Why Your Drains Smell Bad (And How to Fix It)

That sewer smell isn't something you have to live with. Here's what's causing it.

I'm Justin Sims, and "my drains smell terrible" is one of the most common calls I get. It's also one of the most fixable.

That foul odor coming from your sink, shower, or floor drain isn't just unpleasant—it's your plumbing telling you something's not right. Let me explain what's probably happening and how to fix it.

The Most Common Cause: Dry P-Trap

This is the culprit about 50% of the time, and it's the easiest to fix.

What's a P-Trap?

Look under any sink and you'll see a curved pipe—it looks like a "P" on its side (or sometimes a "U"). This is the P-trap.

The trap holds a small amount of water that creates a seal. This seal prevents sewer gases from rising up through the drain into your home.

Why It Dries Out

If a drain isn't used regularly, the water in the trap evaporates. No water = no seal = sewer smell.

Common culprits:

• Guest bathroom sinks or showers
• Floor drains in basements or utility rooms
• Rarely-used bar sinks
• Seasonal home drains

The Fix

Run water in the drain for 30 seconds. That's it. The trap refills, the seal is restored, smell gone.

**For prevention:** Run water in unused drains every couple of weeks.

**Pro tip:** Pour a tablespoon of vegetable oil after filling the trap. It floats on the water and slows evaporation.

Cause #2: Biofilm Buildup

That black, slimy stuff you see when you remove a drain cover? That's biofilm—a combination of bacteria, soap scum, hair, toothpaste, and other organic matter.

Why It Smells

Biofilm is alive. It's a bacterial colony that feeds on the gunk in your drains. And like all living things, it produces waste. That waste smells.

Where It Collects

• Bathroom sink drains (toothpaste, soap)
• Shower drains (hair, body oils, shampoo)
• Kitchen sink drains (food particles, grease)

The Fix

Manual cleaning:

1. Remove the drain cover or stopper
2. Pull out any visible gunk (use needle-nose pliers or a drain tool)
3. Scrub the visible drain surfaces with a brush

Baking soda and vinegar:

1. Pour 1/2 cup baking soda down the drain
2. Follow with 1/2 cup white vinegar
3. Let it fizz for 15 minutes
4. Flush with hot water

Enzyme drain cleaner:

Enzyme-based cleaners (not chemical drain cleaners) use beneficial bacteria to eat the biofilm. Use monthly for maintenance.

**What NOT to use:** Bleach and chemical drain cleaners. They kill bacteria short-term but don't remove the biofilm. Plus they can damage pipes.

Cause #3: Venting Problems

Every drain in your home connects to a vent system—pipes that go up through your roof. These vents:

• Allow air into the system so water drains properly
• Let sewer gases escape outside

When Vents Fail

If a vent is blocked or damaged, sewer gases can't escape. They build up pressure and push back through the water in your P-traps.

Signs of venting issues:

• Multiple drains smell bad
• Gurgling sounds when drains empty
• Slow drains throughout the house
• Smell is worse on windy days

The Fix

Vent problems usually require a plumber. We check:

• Roof vents for blockages (leaves, bird nests, debris)
• Vent pipe connections in the attic
• Vent sizing and configuration

Sometimes the fix is as simple as clearing a clogged vent. Sometimes it requires adding or resizing vents.

Cause #4: Garbage Disposal Funk

If the smell is coming from your kitchen sink, the garbage disposal might be the problem.

Why It Happens

Food particles get stuck under the splash guard, on the impeller walls, and in crevices. They rot. It smells.

The Fix

Ice and salt cleaning:

1. Dump a tray of ice cubes into the disposal
2. Add 1/2 cup of coarse salt
3. Run the disposal with cold water for 30 seconds

Citrus freshening:

1. Cut a lemon or orange into chunks
2. Run through the disposal with cold water

Deep clean:

1. Unplug or turn off the disposal
2. Scrub under the splash guard with a brush
3. Use baking soda and vinegar treatment

Cause #5: Sewer Line Issues

If the smell persists after trying everything else, you might have a bigger problem.

Possible Issues

• **Cracked sewer line:** Allows gases to seep up
• **Partial blockage:** Creates gas buildup
• **Tree root intrusion:** Damages pipe integrity
• **Broken wax ring (toilet):** Gases escape around the base

Warning Signs

• Persistent smell despite cleaning and trap filling
• Smell is strongest in one area
• Multiple drains affected
• Slow drainage throughout the house
• Sewage smell in the yard

The Fix

This requires professional diagnosis. We use camera inspection to see what's happening inside your sewer line and determine the appropriate repair.

Cause #6: Washing Machine Drain

That front-load washer smell isn't always the washer—sometimes it's the drain.

Why It Happens

• P-trap in the standpipe dries out between loads
• Biofilm builds up from soap and fabric residue
• Drain line has partial clog

The Fix

• Run a cleaning cycle monthly with washer cleaner
• Clean the drain line with enzyme cleaner
• Ensure the standpipe trap doesn't dry out

Prevention: Keeping Drains Fresh