Diagnosis

Sewer Gas Smell in the House: Dry Traps to Broken Pipes

By San Diego Sewer Repair · October 23, 2026 · 8 min read

A sewer gas smell in the house is one of the few sewer problems where the cheap explanation is usually the right one. Most of the time it's a trap that's dried out or a wax ring that's gone, and the fix costs nothing or nearly nothing. Sometimes it's a cracked cast iron drain under the slab, and that's a different conversation. The trick is to work through the causes in order and not skip to the expensive one. This guide walks through them for a typical sewer smell in a San Diego house, plus how to test for sewer gas before you pay anyone.

First, what the smell is

Sewer gas is a mix: hydrogen sulfide (the rotten-egg part), methane, ammonia and whatever else is decomposing in the line. Every drain in the house is connected to it. What keeps it out is a few inches of water sitting in the U-shaped trap under each fixture, plus a vent system that lets the pressure equalize through the roof instead of through your traps. When you smell it, one of those two things has stopped working somewhere, or the pipe itself has opened up.

Cause one: a dry p-trap

This is the most common one. A guest bathroom nobody's used since spring, a floor drain in the garage or laundry, a shower in a rental unit that's been empty, a utility sink. The water in a dry p-trap has simply evaporated, and in San Diego's dry air that can take as little as a few weeks. Once it's gone, the drain is an open pipe to the sewer.

Fix: run water down every drain in the house for a minute. If the smell fades over the next day, that was it. For a floor drain nobody uses, pour a cup of water down it monthly, or a little mineral oil on top of the water to slow evaporation.

Cause two: a failed wax ring

The toilet sits on a wax ring that seals it to the flange in the floor. When the ring fails, from age, a rocking toilet or a floor that's moved, sewer gas comes up around the base. The tell is a smell in one bathroom, strongest near the floor, sometimes with a little water at the base after a flush, and sometimes a toilet that rocks when you sit on it.

In slab houses on San Diego's expansive clay, this one is more common than elsewhere: the slab moves a fraction each wet-dry cycle, the flange sits on the slab, and the seal gives up. Fix: a new wax ring, $150–$350 with the toilet pulled and reset. If the flange is broken or below the tile, a bit more.

Cause three: a blocked or missing vent

Every drain line has a vent that runs up through the roof. If the vent is blocked (a bird's nest, leaves, a rodent), or an old remodel removed one and never replaced it, flushing a toilet pulls air through the nearest trap instead. You hear a gurgle, the trap loses its water seal, and the smell follows. The tell is a gurgle in one drain when another fixture runs, and a smell that comes and goes with use.

Fix: clear the vent from the roof, or on a house with a botched remodel, add one. $150–$500 for a clearing. Studio and granny-flat conversions in older neighborhoods are where missing vents tend to turn up.

Cause four: a leak in the drain line itself

Now the expensive one. Cast iron under a slab corrodes from the inside and cracks at the hubs. When a section opens up, sewage soaks into the soil under the house, and the smell comes up through the slab, through cracks in the floor, along a wall, or from a bathroom where nothing else is wrong. The tells are a smell that's steady rather than occasional, that doesn't respond to running water or replacing wax rings, that's often worst in the morning, and that may come with a damp patch on the floor, a musty room, or drain flies.

The same applies to a cracked or separated pipe in a crawlspace under a raised-foundation house, which is what a lot of the 1920s and 30s bungalows in North Park, South Park and Mission Hills sit on. There you can sometimes smell it from the crawlspace vents outside.

Fix: a camera inspection with a sonde locate to find the crack, then a spot repair, a liner, or an under-slab job depending on how much of the line is gone. A leak inside the house is the one cause on this list that gets worse on its own.

How to test for sewer gas at home

Before anyone is paid, a few free checks narrow it down:

1. The water test. Run every fixture, including floor drains and the unused shower. A smell that's gone the next day was a dry trap.
2. The rock test. Sit on each toilet and shift your weight. A toilet that moves has a seal that's probably failed.
3. The nose map. Note where and when the smell is strongest. One room near the floor points to a toilet; a steady smell in a room with no plumbing points to the pipe.
4. The smoke test. A plumber or sewer contractor can push harmless theatrical smoke into the drain system and watch where it comes out: a cracked pipe, a loose fitting, an open vent in the attic. It's the professional way to find a leak that the other tests can't.

If you suspect natural gas rather than sewer gas, don't run tests. Leave the house and call the gas utility or 911.

Working through it

The rainy-season version

In a big winter storm the sewer main under the street can run full, and gas gets pushed back up laterals and through any weak trap or vent. A smell that appears only during the first few storms of the season and goes away when they pass is usually that, and it's a sign your traps and vents are marginal rather than broken. If it happens every time it rains, have the vents checked. The fall checklist covers the pre-rain walk-through.

What not to do

Don't pour bleach down every drain; it doesn't fix any of the causes and it's hard on old cast iron. Don't ignore a steady smell in a slab house because someone said the traps were fine; traps being fine is exactly what points at the pipe. And don't buy a replacement on the strength of a smell alone. Get the camera in the line. If it's a dry trap, you're out $250 and you've got a recorded baseline of a sound sewer, which is not a bad thing to own on a 1950s house. If it's a cracked line, you've found it while it's a spot repair and not a foundation problem.

Common questions

Does sewer gas rise or fall?

Both, because it's a mix. Hydrogen sulfide is heavier than air and tends to collect low, near floors and drains, while methane is lighter and rises. In practice the smell is usually strongest near the source, which is why a failed toilet seal smells worst at floor level.

How do you test for sewer gas in a house?

Start free: run water in every drain, rock each toilet, and note where the smell is strongest and when. If that doesn't find it, a plumber or sewer contractor can run a smoke test to show where gas escapes, and a camera inspection shows whether a drain line has cracked. Household gas detectors are made for natural gas and carbon monoxide, so don't rely on one to rule out sewer gas.

Is sewer gas dangerous to breathe?

A faint smell from a dry trap is mainly an odor problem, but it shouldn't be ignored. A strong or persistent smell, or headaches, nausea or dizziness in the house, means air the place out and get the source found promptly. If you're worried about symptoms, ask a doctor, and if the smell could be natural gas, leave and call the gas utility or 911.