then flush with baking soda and vinegar to scrub the pipe walls and kill odour-causing bacteria.

3. The Hot Water Culprit: The Water Heater

If the smell only appears when you run hot water, but not when you run cold water, the problem is not in the drainpipe—it's in your water heater tank.
  • The Problem: The magnesium or aluminum anode rod inside the water heater (designed to prevent the tank from rusting) can react with naturally occurring sulfates in the water. This reaction produces H2S gas, which is released as hot water flows.
  • The Fix: Call an HVAC or plumbing professional to drain and flush the tank. They may recommend replacing the old rod with a special zinc or aluminum-zinc alloy rod, which resists this reaction.

Step-by-Step Troubleshooting Flow For Shower Drain Problems

1. Run the Water: Pour a gallon of water down the drain. Did the smell stop?
YES: The problem was a Dry P-trap. Regular maintenance (monthly running water) is required.
NO: Proceed to Step 2.
2. Check the Temperature: Run hot water for 30 seconds, then cold water for 30 seconds. Does the smell only happen with hot water?
YES: The problem is the Water Heater Anode Rod. Call a professional plumber or technician.
NO: Proceed to Step 3.
3. Deep Clean: Use a drain snake to physically remove the hair clog, then immediately follow up with a baking soda and vinegar flush. Did the smell stop permanently?
YES: The problem was biofilm and bacterial buildup. Repeat the natural cleaning monthly.
NO: Proceed to Step 4.
4. Call the Pro: If the constant smell persists after cleaning and refilling the trap, the problem could be a Vent Pipe Blockage or a broken sewer line that's releasing gas. This requires professional diagnosis and repair.

Why Does My Shower Drain Smell So Bad?

This quick video highlights the main reason why shower drains can smell so bad, pinpointing the most common causes.

Rotten Egg Smell: DIY Fixes, When to Worry, and When to Call the Professionals

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While the rotten egg smell is the most alarming of all drain odours, the vast majority of cases are resolved by simply refilling the P-trap seal or performing a thorough deep cleaning to eliminate H2S-producing bacteria.
However, because H2S is a component of sewer gas, never ignore a persistent odour. If your three-step DIY troubleshooting fails, prioritise calling a qualified plumber to ensure your home's ventilation and sewer connections are intact.​
Why not check out our Help & Advice section for further guidance on tackling common DIY bathroom jobs? From fixing a smelly P-trap to naturally cleaning a smelly shower drain , we can help you promote a healthy space that everyone in the home can enjoy using. 
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.