Sewer Smell Inside Your Home? Here's What's Causing It

What Is a Sewer Smell In House Problem?

A sewer or drain smell inside your home is unpleasant and can indicate anything from a simple dry drain trap to a cracked sewer pipe releasing hydrogen sulfide gas — a gas that is not only foul-smelling but can be harmful at high concentrations. The smell of rotten eggs or stale drains should never be ignored or masked with air freshener; the source needs to be found and fixed.

All your drains are connected to the sewer system, but the U-shaped traps under every basin, bath, and toilet are designed to hold a water seal that blocks sewer gas from coming back up into your home. If that seal dries out, evaporates, or is lost through a siphoning effect, the odour pathway is open.

In South African homes, particularly those with infrequently used guest bathrooms or holiday properties left empty over summer, dry traps are the most common cause of sewer smells. This is also a more common problem during dry, hot summer months when evaporation from traps accelerates.

Common Symptoms & Warning Signs

• Persistent rotten egg or sewage smell inside the house
• Smell strongest near a specific drain, basin, or toilet
• Smell only noticeable in one room, often a guest bathroom or utility room
• Smell accompanies slow drains or gurgling sounds
• Smell detectable outside near drain inspection chambers
• Smell noticed more frequently in warm dry weather (evaporating traps)

What Causes a Sewer Smell In House Issue?

• Dry drain trap — water seal has evaporated in an unused fixture
• Siphoned trap — negative pressure in the drain system has sucked the water out of the trap
• Cracked or broken sewer pipe allowing gas to escape into walls or subfloor
• Blocked or missing vent pipe allowing gas to escape into the building instead of venting outside
• Faulty toilet wax ring or pan seal allowing sewer gas to escape at the base
• Partial drain blockage creating pressure fluctuations that displace trap water

Properly functioning plumbing relies on an interconnected venting system — vent pipes that run from drain lines through the roof, releasing sewer gases safely outside. If a vent pipe is blocked (by birds nesting, leaves, or debris), sewer gas finds its way back through trap seals instead. This is a system-level problem that requires a plumber to diagnose and fix.

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How Urgent Is This?

A sewer smell from a dry trap is low to medium urgency — simply running water into the affected drain to refill the trap will eliminate the smell immediately. If the smell returns within days, the trap is being siphoned, which points to a venting issue that needs a plumber.

A persistent sewer smell that cannot be traced to a specific drain, or one that is accompanied by a feeling of dizziness or nausea (high hydrogen sulfide concentration), is a higher-urgency health issue. Ventilate the affected rooms, avoid open flames, and call a plumber promptly to locate the source.

DIY vs Professional Repair

Run water into all rarely-used drains, basins, and toilets — this refills dry traps and eliminates the most common cause immediately. For toilets in unused rooms, a small amount of vegetable oil poured into the bowl after flushing will slow trap evaporation. These are safe, effective DIY first steps.

If the smell persists after refilling all traps, the cause is structural — a cracked pipe, vent issue, or failing toilet seal — and requires a qualified plumber. Do not attempt to trace sewer gas with a naked flame or lighter; hydrogen sulfide is flammable at high concentrations.

What Professionals Actually Do

The plumber will systematically work through the possible causes — checking all traps, testing the toilet pan seal, inspecting visible drain joints, and checking vent pipe outlets on the roof for blockages. A smoke test (non-toxic smoke pumped into the drain system) is an effective way to find cracks or gaps in the sewer pipe network, as smoke will emerge wherever there is an escape point.

Once the source is identified, the repair may involve reseating a toilet, replacing a trap, clearing a blocked vent pipe, or repairing a cracked drain pipe. If the drain pipe is buried or in a wall, the plumber will advise on whether pipe repair or relining is more appropriate.

Property Damage Risks

Sewer gas escaping from a cracked pipe inside a wall or subfloor can cause mould growth in addition to the health hazard from the gas itself. A cracked drain pipe leaking both gas and liquid waste into a wall cavity contaminates building materials, requires professional mould remediation, and can cause significant structural damage if the pipe is carrying wastewater.

Hydrogen sulfide at high concentrations is toxic. While domestic sewer concentrations rarely reach dangerous levels with normal ventilation, enclosed spaces like undercroft crawl spaces, poorly ventilated bathrooms, and ceiling voids can accumulate dangerous gas levels. If occupants are experiencing headaches, nausea, or eye irritation alongside the smell, evacuate the affected area and call a plumber urgently.

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