A sewer smell inside rarely comes from nowhere. It comes from one of a handful of places, and you can narrow most of them down with your nose and ten minutes of walking from room to room.

Pay attention to where it is strongest and when it shows up. A smell that lives in the spare bathroom points somewhere different from one that rolls through the whole house every time somebody flushes.

Start with the drain nobody uses

Under every drain is a trap, a bend in the pipe that holds a little water. That water is the only thing standing between your house and the gas in the sewer line beyond it. When a guest shower, a tub or a floor drain goes weeks without use, the water in the trap evaporates and the seal is simply gone.

This is the cause we find most, and it costs you nothing to fix. Run water down every drain in the house for half a minute, including the laundry sink and any floor drain in the garage. If the smell fades over the next hour or so, you found it.

Then keep it from coming back. Run those drains about once a week. A floor drain that almost never sees water can get a cup poured into it on a regular schedule.

A toilet that smells right at the floor

Under each toilet, a wax ring seals the joint between the bowl and the drain opening in the floor. It keeps flush water in and sewer gas out. If a toilet has been bumped, rocks a little, or was set on a thin seal, gas can slip out around the base without a drop of water showing.

The clue is a smell that is strongest at the toilet itself, sometimes worse right after a flush. Put a hand on the bowl and push gently side to side. Any movement at all usually means the wax has been squeezed out of shape.

Cranking down on the bolts will not fix it and can crack the base. The toilet has to come up and go back down on a new seal, which is a short job.

Is the vent doing its job?

Drain pipes tie into vent pipes that climb through the walls and out the roof. The vent lets air into the system so water can flow without sucking traps dry, and it lets sewer gas out well above your windows.

When a vent is blocked, water rushing down one drain pulls the water out of nearby traps as it passes. You hear the sink gurgle after the tub empties, drains run slow and then recover on their own, and a whiff of sewer comes up right after heavy water use. Leaves and debris at the roof opening are the usual culprits. A vent pipe that cracked or separated inside a wall lets gas straight into the room.

Please leave the roof to us. Clearing a vent means working at the edge of the roof with tools down a pipe, and a fall is a far bigger problem than a smell.

Cracked drain lines in older walls and floors

No dry trap, no loose toilet, no vent trouble, and the smell is still there. In homes that have been lived in for decades, that combination starts pointing at the drain pipe itself. Older drain lines can crack along their length or pull apart at a joint, and a crack too small to leak visible water can still leak gas.

A few signs tend to travel with it: a smell that stays in one wall or one corner, a damp or stained patch low on the drywall, or a musty note mixed in with the sewer one. Under a raised floor or a slab, a broken line can also show up as a drain that backs up more and more often.

Tracking it down usually means a camera in the line and a test of the drain system, so we open a wall only where the problem actually is. If the damage turns out to be in the main line outside, our sewer line repair page covers what happens next.

What you can check yourself, and when to call us

Before you call, work through this. Run water in every drain, especially showers and floor drains that sit idle. Rock each toilet gently. Notice whether the smell follows a flush or a draining tub. Open the cabinets under the sinks and look for a trap that has come loose at its slip nut.

Call when the smell survives all of that, when it arrives with gurgling or slow drains around the house, when you see dampness on a wall or floor, or when it is strong enough to give anyone a headache. Sewer gas in a closed-up house is not something to wait out over a long weekend.

Jot down what you noticed before you pick up the phone: the room, the time of day, whether it follows a shower, a flush or the washer, and whether it went away after you ran water. Two minutes of notes like that often tells us which of the causes above we are dealing with before we arrive, so the right parts come in the truck.

Slow drains often turn up alongside this, and our Pearl City drain cleaning page covers them. Toilets, showers and floor drains are all on our bathroom plumbing page, and you can see past work in our gallery.

Most of the time the fix is a cup of water or a fresh toilet seal. When it is not, you want to know where the gas is getting in before it turns into a wall repair.