When your bathroom sink smells like sewer, the cause is usually one of three things: a dried-out P-trap, a clogged plumbing vent, or bacteria building up in the drain. Running water refills a dry trap, a deep clean clears the bacteria, and a blocked vent needs a plumber.
A sewer-like odor can point to a serious issue, so find the source fast. Formica Plumbing & Sewer Co. is a third-generation, family-owned plumber that has served Wickliffe, OH since 1960. Here’s what’s behind the smell and how to stop it.
Key Points
- A dried-out P-trap is a common reason a sink smells like sewer, and running water through the drain refills it.
- A clogged plumbing vent pushes sewer gas back through the sink and often causes gurgling or slow drains.
- Biofilm, a black slime of bacteria and drain waste, grows on the stopper and inside the drain and gives off a foul smell.
- Hot water, a sink cleaner, and a bottle brush remove biofilm; vent problems need a licensed plumber.
What Makes a Bathroom Sink Smell Like Sewer Gas?
Three problems cause most sewer smells from a bathroom sink: a dried-out P-trap, a clogged vent, and built-up bacteria.
A Dried-Out P-Trap
The P-trap is the U-shaped pipe under your bathroom sink. It holds water that forms a barrier against sewer odors coming up from the drain. When the water level in the P-trap drops too low, sewer gas comes straight into your bathroom.
A dry P-trap is most common in guest bathrooms and in homes left empty for an extended time. A P-trap that keeps drying out even with regular use usually points to a slow leak in the trap or a vent problem pulling the water out.
A Clogged Plumbing Vent
Every home plumbing system needs vents that lead outside and release sewer odors away from the house. When a plumbing vent clogs, the smell has nowhere to go but back through your sink and into your bathroom. Dust, leaves, and other debris that land on your roof are the usual causes of vent clogs.
Built-Up Bacteria (Biofilm)
Biofilm is a black slime of bacteria and drain waste that grows inside sink drains. It is a common reason a bathroom sink smells like sewer even when the P-trap is full. Your bathroom sink handles hand washing, shaving, and teeth brushing, and all of that waste feeds bacteria.
To check for biofilm, unscrew the sink stopper and lift it out. Black slime or gunk on the stopper means biofilm is growing in the drain. Check the overflow hole near the rim of the basin too. Soap scum and bacteria collect inside that channel and give off the same sour smell.
My Sink Smells Like Sewer. How Do I Fix It?
When your sink smells like sewer, match the fix to the cause: refill a dry P-trap, call a plumber for a clogged vent, and deep clean the drain to remove biofilm.
Refill a Dry P-Trap
Running water into the drain is the fastest fix for a dried-out P-trap. The water flows into the trap and restores the seal that keeps sewer gas out. In a guest bathroom nobody uses, run the faucet now and then to keep the trap full.
If the smell continues after you refill the trap, check for vent problems, and clean out the sink, you need a professional plumber to find the cause. A whole-home plumbing inspection checks the trap, vent, and drain lines together.
Have a Plumber Clear a Blocked Vent
If your sink smells like sewer because of a blocked vent, you need a plumber. Besides the smell, watch for water that gurgles as it goes down the drain or drains slower than usual. Clearing vent pipes yourself can expose you to toxic gases. Sewer gas can contain hydrogen sulfide, and OSHA’s hydrogen sulfide guidance explains why that gas is dangerous to breathe.
Deep Clean Biofilm From Your Sink
If you find biofilm in your bathroom sink drain, a deep clean takes care of it. Follow these steps:
- Turn on your hot water faucet and let it run for two minutes.
- Spray the sink with a sink-specific cleaner and pour some down the drain.
- Scrub the inside of the drain pipe with a bottle-brush scrubber, rinsing every so often.
- Keep scrubbing until the brush comes out clean. That means you’ve removed all or most of the biofilm.
If your sink still smells like sewer after a deep clean, our professional drain cleaning service clears buildup a brush can’t reach.
How Can I Make My Bathroom Smell Better?
Bathrooms will smell from time to time, and odors can linger after the cause is gone. These habits keep your bathroom smelling fresh.
Keep Your Bathroom Clean
A weekly deep clean improves your bathroom’s smell overall. Cover these tasks:
- Clean the bathtub and wipe down the shower liner
- Scrub the toilet inside and out
- Clean the sink handles and bowl
- Sweep and mop the floor
- Wipe down the counters
- Empty the trash
Let Towels and Bath Mats Dry Properly
Wet towels left in a heap invite mold and mildew, which can create a sour smell within a few hours. A wet bath mat traps water that feeds mildew or mold underneath it. Hang towels and mats so they dry out completely.
Use Scented Bathroom Trash Bags
Bathroom trash rarely holds food waste, but scented trash bags still help keep the room smelling pleasant.
Burn Candles or Use Wall Plug-Ins
Candles and wall plug-ins make your bathroom more pleasant for everyone who uses it. Air freshening cones and oil burners work too. For odors as they happen, keep a spray air freshener handy.ighting candles or using wall plug-ins. Other methods of creating a better smelling room could include placing air freshening cones or oil burners. To address bad smells as they occur, a sprayable air freshener can make a difference.
Sewer Smell From a Bathroom Sink: Common Questions
Is sewer gas from a sink dangerous?
Sewer gas from a sink is more than unpleasant. It can contain hydrogen sulfide and other gases you shouldn’t breathe for long. A faint smell from a dry P-trap usually clears once you run water. A strong or lasting odor needs attention, so if the smell persists after you refill the trap and clean the drain, have a licensed plumber check the vent and drain lines.
Why does only one sink smell like sewer?
When only one sink smells like sewer, the problem is usually at that fixture: a dry P-trap, biofilm in the drain, or gunk in the overflow hole. When several drains smell at once, the cause more often sits further down the line, such as a blocked vent or a sewer line problem. Our sewer line repair team can trace a house-wide odor to its source.
Why does my sink smell like sewer after I’ve been away?
The water in a P-trap evaporates when a sink sits unused. After a long trip, or in a guest bathroom nobody uses, the trap can dry out enough to let sewer gas through. Run the faucet when you get home, and the smell should fade as the seal refills. If the sink smells like sewer a day later, the cause is more likely biofilm or a vent problem.
Keep Your Bathroom in Its Best Condition With Formica Plumbing & Sewer Co.
Searching for the answer to “Why does my bathroom sink smell like a sewer?” often starts with a bad smell and ends with a deeper plumbing issue. Some of these problems have simple fixes. Others need a professional plumber.
Formica Plumbing & Sewer Co. has served Wickliffe, OH since 1960, and our family still stands behind every job. Our sink repair and installation service keeps your bathroom sink working and smelling the way it should. If your sink smells like sewer and the simple fixes don’t help, call the office or contact our team today to schedule your visit, and we’ll find the source of the smell and explain your options in plain terms.