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Every drain in a house has a trap, the U-shaped bend that holds a little water. That water is the only thing between your rooms and the sewer. When the water is missing, or the seal fails, the smell comes up. Most cases are harmless and fixed in a minute, but a sewer smell should never simply be ignored, because sewer gas contains methane and it is unpleasant to breathe.
1. A dry trap
The most common cause by far. A drain that is rarely used, such as a basement floor drain, a spare shower, a laundry sink or a guest bathroom, loses its trap water to evaporation in a few weeks. The seal is gone, and the smell comes through.
Fix: pour a quart of water down the drain. If the drain is used only rarely, follow it with a tablespoon of cooking oil, which floats on top and slows the evaporation. Make a habit of running water in unused fixtures once a month.
If the trap dries out within days, water may be being sucked out of it by a venting problem, see below.
2. A toilet that is not sealed to the floor
The wax ring between a toilet and the drain pipe can fail, often after the toilet has been rocking on an uneven floor. Sewer gas seeps out around the base. Signs: a smell that is strongest at floor level near the toilet, a toilet that rocks, or discoloration of the floor around the base.
Fix: the toilet has to be lifted and the ring replaced. It is a job a confident homeowner can do in an afternoon, and a plumber can do in an hour. Do not just caulk around the base; that hides a leak without stopping it.
3. A blocked or missing vent
The vent pipe that runs up through the roof lets air into the drains as water flows. If it is blocked, by leaves, ice or a bird's nest, water draining from one fixture can suck the water out of another fixture's trap, leaving it dry. The signs are gurgling from drains when a toilet is flushed, drains that empty slowly, and traps that dry out again soon after being refilled.
Fix: look at the vent from the ground for anything obvious. Clearing it means going onto the roof with a garden hose or a plumber's snake, which is a job for someone who is safe on a roof, or a plumber.
4. Biofilm in the drain
Sometimes the smell is not sewer gas at all but the film of soap, hair and bacteria that lines a bathroom sink or shower drain. It smells musty rather than like sewage, and it is strongest when water first runs.
Fix: remove the stopper and clean it, then scrub the inside of the drain opening with a drain cleaning brush. Pour a kettle of hot water down, followed by an enzyme drain cleaner, and repeat monthly.
5. A real sewer problem
A cracked drain pipe under the floor or in a wall, a broken sewer line under the yard, or a failing septic system leaks gas and sometimes sewage. The signs are a smell that is present in more than one room or outdoors, wet patches in the yard, lush grass over the sewer line, or sewage backing up into the lowest drain in the house. This is a plumber's job, and possibly a camera inspection of the line.
A note for the many Rockland homes on septic: a strong smell near the tank or the leach field means the system needs attention from a septic contractor. Have the tank pumped on the schedule they recommend, usually every three to five years.
A safety note
Sewer gas can contain hydrogen sulfide as well as methane. If the smell is very strong, open windows and keep flames away until the cause is found. A rotten egg smell can also be natural gas, which has an odorant added to it: if there is any doubt, leave the house and call your gas utility or 911 from outside.
When to call a plumber
- The smell persists after every trap has been refilled
- A toilet that rocks or leaks at the base
- Gurgling drains together with traps that keep drying out
- Sewage backing up, or wet ground over the sewer line
Find a licensed plumber near you.