Why Does My Apartment Smell Like Sewage Intermittent Smells Explained

If you’ve noticed a foul, rotten-egg-like odor drifting through your apartment at unpredictable times, you’re not alone. Intermittent sewage smells can be unsettling, especially when they appear without warning and then vanish just as quickly. These odors aren’t just unpleasant—they could signal underlying plumbing issues that, if ignored, might lead to health risks or costly repairs. Understanding the root causes and knowing how to respond is essential for maintaining a safe, comfortable living environment.

Sewage gas contains hydrogen sulfide and methane—both hazardous in high concentrations. While occasional whiffs are unlikely to cause harm, persistent exposure should not be taken lightly. The challenge with intermittent smells is their unpredictability, which often delays diagnosis. This guide breaks down the most common causes, offers practical steps for detection and resolution, and helps you determine when professional help is necessary.

Common Causes of Intermittent Sewage Smells

Unlike constant odors, which may point to an obvious leak or blockage, intermittent sewage smells suggest conditions that fluctuate based on usage, pressure changes, or environmental factors. Below are the primary culprits behind these sporadic odors:

• Dry P-traps: Every drain in your apartment connects to a U-shaped pipe called a P-trap, which holds water to block sewer gases from rising into your home. If a fixture isn’t used regularly (like a guest bathroom sink or floor drain), the water in the trap can evaporate, allowing gases to escape.
• Vent Pipe Blockages: Plumbing systems rely on vent pipes that extend through the roof to release gases and maintain air pressure. Leaves, bird nests, or ice can obstruct these vents, causing gases to back up into living spaces, especially when multiple fixtures are in use.
• Cracked or Loose Pipes: Over time, aging pipes can develop hairline cracks or loose seals. These defects may only emit odors under certain conditions—such as when water pressure increases during peak usage hours.
• Toilet Seal Failure: The wax ring sealing the base of your toilet to the floor flange can degrade or shift. Movement or improper installation can create gaps that allow gas to seep out, particularly after flushing.
• Sewer Line Issues in Multi-Unit Buildings: In apartments, shared sewer lines mean problems in neighboring units or building-wide stacks can affect your space. A clog or collapse downstream might force gases upward when wastewater flows past the obstruction.

Tip: Run water in unused sinks, showers, and floor drains every few weeks to keep P-traps filled and functional.

How to Diagnose the Source of the Smell

Pinpointing the origin of an intermittent odor requires observation and systematic testing. Since the smell comes and goes, timing and context matter.

1. Note When It Occurs: Keep a log for several days. Does the smell appear after running the washing machine? During evening hours when neighbors flush toilets? After heavy rain? Patterns can reveal whether usage, weather, or building dynamics are contributing.
2. Check All Drains: Pour a cup of water into every drain, including those in utility rooms, under sinks, and floor drains. If the odor diminishes afterward, a dry trap was likely the issue.
3. Inspect the Toilet Base: Gently rock the toilet side to side. Any movement may indicate a compromised wax seal. Also, check for discoloration or moisture around the base.
4. Sniff Test at Fixtures: On a day the smell appears, go room by room. Hold a damp cloth near each drain and fan vapors toward your nose. Stronger odors near specific fixtures narrow the search.
5. Test Ventilation: If accessible, inspect roof vents for debris. In winter, snow or ice buildup can temporarily block airflow until temperatures rise.

“Intermittent sewage odors are often misdiagnosed because they don’t present continuously. Homeowners need to act early—even brief exposures can indicate structural flaws.” — James Rolen, Master Plumber & Building Inspector

Step-by-Step Guide to Eliminate Sewage Odors

Once you’ve gathered clues about the source, follow this structured approach to resolve the issue:

1. Replenish Dry Traps: For infrequently used drains, pour one quart of water down each to refill the P-trap. Consider adding a small amount of mineral oil to slow evaporation in seasonal spaces.
2. Disinfect Drains: Use a mixture of baking soda and vinegar followed by hot water to clean organic buildup that may harbor bacteria and amplify odors. Avoid chemical drain cleaners, which can damage pipes over time.
3. Inspect and Replace Wax Ring (if needed): If the toilet rocks or shows signs of leakage, shut off the water supply, disconnect the toilet, and replace the wax ring. Reinstall carefully to ensure a tight seal.
4. Clear Vent Pipes: Safely access the roof (or hire a professional) to remove obstructions from vent stacks. Use a plumber’s snake or garden hose to flush debris from the top down.
5. Conduct a Smoke Test: In complex cases, licensed plumbers use non-toxic smoke machines to pressurize the system and visually identify leaks. This method detects hidden cracks or faulty joints invisible to the naked eye.
6. Monitor After Repairs: Continue tracking odor occurrences for at least a week post-treatment. Recurrence suggests deeper issues requiring further investigation.

