You walk into the bathroom or kitchen and catch a whiff of something unpleasant coming from the drain. It might smell like rotten eggs, sewage, or just something musty and stale. Drain odors are more than just unpleasant — they can indicate a plumbing issue that needs attention. Here's what causes them and how to fix them.
Cause #1: Dry P-Trap
This is the most common cause of drain odor and the easiest to fix. The P-trap is the U-shaped section of pipe underneath every sink, tub, and shower drain. It holds a small amount of water at all times, and this water acts as a seal that blocks sewer gases from rising up through the drain into your home.
When a drain isn't used for a while — a guest bathroom, a utility sink in the garage, a basement floor drain — the water in the P-trap evaporates. Once it's gone, sewer gas has a direct path into your living space.
The fix: Run water in every drain in your house for 30 seconds, including ones you don't use often. This refills the P-trap. For floor drains you rarely use, pour a cup of water down them once a month. Some homeowners pour a thin layer of mineral oil on top of the water in rarely-used P-traps — the oil slows evaporation and keeps the seal intact longer.
Cause #2: Biofilm Buildup
Biofilm is the slimy, often black coating that forms inside drain pipes over time. It's a colony of bacteria feeding on soap residue, toothpaste, hair, skin cells, and other organic material that goes down the drain. Biofilm itself has a musty, unpleasant smell — and as bacteria break down organic material, they produce hydrogen sulfide gas, which smells like rotten eggs.
Biofilm is most common in bathroom sink drains and shower drains because of the constant flow of soap, shampoo, and organic material. Kitchen sinks develop it too, especially if grease goes down the drain regularly.
The fix: Remove the drain stopper or cover and clean it — biofilm often coats the underside of the stopper. Then pour a mixture of 1/2 cup baking soda followed by 1/2 cup white vinegar down the drain. Let it fizz for 15 minutes, then flush with boiling water. For stubborn biofilm, use an enzyme-based drain cleaner (like Bio-Clean or Green Gobbler) that eats organic material without damaging pipes. Repeat monthly as prevention.
Important: Avoid chemical drain cleaners like Drano for odor problems. They're designed to dissolve clogs, not biofilm, and they can corrode older pipes — especially the galvanized and cast iron pipes common in older Central Texas homes.
Cause #3: Sewer Gas Leak
If the smell is persistent, strong, and smells distinctly like sewage (not just musty), sewer gas may be entering your home through a path other than a dry P-trap. Common entry points include cracked or broken drain pipes in the wall or under the slab, loose or failed connections between drain pipes, a broken or missing wax ring at the base of a toilet, and a damaged sewer cleanout cap in the yard.
Sewer gas contains methane, hydrogen sulfide, and ammonia. In small concentrations it's just unpleasant. In large concentrations it can cause headaches, nausea, and in extreme cases is a health and fire hazard.
The fix: This requires a professional. A plumber can perform a smoke test — where non-toxic smoke is pumped into the drain system — to identify exactly where the gas is escaping. Once the leak point is found, the repair is usually straightforward: tightening a connection, replacing a wax ring, or patching a cracked pipe.
Cause #4: Blocked Plumbing Vent
Every drain in your home is connected to a vent pipe that runs up through the roof. These vents allow air to enter the drain system so water flows freely (like releasing the finger on top of a straw). They also vent sewer gases up and out above your roofline.
If a vent pipe gets blocked — by leaves, bird nests, ice (less of a concern in Central Texas), or even a dead animal — sewer gas can't escape through the roof and instead backs up into the house through drains. You might also notice slow draining or gurgling sounds when water goes down the drain.
The fix: Vent pipe blockages need professional clearing. The plumber may snake the vent from the roof or use a camera to identify the blockage. If the vent pipe is damaged or improperly installed, it may need repair or replacement.
Cause #5: Garbage Disposal Buildup
If the smell is coming specifically from your kitchen sink and you have a garbage disposal, food debris may be stuck inside the disposal or in the splash guard (the rubber baffle at the drain opening). The disposal grinds food but doesn't always flush everything out — bits of food rot in the crevices and smell.
The fix: With the disposal off, use a flashlight to look inside and remove any visible debris with tongs (never put your hand in a disposal). Clean the splash guard by pulling it up and scrubbing both sides with dish soap and an old toothbrush. Then grind ice cubes and a quartered lemon in the disposal with cold water running — this cleans the grinding chamber and leaves it smelling fresh.
When to Call a Plumber
If the smell persists after refilling P-traps and cleaning drains, or if you notice the smell throughout the house (not just at one drain), it's time to call a professional. Persistent sewer gas usually points to a cracked pipe, blocked vent, or failed seal that won't resolve with DIY cleaning.
At Kimco, we diagnose drain odors with camera inspections and smoke testing. Our drain cleaning and sewer line services are flat-rate — you know the price before work begins. We serve homeowners in Lexington, Giddings, Taylor, Rockdale, Bastrop, Elgin, and communities across Central Texas. Call (737) 260-7255.
Quick Reference: Drain Smell Diagnosis
Smell only from rarely-used drains: Dry P-trap — run water for 30 seconds. Musty smell from bathroom/kitchen drains: Biofilm — clean with baking soda and vinegar. Strong sewage smell from one drain: Possible cracked pipe or failed wax ring — call a plumber. Sewage smell throughout the house: Blocked vent or broken sewer pipe — call a plumber immediately. Rotten egg smell only from hot water: Likely your water heater's anode rod — see our post on water heater odors.