Where does a sewer smell come from?
It's sewer gas getting past one of the seals that keep it out of the house. Your drains connect to the sewer or a septic tank, so that gas is always in the pipes, and 3 things keep it out:
- Traps. The curved pipe under almost every drain (the trap) holds a little water that blocks the gas.
- Toilet seals. Most toilets sit on a seal over the drain in the floor.
- Vents. Pipes through the roof let gas out and air in, so traps don't get pulled dry.
When one of those fails, or a pipe cracks, you smell it.
Could a dry drain be the cause?
Yes, and it's the one we'd check first, because it's free and fast. A trap that sits unused can dry out in weeks or months, depending on the humidity. Guest baths, a shower nobody uses, laundry sinks and floor drains are the usual spots.
Run water in each rarely used drain for a minute, and pour about a quart into each floor drain. If the smell fades within a day or so, that was likely it. Running them every few weeks usually keeps it away.
One catch: noses get used to a smell fast. Have someone who's been out of the house sniff when they walk in.
Why does just one sink smell?
Often because the drain itself is dirty. Hair, soap and toothpaste leave a slimy film on the stopper and in the pipe. Bacteria live in it, and it can smell like sewer even with a full trap.
If a chemical drain cleaner went down that sink recently, leave it alone and call. Otherwise, put on gloves, pull the stopper if it lifts out, and clean it. Scrub what you can reach in the drain opening and the overflow.
If the smell hangs on, the buildup is farther down or the line is partly clogged. Some sinks also have a one-way vent valve underneath (an air admittance valve) that can stick open. That's the point to call.
Why does it smell around the toilet?
Most likely, the seal under the toilet (the wax ring) has failed. It's often a ring of actual wax. Low tech, and fine until the toilet starts to move.
Easy check: grab the bowl with both hands and try to rock it. If it moves, the seal is suspect, and every wobble works it looser.
The fix is resetting the toilet on a new seal. If you're handy, that's a fair DIY job. If the floor under it feels soft, stop and call. The floor needs fixing first, or the new seal won't hold either.
Why do my drains gurgle and smell?
Gurgling plus a smell points to a blocked vent or a partial clog. If leaves or a nest block the roof vent, draining water has to pull air from somewhere else. It pulls that air through the nearest trap, and the water goes with it. Then gas gets in.
A partial clog can do the same, and the buildup inside smells on its own. A smell that shows up on windy days, or right after the washer drains, fits either one.
Finding out which one it is comes first. Don't climb up on the roof to check the vent yourself. It's not worth the fall.
Could it be a cracked pipe or the sewer line?
Yes, especially if it's strongest in the crawl space, near one wall or out in the yard. Drain pipes in walls and under the house crack, and joints pull apart. In the yard, tree roots and shifting soil break sewer lines. Any of those lets sewage and gas out.
Over a crawl space, a leak can go unnoticed until the smell reaches the rooms above. Several drains backing up at once, or a soggy, extra-green strip of yard, points to the sewer line.
Before anyone digs, we'd want a camera run through the line. It shows what's wrong and where, so nobody guesses with your yard. A camera inspection is the first step in sewer line repair.
What if the smell is only in the hot water?
Then it's probably your water heater, not your drains. A rotten egg smell in the air near a gas water heater or furnace is different. Treat that as gas and get out.
To be sure, fill a glass from the tap that smells and sniff it in another room. If the glass smells, it's the water.
Most tanks have a magnesium rod inside that corrodes so the tank doesn't (the anode rod). Sulfate in the water reacts with that rod, often helped by bacteria in the warm tank. That makes hydrogen sulfide, the rotten egg smell.
A plumber can disinfect and flush the tank and swap in a rod the manufacturer approves, often aluminum. Don't just pull the rod out. The smell may go, but so does the tank's rust protection, and it can void the warranty. We check the anode rod during water heater maintenance.
If hot and cold both smell, suspect the water supply. Call your water utility, or on a well, have it tested.