Do’s and Don’ts: Managing Sewage Odors in Apartments

Do’s	Don’ts
Run water in all drains monthly to maintain P-trap seals	Ignore recurring odors hoping they’ll disappear
Contact building management promptly about shared system concerns	Pour bleach or strong chemicals down drains frequently (can corrode pipes)
Use enzyme-based drain cleaners for routine maintenance	Attempt major pipe repairs without permits or expertise
Keep records of complaints and repair attempts	Assume it’s “just old plumbing” without verification
Hire licensed professionals for vent or sewer line inspections	Block vent pipes with covers or screens that restrict airflow

Real Example: Tracking Down a Mysterious Apartment Smell

In a six-story Chicago apartment building, tenants on the third floor reported a sewage smell that appeared every few days, mostly in the evening. The odor was strongest near the bathroom sink but couldn’t be replicated on demand. Maintenance initially dismissed it as a dry trap, refilled it, and closed the ticket.

Two weeks later, the tenant logged the occurrences: always between 7–8 PM, lasting 15–30 minutes. She also noted that the smell coincided with her downstairs neighbor doing laundry. A follow-up inspection revealed that the second-floor washing machine drain was partially blocked, causing wastewater to back up slightly into the third-floor unit’s secondary drain line during high-volume discharge. The intermittent nature stemmed from the neighbor’s inconsistent laundry schedule.

After clearing the blockage and resealing a cracked joint in the stack pipe, the odor vanished completely. This case underscores how timing, neighbor activity, and shared infrastructure play critical roles in diagnosing apartment-specific plumbing issues.

When to Call a Professional

While some fixes are DIY-friendly, others require expert intervention. Here are red flags indicating it’s time to call a licensed plumber or building engineer:

• The smell persists despite refilling traps and cleaning drains.
• You detect gurgling sounds in multiple fixtures during flushing or draining.
• There’s visible mold, warping, or moisture near baseboards or under sinks.
• More than one unit in the building reports similar issues (suggesting a main line problem).
• You suspect vent damage due to recent storms or rooftop work.

Professionals have tools like camera scopes, pressure testers, and smoke generators to diagnose issues invisible to untrained eyes. In rental situations, document all communication with management—your lease may entitle you to habitable living conditions free of hazardous fumes.

Tip: If renting, send written notices via email or certified mail to create a paper trail in case disputes arise over unresolved maintenance issues.

Frequently Asked Questions

Is it dangerous to breathe sewage smell occasionally?

Short-term exposure to low levels of sewer gas is generally not harmful for healthy adults, though it may cause headaches or nausea in sensitive individuals. However, hydrogen sulfide is toxic at high concentrations, and prolonged exposure—even at low levels—can lead to respiratory irritation. Eliminating the source is always recommended.

Why does the smell come and go only at night?

Nighttime patterns often relate to increased plumbing usage across the building. As neighbors run showers, flush toilets, or operate dishwashers in the evening, pressure changes in shared stacks can push gases into weak points like dry traps or cracked fittings. Lower ambient noise and airflow at night may also make odors more noticeable.

Can a dirty garbage disposal cause a sewage-like smell?

Yes. While not actual sewage, decomposing food waste in a disposal can produce sulfur compounds that mimic sewage odor. Clean it regularly with ice cubes and salt, or a mix of lemon peels and baking soda. If the smell lingers after cleaning, the issue may lie deeper in the drain line.

Prevention Checklist for Tenants and Owners

• ✅ Pour water into all drains (including basement floor drains) every 3–4 weeks
• ✅ Inspect toilet base for wobbling or stains monthly
• ✅ Schedule annual plumbing check-ups in older buildings
• ✅ Report persistent odors to building management in writing
• ✅ Install drain strainers to reduce organic buildup
• ✅ Avoid using chemical drain openers; opt for enzymatic treatments instead
• ✅ Monitor roof vent areas after storms or heavy snowfall

Final Thoughts: Take Action Before It Escalates

Intermittent sewage smells are more than a nuisance—they’re warning signs. Whether caused by a simple dry trap or a hidden crack in a shared vent stack, ignoring them risks long-term damage and potential health effects. The temporary nature of the odor makes it easy to dismiss, but consistency in maintenance and prompt attention to changes can prevent minor issues from becoming emergencies.

If you're a renter, assert your right to a safe and sanitary living environment. If you own your unit, treat your plumbing system with the same care as any major appliance. Small efforts today—like running water down a forgotten drain or scheduling a vent inspection—can save thousands in repairs tomorrow.

💬 Has your apartment had mysterious sewage smells? Share your experience or solution in the comments—your insight could help someone solve their own stinky puzzle!