There is a sewer smell in the house and no backup.

Pour a couple of quarts of water down every drain in the house that does not get used often: the guest bathroom, the basement floor drain, the laundry standpipe, the shower nobody uses. A dry trap is the most common cause of a sewer smell by a wide margin, and refilling one takes thirty seconds.

If the smell is gone the next morning, that was it.

Why an unused drain smells

Every fixture has a trap, the U shaped bend under it, and the water sitting in that bend is the only thing between the room and the sewer. It is a water seal, not a valve. When a fixture goes unused for weeks the water evaporates, the seal breaks, and sewer gas comes up through the opening.

Floor drains are the usual offender because nothing ever runs into them deliberately. Guest bathrooms are second. Basement laundry standpipes are third.

Two refinements once you have refilled them:

• Add a small amount of cooking oil after the water in a drain that will stay unused. It floats and slows the evaporation.
• If a trap empties again within days, it is not evaporating. It is being siphoned, which is a venting problem rather than a disuse problem.

Work through it in this order

1. Every trap refilled. Above. Do this first because it is free and it is usually the answer.

2. Where is the smell, and when? A smell in one room points at that room’s fixture. A smell everywhere points at the vent system or the main line. A smell that appears only when the washing machine drains, or only when someone showers, points at a vent that cannot supply air fast enough, and the trap is being sucked dry each time.

3. A smell only when it rains, or only on windy days, usually means the vent stack outlet on the roof is blocked or too close to a window, and the wind is pushing air down it rather than letting it out.

4. Look at the toilet base. If a toilet rocks when you push it, the seal underneath it has failed. That lets sewer gas past the trap entirely, and it also lets water into the floor. A slow leak at a toilet base is one of the classic hidden wet patches.

5. Check the cleanout caps. A missing or cracked cap on a cleanout inside the house, in a basement or a utility room, is an open pipe to the sewer.

6. Under sinks. A trap arm that has come loose, a dishwasher hose connected without a trap, or a washing machine standpipe that is too short. Look and smell directly at each one.

The ones that need a professional

• A blocked vent stack. Usually a nest, leaves, or ice. It is on the roof, which is where the job stops being yours.
• A cracked or disconnected drain line inside a wall or under a slab. The tell is a smell that is strongest at one wall, or a persistent smell with no fixture near it.
• A broken sewer line under the house. Often shows up as a smell in a crawl space, with damp ground under it, and it needs a camera to confirm.
• A dry trap primer that has failed, on a floor drain in a newer building that is supposed to be topped up automatically.

A plumber can smoke test the system, which pushes harmless artificial smoke through the drains and shows exactly where it escapes. If you have chased this twice without finding it, that test is the thing to ask for by name.

What not to do

• Do not pour bleach down the drain to mask it. It does not fix a broken seal, and mixed with anything containing ammonia it produces a toxic gas.
• Do not seal a floor drain shut. That drain exists because water has to go somewhere, and blocking it turns the next backup into a flooded room.
• Do not ignore it because it comes and goes. Intermittent usually means siphoning or wind, both of which are real faults.

If the smell arrives with anything else

A smell plus a slow drain, or a gurgle in another fixture, is not a trap problem. That is a line starting to block, and waste appearing in a fixture you did not use is the next stage of it.

A smell plus damp under the house, or a patch of ground that stays wet in dry weather, points at a broken line rather than a dry trap.

The short version

Refill every unused trap first, including the floor drain. If it returns within days it is being siphoned, not evaporating. Chase it by where and when it smells. Leave and call the utility if it is strong and sudden or if anyone feels ill. Ask for a smoke test rather than guessing a third time.