Sewer Smell in the Basement: Causes

The basics

Sewer gas is kept out of your home by water sitting in traps under every drain, and by vent pipes that carry gases up through the roof. When either fails, the smell gets in. The most common causes are:

• Dry floor drain trap. Water evaporates from rarely used drains.
• Unused fixtures. A basement shower or laundry tub that sits idle.
• Loose cleanout cap. A missing or cracked plug on a drain access.
• Failed toilet seal. A wax ring that no longer seals at the flange.
• Vent problems. A blocked roof vent or a failed air admittance valve.
• Cracked drain pipe. Older cast iron can crack and leak gas.

Step by step

1. Pour water into every floor drain. A litre or two refills the trap.
2. Run unused fixtures. Run each basement tap and flush unused toilets.
3. Check cleanout caps. Make sure each is tight and intact.
4. Look at toilets. A toilet that rocks or has staining at the base may have a failed seal.
5. Add a little oil. A spoonful of mineral oil on top of floor drain water slows evaporation.

Time to bring in the shop

Call us if the smell persists after refilling traps, if it is worse when fixtures upstairs drain, or if you see moisture around pipes. We can smoke-test the system to find the source and check the lateral with a camera. A cracked stack or failed seal is a straightforward repair. Read about sewer line repair if the problem is in the main line.

New smell after a renovation?

Check that new fixtures have traps and that vents were connected properly. Missing vents are common in DIY basement work.

Questions about this job? Call (365) 413-9125 and talk to a plumber.