Sewer Gas Smell in House: Causes and Fixes
Sewer gas contains hydrogen sulfide, methane, ammonia, and other compounds that produce the distinctive rotten egg odor. Beyond being unpleasant, sewer gas can cause headaches, nausea, and dizziness at low concentrations, and methane is flammable in higher concentrations. Finding and fixing the source promptly is important for both comfort and safety.
Step 1: Check for Dried-Out Drain Traps
Every drain in your house has a P-trap or S-trap, a curved section of pipe that holds a small amount of water. This water creates a seal that blocks sewer gas from traveling up through the drain into your living space. When a drain goes unused for several weeks or months, the water in the trap evaporates, breaking the seal and allowing gas to enter.
Walk through every room in your house and run water for 30 seconds in every drain, including bathroom sinks and tubs in guest rooms, basement floor drains, laundry sinks, utility room drains, and any drain you do not use regularly. The most commonly dried traps are floor drains in basements and garages, guest bathroom fixtures, and laundry standpipes.
If the smell disappears within an hour of refilling all traps, the dried trap was the cause. For drains that go unused for long periods, pour a few tablespoons of mineral oil into the drain after filling the trap. The oil floats on top of the water and significantly slows evaporation, keeping the trap sealed for months.
Step 2: Inspect Toilet Wax Seals
The wax ring (or wax-free seal) between the toilet base and the floor flange creates an airtight and watertight connection. When this seal fails, sewer gas escapes around the base of the toilet, and you may also see water seeping onto the floor during or after flushing.
Check each toilet by looking for water stains around the base, feeling for moisture at the floor joint, and gently pushing the toilet side to side to see if it rocks. A toilet that rocks has a broken seal. The wax ring itself costs $5 to $30, and professional replacement (including removing and resetting the toilet) costs $150 to $300. If you are comfortable with basic plumbing, this is a manageable DIY project that takes about an hour.
Step 3: Check the Plumbing Vent Stack
Your plumbing vent system is a network of pipes that extend from the drain system up through the roof. The vent allows air into the drain system to equalize pressure, which is what enables water to flow smoothly through the pipes. If the vent is blocked, draining water creates suction that pulls water out of the traps, breaking the sewer gas seal.
If you notice gurgling sounds from drains, slow drainage in multiple fixtures, or the water level in toilet bowls fluctuating, a blocked vent is a likely cause. Common vent blockages include bird nests, leaves, dead animals, and ice during cold weather. Inspect the vent opening on your roof visually, and if a blockage is visible, remove it carefully. If the blockage is deeper in the vent stack, a plumber can clear it with a snake or water jet ($150 to $400).
Step 4: Look for Cracked or Loose Drain Connections
Inspect all visible drain pipes for cracks, gaps, or loose connections. Check under every sink, in the basement or crawl space where drain pipes are exposed, and around any cleanout fittings. A cracked pipe, a slip joint that has loosened, or a cleanout cap that is missing or not fully tightened can all allow sewer gas to escape into the living space.
Common locations for gas leaks include the joints in PVC drain pipes under sinks (where the slip nuts may have loosened), cleanout plugs in the basement floor or wall that are cracked or missing, and connections where new drain work ties into old pipe of a different material. Tightening a loose connection or replacing a cracked fitting is typically a $50 to $200 repair.
Step 5: Schedule a Sewer Camera Inspection
If the first four steps do not identify the source of the sewer gas smell, the leak is likely in the buried portion of the drain system or the sewer lateral. A break, crack, or separated joint in the underground pipe allows sewer gas to escape into the surrounding soil. The gas then migrates through the soil and enters the house through foundation cracks, utility penetrations, or the slab itself.
A sewer camera inspection ($200 to $500) can identify breaks and separations in the lateral that are releasing gas. If the camera finds a damaged section, the repair options include spot repair ($1,000 to $4,000) for a localized break, CIPP lining ($5,000 to $15,000) for multiple joint separations, or full replacement if the damage is extensive.
When Sewer Gas Smell Indicates a Serious Problem
In most cases, sewer gas smell is caused by a simple dried trap or loose fitting. However, certain patterns indicate a more serious underlying problem.
If the smell persists after refilling all traps and checking all visible connections, it likely originates from the buried portion of the plumbing system. If the smell is strongest in the basement, especially near the foundation walls, gas may be entering through soil around a damaged exterior sewer line. If you also notice slow drains, recurring backups, or yard symptoms (lush patches, depressions), the sewer lateral itself is likely compromised.
Persistent sewer gas smell combined with other sewer line warning signs should prompt a camera inspection sooner rather than later. The same pipe damage that releases gas today can cause a full backup or structural soil erosion tomorrow.
Start with the simplest fix: run water in every unused drain to refill dried traps. If that does not solve it, check toilet seals, vent stacks, and visible drain connections. If the smell persists, a camera inspection of the sewer line will identify breaks or separations in the buried pipe that are releasing gas.