A sewage smell in the house is sewer gas getting past a barrier that should stop it. It is unpleasant and, in concentration, unhealthy. Here is where it comes from.
1. A dry trap
Every drain has a U-shaped trap that holds water to block sewer gas. A floor drain, guest shower or basement sink that goes unused dries out. Pour a quart of water in and the smell is gone in an hour.
2. A failed toilet seal
If the smell is strongest at the toilet, or the toilet rocks, the wax ring has failed. That is a quick toilet repair.
3. A blocked or broken vent
Vents carry sewer gas out through the roof. A nest, ice or a cracked pipe in the wall sends it indoors. Gurgling drains are a clue.
4. A cracked drain pipe
Old cast iron splits along the top where you cannot see it. Smell in the basement with no visible water is typical.
5. A sewer line problem
A partial blockage or a break in the lateral pushes gas back. Slow drains and smell together call for a sewer camera inspection and possibly sewer line repair.
6. A backup on the way
Smell plus water at a floor drain is the start of a sewer backup. Stop using water and call.
How a plumber finds it
A plumbing inspection checks traps, seals and vents. For hidden leaks in the drain system, a smoke test fills the pipes with harmless visible smoke that escapes wherever sewer gas does.
One caution: a rotten egg smell near a gas appliance may be natural gas, not sewage. If in doubt, leave the house and call the gas utility, then see gas line repair.
Common questions
Is sewer gas dangerous?
In the small amounts that cause household odors it mostly causes headaches and nausea, but it should be fixed. High concentrations are hazardous.
Why does the smell get worse when it rains?
Changes in air pressure and a rising water level in the sewer push gas back through weak points such as dry traps and cracked pipes.