Sewer Smell in the Basement

Why it matters

Sewer gas is a mix of gases from decomposing waste, including hydrogen sulphide, which gives it the familiar smell. Each drain includes a trap, a water-filled bend that seals sewer air out. When the water in a trap evaporates, or a seal or pipe is damaged, gas finds its way in. Besides being unpleasant, a persistent sewer smell can point to a cracked pipe or a venting problem that will not fix itself. In strong concentrations, sewer gas is a health concern, so a smell that is heavy or makes anyone feel unwell should be treated seriously.

Step by step

1. Refill dry traps. Pour a few litres of water into the basement floor drain, any unused shower or sink, and the laundry standpipe. A rarely used basement bathroom is the most common source.
2. Add a little oil to the floor drain. After refilling, a spoonful of mineral oil floats on the water and slows evaporation.
3. Check the cleanout caps. A loose or missing cap on the main cleanout lets gas straight into the basement.
4. Look at the toilet base. A failed wax ring can leak gas without leaking water. A toilet that rocks is a clue.
5. Inspect the sump pit lid. Some pits collect water that smells; a sealed lid helps.
6. Think about recent changes. New fixtures, renovation work or a recent backup can all disturb seals and vents.

Common mistakes

• Masking the smell with air fresheners and hoping it fades.
• Pouring bleach or chemicals into the floor drain, which does nothing for a dry trap and can damage seals.
• Sealing a floor drain completely, which removes a safety outlet for water.
• Ignoring a smell that returns within days of refilling the trap.
• Overlooking a vent pipe blocked by a bird nest or ice on the roof.

When to call a plumber

Get in touch if the odour lingers after you have refilled every trap and checked the cleanout and toilets. A plumber can smoke-test the drain system to find cracks, inspect the lateral with a camera, and check that vents are clear. Strong smells along with gurgling drains or slow flushing often mean a blockage or a damaged section of drain. Problems with the water supply side, such as an old service line, are a different issue entirely; see water service line replacement if that applies.

If you smell gas that is more like a chemical odour added to natural gas, get everyone outdoors and phone the gas utility from there. That is not a plumbing smell.