What Causes Persistent Sewer Smell Inside a House?
Last updated October 19, 2026
The short answer
Dried-Out Plumbing Traps
The most frequent cause is a dry P-trap. This U-shaped pipe under sinks, showers, and floor drains holds water to block gases from the sewer line. If a fixture is rarely used, the water can evaporate, breaking the seal.
To fix a dry trap, simply run water down the drain for a minute to refill it. For floor drains in a basement or garage that see little use, pouring a quart of water down them every few months maintains the seal. If the smell returns quickly after refilling, the problem is likely elsewhere in your plumbing system.
Vent Pipe Issues
Your home's plumbing system has vent pipes that exit through the roof to equalize air pressure and allow sewer gases to escape outdoors. If these vents become blocked by leaves, bird nests, or ice, the gases can be forced back down into your home. A broken or disconnected vent pipe inside a wall will also release odors directly into living spaces.
Locating a vent blockage often requires a visual inspection of the roof vents. You can check for obvious debris, but repairing a broken pipe inside a wall is a job for a plumber. Persistent smells that worsen when you run multiple fixtures strongly suggest a venting problem that needs professional diagnosis.
Sewer Line or Seal Breaches
Cracks or breaks in the main sewer line under your home or yard can allow gases to seep into the soil and then into your basement or crawl space. Similarly, worn wax rings sealing toilets to the floor drain can leak gas. Old, corroded cast iron pipes are especially prone to developing holes that release odors before a full clog or backup occurs.
A plumber can perform a camera inspection to pinpoint the exact location and nature of the damage. This repair is critical, as a broken line can also lead to significant water intrusion and contamination under the home.
- Cracked main sewer line under the slab or yard
- Failed wax seal beneath a toilet
- Loose or damaged cleanout plug
- Degraded pipe connections in crawl spaces
- Holes in old cast iron or clay pipes
- Improperly installed or shifted plumbing fittings
Ejector Pump or Specialty Fixture Problems
Homes with basements or bathrooms below the main sewer line use an ejector pump in a sealed basin to lift wastewater. If the pump's seal fails, the lid is loose, or the vent check valve is stuck, sewer gas can escape.
Listen for unusual pump cycling or check for odors near the pump basin. The seal around the lid should be intact and tight. For any complex pump or below-grade plumbing issue, consult a plumber familiar with these systems. Attempting repairs without proper knowledge can lead to exposure to raw sewage and significant health risks.
- Failed seal on ejector pump basin lid
- Malfunctioning vent valve on pump discharge line
- Dry or missing trap on a floor drain in utility room
- Cracked sump pump basin allowing gas seepage
- Improperly vented basement bathroom or laundry
- Decomposing debris in a seldom-cleaned pump basin
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Call Now (469) 209-4452Frequently asked questions
Is sewer gas smell dangerous to my health?
Yes, sewer gas contains methane, which is flammable, and hydrogen sulfide, which is toxic. Low levels can cause headaches, dizziness, and eye irritation. Prolonged exposure or high concentrations are serious health hazards. If you feel unwell, get fresh air and see a doctor. Always treat the smell as a warning to find and fix the source promptly.
Could the smell be coming from my washing machine?
Yes, washing machines have a drain hose and a standpipe trap that can dry out if not used regularly. Mold and debris in the machine's drum or drain pump filter can also cause odors. Running a hot wash cycle with vinegar can help, and checking the drain hose installation maintains a water seal.
Will my homeowners insurance cover repairs for a sewer smell?
Coverage depends on the cause. Sudden, accidental damage like a pipe bursting may be covered. Damage from wear and tear, like a slowly deteriorating pipe, typically is not. You must file a claim for your insurer to make a determination. Policies vary, and your insurer decides what is covered. Always check your policy details and document the issue.