Why Your Drain Smells Like Sewage — 6 Causes and Their Fixes

A sewage smell indoors means sewer gas is entering living space. Air freshener masks it; it does not fix it. Here are the six causes, roughly in order of how often we find them.

Cause 1: A dry [P-trap](/glossary#p-trap)

Every drain has a U-shaped trap holding a plug of water that blocks gas from the sewer. In a guest bathroom, a basement floor drain, a laundry tub or a floor drain in a dry basement, that water evaporates over weeks. Fix: pour a couple of litres of water down the drain, plus a tablespoon of cooking oil in rarely-used drains to slow evaporation. If the smell stops, you are done.

Cause 2: Biofilm inside the drain

A slimy layer of bacteria, soap, hair and organic matter coats the inside of the pipe just below the opening. It smells of rot rather than sewage and is often worst at bathroom sinks. Fix: mechanical cleaning — the coating has to come off the wall, which is why chemicals do not touch it.

Cause 3: A blocked or frozen vent stack

Your plumbing vents to the roof so air can enter behind draining water. If that vent blocks — leaves, a nest, or a frost cap in a Winnipeg January — the draining water siphons the water out of your traps, and gas comes straight in. The tell: drains gurgle or glug rather than flowing smoothly, and the smell moves around the house. Fix: clear the vent. Not a DIY job in winter, on a roof, in Winnipeg.

Cause 4: A failed toilet wax ring

If the seal under the toilet has failed, gas escapes at the base — and so does water, into your subfloor. The tell: smell strongest right at the toilet, and the toilet rocks slightly. Fix: pull, replace the wax ring, check the flange, reset.

Cause 5: A cracked or disconnected drain line

A broken line under a slab or in a wall leaks gas — and waste — continuously. The tell: persistent smell with no obvious source, often worse in one room, sometimes with damp or staining. Fix: camera inspection to find it, then repair.

Cause 6: A problem in the main sewer line

Roots, a partial blockage or a break lets gas back up through the system. The tell: the smell is worse after rain, and there are drainage symptoms elsewhere too. Fix: camera the main line.

How we diagnose it

Systematically, not by guessing. Every trap checked and refilled, every vent tested for draw, suspect lines cameraed, and where necessary a smoke test — non-toxic smoke pushed through the system so it escapes wherever there is a defect and shows us exactly where. Then you get a written diagnosis with the actual cause.

When to call a pro

If pouring water down every drain does not stop the smell within a day, call us. We serve Winnipeg, Selkirk, Steinbach, and everywhere within 100 km, 24/7. Call +1 (204) 399-4413.