Sewage smell in the basement: causes and fixes

What you are smelling

Sewer gas is a mix of gases from decomposing waste. The rotten-egg smell is hydrogen sulphide, noticeable at very low concentrations. Plumbing is designed to keep these gases out of the house with two things: water-filled traps under every drain, and vent pipes that carry the gas up through the roof. A smell indoors means one of those has failed, or sewage is somewhere it should not be.

Start with the simplest cause

A dry floor drain trap

Every floor drain has a U-shaped bend that holds water. In a drain that rarely gets used, the water evaporates over weeks, especially in winter when the furnace dries the air. Sewer gas then comes straight up.

The fix: pour a couple of litres of water into the drain. The smell should fade within a few hours. A spoonful of mineral or cooking oil on top slows evaporation. Repeat every month or two.

Check other seldom-used drains too: a basement shower, a laundry tub, a spare bathroom sink, a bar sink.

Other common causes

A missing or loose cleanout plug. Inside many floor drains, and on the main stack, there is a cleanout opening with a threaded plug. If a drain cleaner left it off, or it has corroded, gas bypasses the trap. A replacement plug costs very little.

A failed toilet wax ring. The seal between a toilet and the drain flange can dry out or shift, particularly on a toilet that rocks. Signs are a smell around the base and sometimes staining or dampness. The toilet has to be lifted and the seal replaced.

A blocked or faulty vent. A vent blocked by a nest, leaves or ice makes drains gurgle and can siphon the water out of traps. An air admittance valve, used in place of a vent pipe on some fixtures, can fail and stick open.

The sump pit. Stagnant water, or a pit that receives laundry or other waste water, smells. Clean the pit, fit a sealed lid and make sure only ground water drains to it.

An ejector pit. A basement bathroom below the sewer level pumps its waste up from a sealed pit. A loose lid, failed gasket or blocked vent lets the smell out.

A cracked or leaking drain pipe. Old cast iron stacks corrode and crack, and joints fail. A leak inside a wall or under the slab gives a persistent smell, sometimes with damp patches, stains or, under a floor, settlement.

A partial sewer blockage. Slow drains, gurgling and a smell that comes and goes can mean the main line is partly blocked and waste is sitting in it. A full backup may follow.

Residue from an earlier backup. If sewage once reached the basement and porous materials were dried instead of removed, the smell returns in humid weather.

Things mistaken for sewer gas

• Natural gas. The added odorant smells of rotten eggs or sulphur. If you suspect gas, leave the house and call your gas utility’s emergency line from outside. Do not use switches or phones indoors.
• The water heater. Hot water that smells of rotten eggs points to a reaction in the tank, often to do with the anode rod, and is commoner with well water.
• Mold and damp. Musty, earthy smells come from moisture problems, not drains. See musty smell in the house.
• A dead animal in a wall, duct or window well. A strong, sweetish smell that peaks and then fades over a couple of weeks.
• A dirty drain. Slime in a laundry tub or shower drain smells without any sewer gas involved.

Finding the source

1. Follow your nose to the strongest point.
2. Refill every trap in the basement and wait a day.
3. Check that the floor drain’s cleanout plug is in place.
4. Look at the base of each toilet for movement or staining.
5. Check that sump and ejector pit lids are sealed.
6. Look along exposed drain pipes for stains, drips and corrosion.
7. Note when the smell is worst: after rain, when the laundry drains, when it is windy, or all the time.

If that does not find it, a plumber can run a smoke test, which fills the drain system with visible, harmless smoke that escapes wherever gas does, or a camera inspection of the drains.

Is it harmful?

At the levels usually found in a house with a dry trap, sewer gas is unpleasant and can cause headaches and nausea, but it is not an emergency. Open windows and fix the cause.

Take it more seriously when:

• The smell is strong and persistent
• People feel dizzy, sick or have irritated eyes
• There is visible sewage or a wet area

Hydrogen sulphide is dangerous at high concentrations, which occur in confined spaces such as pits and tanks, not normally in living areas. Sewer gas also contains methane, which is flammable. Never enter a sewage pit or tank.

When sewage has leaked

If the cause is a leaking pipe or a past backup, the smell will not go until the contaminated material does. That means repairing the pipe, removing affected drywall, insulation, flooring and sometimes soil or concrete, cleaning and disinfecting what remains, and drying it. Deodorisers and foggers only mask it.

Prevention

• Top up seldom-used traps every month or two
• Keep cleanout plugs and pit lids in place
• Fix rocking toilets
• Have slow, gurgling drains looked at before they back up
• Have the main drain camera-inspected in an older house

More: sewage backup cleanup.