How do you find the source of a sewer smell?

### Step 1: Rule out natural gas first

Natural gas is odorized to smell like sulfur or rotten eggs, which is close to how sewer gas smells. Southwest Gas says its gas carries that odor on purpose so leaks get noticed. If the smell is near a gas appliance, hisses, or comes with dizziness or a headache, treat it as gas. Leave the building right away, leave the doors open, and from a safe place call 911 and Southwest Gas at 877-860-6020. See what to do if your house smells like gas or rotten eggs.

Hydrogen sulfide in sewer gas can irritate the eyes, nose and throat, and the CDC notes people stop smelling it at higher levels, so the nose is not a safe gauge. If anyone feels sick, read the first signs of sewer gas poisoning and ventilate.

### Step 2: Map the smell

Write down three things before you touch anything:

• Where. One drain, one room, one floor, the whole building, or only outside?
• When. Always, after big water use, only when the AC runs, only after a rainstorm or a cleaning?
• What changed. A remodel, a vacancy, a long trip, new work on the roof or floor.

That answer points you to the right step below.

### Step 3: Refill every trap

A trap holds a water seal, and the plumbing code summary from ASPE puts it at 2 to 4 inches deep. That seal is what keeps sewer gas out. It is lost most often by evaporation in rarely used fixtures, which Wikipedia lists as the most common source of indoor sewer gas, with floor drains near furnaces, utility sinks and showers as typical culprits.

Pour water into every sink, tub, shower and floor drain, and flush every toilet. Include the drains you forgot about: a mop sink, a laundry standpipe, a garage or mechanical room floor drain. Monitor the result. Improvement supports a dry-trap explanation, but returning odor needs further checks.

For a single bathroom drain, see why a bathroom drain smells like sewer. If the same drain keeps drying out, the lasting fix is a trap primer or barrier device, not a reminder. New to traps? Start with what a P-trap is.

### Step 4: Check the toilets

If the smell is worst at one toilet, push on the bowl. A rocking toilet or a stain around the base points to a failed seal. Oatey lists water at the base, a wobbling toilet and a persistent smell near the toilet as the signs. A failed wax ring can smell with no visible water. Read toilet wax ring smells for the fix.

### Step 5: Listen for the vent

Gurgling when another fixture runs, slow drains on the same branch, and a smell that keeps coming back after you refill the traps all point to a vent that is blocked, cracked or disconnected. Oatey names vent blockage as a main cause. Vent work happens on the roof and in the attic, so plan for a plumber. See what a plumbing vent is and blocked roof vents.

### Step 6: Check the kitchen and the AC

A heavy, rancid smell near a kitchen is a neglected grease trap, not a trap seal. See why a restaurant smells like sewer and gas smell in a restaurant.

If the smell shows up only when the AC runs, check the floor drain near the air handler and the condensate line. Codes bar a condensate drain from connecting directly to a plumbing drain, waste or vent pipe. See smell only when the AC runs and AC condensate drain rules.

### Step 7: Look outside

A faint smell right under a roof vent is normal, because vents release gas on purpose. A smell at ground level near a cleanout points to a loose cap or a leak. See smell outside near the cleanout or vent and what a sewer cleanout is.

### Step 8: Camera or smoke test

If you have done steps 3 through 7 and the smell is still there, gas is escaping from the line itself or from a joint you cannot see. Two tools find it:

• A drain camera. A flexible cable with a camera goes in through a cleanout and shows cracks, collapsed sections, roots, offsets and corrosion on live video.
• A smoke test. A plumber seals the openings and pushes smoke through the drain and vent pipes. Unexpected escaping smoke can identify an air path that the crew must investigate and confirm.

We do smoke testing. Our equipment uses liquid smoke and forces air into the vent system, so the smoke fills the drain and vent pipes at a steady, controlled flow. Test flow and visibility depend on access, isolation and obstructions. Liquid-smoke equipment does not guarantee detection of every small or concealed leak. Either way, the plumber decides which tool fits the building, and often uses both a camera and smoke. Have them mark the spot and keep the video. See cracked lines under the slab and the commercial building version.

### If you manage a commercial building

A large building has too many traps for guesswork. Number every floor drain and mop sink on a floor plan, assign someone to fill them and initial a sheet, and do it the same day so you know the whole building was covered. Then compare complaint locations against the plan. A cluster of complaints around one drain, one restroom group or one mechanical room narrows the search fast. If complaints are spread across the building and traps are full, the problem is more likely in the main line or the vents, and that is the point to bring in a plumber with a camera.

### What each step costs you in time

The time needed depends on the building, accessibility, odor severity and test findings. Start with safe, accessible checks, but do not delay professional help for a strong odor or symptoms. The plumber can explain which paid diagnostic work is justified and provide an estimate before starting.

### Step 9: Write it down

Note the date, where you smelled it, what you tried and what changed. In a rental or commercial building, that record settles who does what next. If the smell followed drain work, read why it smells like sewer after drain cleaning before you call it a new problem.

We are licensed and insured, and you get a free estimate and an upfront price before work starts, day or night.