Sewer Smell in Your House? It Could Be a Hidden Drain Leak

 🔄 Reviewed and maintained by the AnyLeak team — August 2026. Rated 5 stars by BC customers.

A sewer smell inside your home is one of those problems people quietly tolerate for weeks — cracking windows, blaming the drain, hoping it fades. It deserves better than that. The odour is sewer gas, and it is only supposed to exist inside your drain pipes; if you can smell it in a hallway or basement, it has found a way out. Sometimes the explanation is completely innocent and fixable with a jug of water. Sometimes it is a cracked drain line or a failed seal quietly leaking wastewater into a wall cavity or under a slab. The two situations smell identical, which is why guessing is a bad plan. Here is how to rule out the harmless causes yourself in an afternoon, what it means if the smell survives those checks, and how the real source gets found without cutting open walls.

What Sewer Gas Is and How It Gets In

Every fixture in your home connects to the drain system, which carries wastewater out and, inevitably, contains sewer gas — a mix of moisture, methane, hydrogen sulphide, and other compounds produced by decomposition. Your home is protected from it by an elegantly simple device: the P-trap, the curved section under every fixture that holds a plug of water as a seal. Vent stacks through the roof give the gas a harmless escape path above the house. Smelling sewer gas indoors means one of three things: a trap seal has failed, the venting is compromised, or the drain system itself is leaking somewhere between the fixtures and the sewer. The first is trivial. The last is not. The order you investigate matters.

Rule Out the Innocent Causes First

We tell every caller the same thing, because it is true: the most common cause of sewer smell is a dried-out P-trap, and it costs nothing to fix. Any fixture that goes unused for weeks — a guest bathroom, a basement shower, a laundry sink, and especially floor drains — loses its trap water to evaporation, opening a direct path for sewer gas. Pour a litre of water into every floor drain and run every seldom-used fixture for a minute. Check that cleanout caps in the basement or crawlspace are present and snug. Then wait a day. If the smell disappears, you are done, and we would rather tell you that than book an unnecessary visit.

Signs It Is a Genuine Drain Leak

If the smell survives the trap-priming test, take it seriously. A real drain leak tends to announce itself in patterns: the odour is persistent rather than intermittent, and localized — strongest along one wall, one stretch of floor, or one room — because the gas is escaping at a specific failure point. Other evidence often accompanies it: unexplained damp patches or staining on walls or floors, lifted or discoloured flooring, and pest activity, since drain flies and rodents are drawn to broken drains. The smell may strengthen when upstairs fixtures run, as wastewater passes the failure point. Under-slab leaks from a broken lateral can be subtler: odour seeping from floor cracks, or a patch of floor that stays inexplicably damp.

How We Find It Without Opening Walls

Drain leaks hide well because the pipes are buried in walls, floors, and slabs — but the drain system can be tested as a system. Smoke and tracer testing fills the drain lines with a visible, harmless indicator under gentle pressure; it escapes exactly where the gas does, revealing the failure point at a wall, floor, or fitting. Camera inspection travels the pipes from inside, documenting cracks, failed joints, and broken sections along with their measured position on the line. Acoustic methods add confirmation where wastewater is actively escaping. Between them, we can typically hand you the failure’s location and nature — a failed seal at a floor-mounted fixture, a cracked stack, a broken lateral — without a single exploratory hole.

The Stakes, Stated Calmly

There is no need for alarm, but there is a need for follow-through. At the concentrations typical of household drain problems, sewer gas is primarily a comfort and habitability issue — though prolonged exposure to hydrogen sulphide can cause headaches, nausea, and fatigue, and methane in confined spaces is worth taking seriously. The quieter cost is what the leak is doing where you cannot see it: wastewater escaping into a wall cavity or under a slab keeps materials chronically wet, invites mould, and in the case of a broken lateral can wash away supporting soil over time. None of this happens overnight. All of it is easier to address this month than next year.

Do This Before You Call

• Pour a litre of water into every floor drain and run every rarely used fixture for a full minute, then reassess after a day.
• Check basement and crawlspace cleanout caps — a missing or loose cap is an open door for sewer gas.
• Track the smell: note the room, the wall, the time of day, and the weather each time you notice it.
• Look for companions: damp patches, stained or lifted flooring, or drain flies near where the odour is strongest.
• Note whether running upstairs fixtures makes the smell stronger — that pattern points to a leak on the drain path.

Why Homeowners and Property Managers Across BC Call AnyLeak

• Non-invasive acoustic, thermal, and moisture-mapping methods — no exploratory demolition
• Certified leak detection specialists serving BC since 1999
• Written reports that document the source for insurance and repair scoping
• Metro Vancouver, Fraser Valley, Victoria, Kelowna, Kamloops, and across BC

Related Services and Guides

• Sewer Leak Detection Specialists
• Residential Sewer Leak Detection
• Hydrostatic Pressure Testing Services in BC

Not sure what you are looking at? Call AnyLeak at (604) 269-5325 or email info@anyleak.ca — describe the symptom and we will tell you honestly whether a professional investigation makes sense.

Frequently Asked Questions

Why does my house only smell like sewage sometimes?

Intermittent odour usually tracks conditions rather than the leak itself. Wind can push vent-stack gases back down toward the house. Rain saturates soil around a leaking lateral and displaces gas indoors. Heavy fixture use sends wastewater past a failure point that is otherwise dry. And a marginal trap — one barely holding its seal — can siphon when large drains flow nearby. The pattern is worth recording, because when the smell happens is often as diagnostic as where.

Can a sewer leak make my family sick?

At typical household concentrations, sewer gas is more unpleasant than dangerous, and there is no need for panic. That said, prolonged exposure to hydrogen sulphide can cause headaches, nausea, and fatigue, and a wastewater leak inside the structure creates chronic dampness that supports mould growth — a more common health concern in practice. Treat a persistent smell as a solvable maintenance problem that deserves prompt attention, ventilate the affected area, and get the source located.

How do you test for a drain leak without breaking open walls?

By testing the drain system as a sealed network. Smoke or tracer testing introduces a harmless, visible indicator into the drain lines; it emerges precisely where sewer gas does, marking the failure point. A drain camera then inspects the suspect section from inside the pipe, confirming the defect and measuring its exact position along the line. The combination tells your plumber what to fix and exactly where — before anything is opened.

Could it be the seal under a bathroom fixture?

Yes — the wax seal where a floor-mounted fixture meets its drain flange is a frequent offender, especially in older homes or after any flooring work that changed the floor height. When that seal fails, sewer gas escapes at floor level and small amounts of wastewater can wick into the subfloor around the flange, sometimes staining the ceiling below. A rocking fixture is a strong clue. Smoke testing confirms it without removing anything.

I primed all the drains and the smell is still there. What now?

You have done the valuable part — the innocent causes are ruled out, and the smell now deserves professional attention. Persistent odour after trap priming points to a leak or venting failure somewhere in the drain system itself. Call us at (604) 269-5325 or email info@anyleak.ca. We will test the system with smoke, tracer, and camera methods, and give you the failure’s location and nature in a report your plumber or strata can act on directly.