Why Does My Drain Smell Like Rotten Eggs?

Quick Answer

A rotten egg smell from your drain is caused by hydrogen sulfide gas. The most common source is a dry P-trap that is no longer blocking sewer gas from entering your home. Running water for 30 seconds usually fixes it. Other causes include bacterial buildup in the drain, a blocked vent pipe, or a failing wax ring on a toilet.

Identify the Source Fast

That smell is hydrogen sulfide gas, and you need to figure out exactly where it's coming from before you can fix it. Here's the diagnostic sequence.

Cause 1: Dry P-Trap

This is the simplest problem and the simplest fix. Every drain in your house has a P-trap -- that U-shaped section of pipe under the sink or in the floor. It holds a small amount of water that acts as a seal against sewer gas. A similar issue can happen with your washing machine smelling like sewage, where a dry or missing P-trap is often the culprit there too.

When a drain goes unused for weeks, that water evaporates. Sewer gas (which contains hydrogen sulfide) flows freely up through the pipe and into your room.

The fix: Run water for 30 seconds. That's it. The trap refills and the seal is restored.

Tip

For drains you rarely use -- basement floor drains, guest bathrooms, utility sinks -- pour a tablespoon of mineral oil into the drain after running water. The oil floats on top of the water in the P-trap and dramatically slows evaporation, keeping the seal intact for months.

Cause 2: Bacterial Buildup in the Drain

Kitchen sinks and shower drains accumulate organic material -- grease, food particles, soap scum, hair. Bacteria feed on this gunk and produce hydrogen sulfide gas as a byproduct.

You'll notice this smell is strongest when you lean close to the drain and it persists even after running water.

The fix:

Pour half a cup of baking soda down the drain.
Follow it with one cup of white vinegar.
Let it fizz for 15 minutes.
Flush with boiling water.

For stubborn buildup, repeat this process three days in a row. If the smell persists after that, use an enzymatic drain cleaner (not a chemical one). Enzymatic cleaners use bacteria to break down organic material without damaging pipes.

Avoid pouring bleach down your drain as a regular habit. It kills the beneficial bacteria in your septic system (if you have one) and can damage older pipes. It also doesn't address the underlying buildup -- it just masks the smell temporarily.

Cause 3: Water Heater Issues

If the rotten egg smell only appears with hot water, the problem is in your water heater, not your drain.

Two common causes:

Magnesium anode rod reaction. Most water heaters have a magnesium anode rod that prevents tank corrosion. When this rod reacts with sulfate-reducing bacteria in the water, it produces hydrogen sulfide. Replacing the magnesium rod with an aluminum or zinc anode rod typically eliminates the smell.

Low temperature setting. Water heaters set below 140 degrees Fahrenheit can harbor sulfate-reducing bacteria. Raising the temperature to 140 degrees kills the bacteria. Be aware that temperatures above 120 degrees increase scalding risk, so consider installing anti-scald valves if you have young children.

Cause 4: Blocked Vent Pipe

Every plumbing system has vent pipes that exit through the roof. They serve two purposes: they allow sewer gas to escape outdoors, and they let air into the system so drains flow properly.

When a vent pipe is blocked -- by a bird's nest, leaves, ice, or even a dead animal -- sewer gas has nowhere to go but back into the house through your drains.

Signs of a blocked vent:

Multiple drains smell simultaneously.
Drains gurgle when other fixtures are used.
Drains are slow even though individual pipes are clear.

The fix: Vent pipe blockages require getting on the roof to inspect and clear the vent opening. If you're comfortable on a roof and can see the blockage, you may be able to clear it with a garden hose or plumber's snake. Otherwise, this is a job for a plumber.

Tip

In cold climates, vent pipes can ice over during prolonged freezing temperatures. If the smell only appears during winter cold snaps, this is likely the cause. Some homeowners install vent pipe extensions or insulated caps to prevent ice formation.

Cause 5: Failing Wax Ring (Toilets Only)

If the smell is coming from around the base of a toilet rather than from a sink or shower drain, the wax ring that seals the toilet to the floor flange may be failing.

Signs include:

Smell concentrated around the toilet base, not the bowl.
The toilet rocks slightly when you sit on it.
Discoloration or water stains on the floor around the toilet base.

Replacing a wax ring costs under $10 for the part but requires removing the entire toilet. It's doable as a DIY project, but it's more involved than the other fixes here.

When to Call a Plumber

Most drain odor problems are straightforward fixes. Call a plumber if:

You've tried all the steps above and the smell persists for more than a week.
Multiple drains are affected and you suspect a vent blockage you can't reach.
You notice sewage backing up into any drain.
The smell is accompanied by slow drains throughout the house, which can indicate a main sewer line issue.