Why Does Sewage Smell Come and Go in a House?

Every drain under a sink, tub, or floor drain has a curved pipe called a P-trap that holds water. That water acts as a seal, blocking sewer gas from rising into the room. If the trap dries out, odor slips through. This often happens in bathrooms or laundry rooms that are rarely used.

When you run water in that fixture, the seal refills and the smell disappears. But if the trap dries out again, the odor returns. You can test this by pouring a cup of water down each drain and waiting a day. If the smell fades, a dry trap is the likely culprit.

• Check floor drains in utility rooms and crawl spaces
• Run water in guest bathrooms weekly
• Pour water into unused drains every few weeks
• Look for cracks in the P-trap itself
• Consider a trap seal primer if the problem repeats

A crack or leak in the sewer line under your slab or in the yard can release gas into the soil. That gas may seep into your home through cracks in the foundation or around pipes. The smell might come and go as the ground shifts or as water moves through the line.

A partial blockage in the sewer line can also cause pressure to build up, forcing gas out through the nearest fixture. Gurgling sounds from drains often accompany this. If you hear gurgling or notice slow drains, the sewer line may need a professional inspection with a camera.

If the smell persists after you refill traps and check for obvious blockages, it is time to call a plumber or water damage professional. They can inspect the vent, camera the sewer line, and pinpoint the source. Do not try to open a sewage line yourself; that is dangerous and can spread contamination.

If you already have sewage water on the floor or walls, do not touch it. Sewage contains bacteria and viruses. Keep pets and children away, turn off the water if you can safely, and call the pros. They have the equipment to clean and disinfect the area properly.