A foul sewage smell wafting through your bathroom isn’t just unpleasant—it could also be a sign of deeper plumbing problems. Whether it’s a faint odour near the sink or a persistent stench rising from your shower drain, ignoring it could lead to costly repairs and potential health hazards. In this comprehensive guide, we uncover the top five reasons your bathroom smells like sewage and provide expert solutions for each.
What Causes Sewage Smells in Bathrooms?
Sewage odours often result from poor ventilation, dried-out traps, leaking seals, or blockages within your home’s plumbing system. These problems create pathways for sewer gas to escape into your living spaces. Sewer gas primarily contains hydrogen sulphide and methane, both of which are not only smelly but potentially hazardous if inhaled in large quantities over time.
1. Dry or Faulty P-Traps
What Is a P-Trap?
The P-trap is the curved section of pipe found under your sink, bath or shower. Its function is to trap a small amount of water that acts as a seal, preventing gas from the sewer line from rising into your bathroom.
Common Issues
- Evaporation in unused sinks, showers or floor drains
- Cracked or leaky pipes allowing the water to escape
- Poor installation resulting in water not settling properly
Signs Your P-Trap May Be the Problem
- Smell gets stronger near the floor drain or under the sink
- You’ve recently returned from a holiday or unused a guest bathroom
How to Fix It
- Run water for 20–30 seconds in all fixtures to refill traps.
- Pour a small amount of mineral oil into the drain to slow evaporation.
- Inspect for visible leaks under sinks or in accessible drains.
- Replace the P-trap if you notice persistent dampness or cracking.
2. Clogged or Contaminated Drains
Why Do Blocked Drains Smell?
Hair, soap scum, toothpaste and other organic waste build up inside drains. Over time, these create a breeding ground for bacteria that release hydrogen sulphide—a gas that smells like rotten eggs.
Where to Check
- Shower and bath drains
- Bathroom sink
- Overflow holes in basins and baths
DIY Fixes
- Use a plunger to dislodge surface clogs.
- Pour a combination of baking soda and vinegar down the drain. Wait 15 minutes, then flush with boiling water.
- Use a drain snake for deeper blockages.
- Install drain guards to prevent future build-up.
When to Call for Help
If odours persist despite cleaning, or if multiple fixtures are draining slowly, a professional clean-out may be needed. This can include hydro jetting or mechanical augers to remove blockages.
3. Worn-Out Toilet Seals (Wax Ring Issues)
What Is a Wax Ring?
The wax ring forms an airtight seal between the base of your toilet and the drain pipe. Its job is to prevent water and gas from leaking out when you flush.
Warning Signs
- Persistent smell near the toilet base
- Wobbling or rocking toilet
- Water leaking around the floor after flushing
Step-by-Step Repair
- Shut off water to the toilet and flush to empty.
- Unscrew and remove the toilet carefully.
- Scrape off the old wax ring from the toilet and floor flange.
- Fit a new wax ring and re-seat the toilet firmly.
- Reconnect water supply, tighten bolts and check for movement.
Pro Tip
Use plastic shims to level the toilet if it rocks slightly. Always caulk around the base once reinstalled to provide extra sealing and stability.
4. Vent Pipe Blockages or Faults
What Do Vent Pipes Do?
Vent pipes allow air into your plumbing system so that water flows smoothly and gases escape outside, usually via a pipe extending through the roof.
What Happens When They’re Blocked?
When vent pipes are obstructed—by debris, leaves, animal nests or ice—air pressure can’t equalise. This causes drain gurgling, water siphoning from traps, and the infamous sewer stench.
How to Detect Vent Issues
- Gurgling noises after flushing or draining
- Water draining slowly in multiple fixtures
- Sewer smell with no visible source
Fixing Vent Problems
- Inspect the roof vent with binoculars or from a ladder
- Use a plumber’s snake or garden hose to clear light debris.
- Call a plumber for a smoke test to locate internal cracks or faults.
5. Leaking or Damaged Waste Pipes
How Do Pipes Leak?
Cracks or loose fittings in the waste pipes can occur from physical impact, corrosion, or faulty joints. These leaks may release raw sewage or sewer gases into the home.
What to Look Out For
- Unexplained mould or water stains near walls or floors
- Constant sewage smell with no clear fixture cause
- Sound of dripping water behind walls or under the floor
Solutions
- Minor leaks: use plumber’s tape or epoxy repair kits for temporary fixes.
- Hidden leaks: use thermal cameras or acoustic sensors to trace them.
- Major issues: re-pipe affected sections or use trenchless pipe lining for underground lines.
Hidden Sources Outside the Bathroom
Consider These Possibilities:
- Main sewer line backups can push smells into every fixture.
- Tree root intrusion into older clay pipes may block flow.
- Septic tank overflow or city sewer issues during heavy rainfall.
If more than one room smells, especially at the same time, your issue may be with the mainline and requires professional inspection.
How Sewer Gas Can Affect Your Health
Sewer gases contain hydrogen sulphide, ammonia, methane, and other chemicals. Prolonged exposure can cause:
- Eye and respiratory irritation
- Headaches, fatigue and nausea
- In rare cases, neurological damage (with high levels over time)
- Risk of explosion from methane buildup (extremely rare but possible)
Always ventilate the area and resolve odour issues quickly to avoid health complications.
Preventing Sewage Smells in the Future
Maintenance Checklist
- Run water in all drains weekly to maintain full P-traps.
- Clean sink and bath drains with vinegar and baking soda monthly.
- Ensure toilets are stable and sealed; re-caulk as needed.
- Clear roof vents each spring and autumn.
- Inspect visible pipes for leaks or corrosion signs.
- Install backflow valves to protect against city sewer backups.
When to Call a Professional Plumber
Call a plumber if:
- The smell persists after trap refills and drain cleaning
- Water drains slowly across multiple rooms
- You suspect damage behind walls or under floors
- You’ve had repeat issues with backups or sewer smells
- You notice raw sewage or standing water
Early detection and professional support can save thousands in repairs.
Final Thoughts
Sewage smells in the bathroom are more than just an inconvenience—they can signal underlying problems that affect your entire plumbing system. By understanding the key causes and how to fix them, you can maintain a fresh-smelling, hygienic space. From simple DIY measures to calling in professional help, taking action early saves you time, money and hassle.Need further help with your bathroom odour issue? If you’re in Scotland, Drainmaster Scotland is your trusted local expert for all drainage and emergency plumbing needs. With 24/7 availability and rapid response times from our Perth headquarters, our engineers are always ready to help resolve sewage odours, blockages, leaks and more. Get in touch with us today to restore freshness and safety to your home.