Skip to content
24/7 emergency
778-938-5311
Paradigm Services
← All articles

Sewer Smell in Bathroom? Causes, Fixes and When to Call

Sewer Smell in Bathroom? Causes, Fixes and When to Call

Curved P-trap beneath a bathroom sink

A sewer smell in the bathroom almost always comes from one of four places: a dry drain trap, a failed toilet seal, a blocked or damaged vent, or buildup inside the drain. Most of these take a few minutes to check. Some are a quick free fix. A few need a licensed plumber. This guide shows you how to tell them apart, what you can safely do yourself, and when the smell means you should stop and call.

Paradigm Services and Installations
Persistent Bathroom Odour? Get It Checked
If simple fixes do not clear the smell, Paradigm Services and Installations provides plumbing service across the Tri-Cities, with on-site quotes.
Visit Paradigm Services

TL;DR

  • Start with the drain you use least. A dry P-trap lets sewer gas rise into the room, and a jug of water often fixes it.
  • Rock the toilet. If it moves side to side, the wax ring has likely failed and the smell is leaking from the base.
  • Gurgling plus smell points to venting or a clog in the drain line, which is plumber territory.
  • Headache, nausea or dizziness with the smell: open windows, leave the building and get fresh air. A sudden rotten-egg smell can also be natural gas, and that is a different emergency.
  • Persistent smell after the simple fixes? Call a plumber. We offer on-site quotes across the Tri-Cities.

Table of Contents

Match the smell to the likely source

Before you pour anything down a drain, work out where the smell is strongest. This table covers the usual suspects and what each one tells you.

Where it is strongest Most likely cause First thing to try
One shower, tub or sink you rarely use Dry P-trap Run water for 30 seconds
Around the base of the toilet Failed wax ring seal Check for rocking, look for staining on the floor
Shower drain, after regular use Biofilm in the drain Clean the drain, then flush with hot water
Whole bathroom, with gurgling Blocked or damaged vent, or a clog in the line Call a plumber
Floor drain in the basement Dry trap or a line problem Refill the trap, watch for backup
Rotten-egg smell, sudden, with hissing Possible natural gas leak Leave the house and call from outside

The rest of this article goes through each one.

Why a bathroom smells like sewage

Every drain in your home has a trap, the U-shaped or curved section of pipe under a sink, tub or shower. It holds a small plug of water. That water is a seal: it stops gas from the sewer line and the drain pipes from rising into your room. Almost every sewer smell problem is that seal failing, or something near it going wrong.

Here are the main reasons it happens.

A dry P-trap

Water evaporates. If a shower, tub or floor drain goes weeks without use, the trap dries out and the gas has a clear path up. This is common in guest bathrooms, basement bathrooms and homes that sat empty over a holiday. The County of Brant lists a dry trap in a rarely used drain as a typical cause of sewer odours in the home.

It is also the first thing we would check on a call like this, because it costs nothing to test.

A failed toilet seal

Your toilet sits on a wax ring that seals it to the drain flange in the floor. When that seal breaks or loosens, gas escapes around the base. The County of Brant notes that a broken toilet seal can let sewer smells into the home, and that a toilet that rocks from side to side tells you the wax ring has failed.

You may also see water at the base, a soft spot in the floor, or staining around the toilet. If that sounds familiar, read our guide on a toilet leaking at the base, because a leaking seal can damage the subfloor quietly for months.

Biofilm and buildup in the drain

Hair, soap residue and skin cells coat the inside of drain pipes. Bacteria feed on that layer and produce a sewage-like odour, even when the trap is full of water. HowStuffWorks describes this buildup as a source of bathroom sewage smells separate from the trap seal.

This is the usual story behind a shower drain that smells like sewer, especially when the smell is worst right after a hot shower. If the shower also drains slowly, a partial clog is likely part of it. Our piece on a slow bathtub drain explains why we try a plunger before chemicals.

A blocked or damaged vent

Plumbing vents run up through your walls and out the roof. They let air into the system so water drains smoothly and gases escape outside rather than into your home. When a vent is cut short, installed wrongly, cracked or clogged, drains can pull the water out of traps, and gas finds its way in. Gurgling, bubbling toilets and slow drains alongside the smell are the clues. Family Handyman notes that bubbling in a toilet or a changing water level can indicate a blocked vent, though a clogged waste line can cause similar symptoms, which is why diagnosis matters.

If your toilet gurgles after flushing, we cover the checks in Toilet Gurgles After Flushing?

A blocked or damaged vent

Cracked or leaking pipes, and septic systems

A cracked drain pipe inside a wall or under the slab can leak gas where you cannot see it. The County of Brant lists missing traps or vents, a broken toilet seal, vent leaks or clogs, and cracks in piping as reasons to bring in a plumber, and says a smoke test can help locate leaks in the drain-and-vent system.

If your home is on a septic system instead of a municipal sewer, an outdoor smell matters too. The Columbia Shuswap Regional District’s septic guide for BC homeowners lists wastewater surfacing, soggy ground over the treatment area and foul odours outdoors as possible symptoms. In BC, septic systems must be installed and repaired by an Authorized Person, such as a Registered Onsite Wastewater Practitioner. It also helps to understand septic or municipal sewer differences when you are buying or maintaining a home. Most Tri-Cities homes are on municipal sewer, but older rural-edge properties may not be.

How to find the source in 15 minutes

Work through these in order. Each step is free or close to it.

  1. Sniff test. Put your nose near each drain in turn: shower, tub, sink, floor drain. Then the base of the toilet. The strongest spot is your lead.
  2. Run the water. Run every tap and shower in the bathroom for 30 seconds, including the ones you never use. If the smell fades, you probably had a dry trap.
  3. Rock the toilet. Gently push it side to side. Any movement means the seal is not holding.
  4. Look at the floor. Dark staining, soft flooring or caulk gaps at the toilet base are warning signs.
  5. Listen. Gurgling after a flush or while the sink drains suggests the system is not venting properly.
  6. Check the rest of the house. If the smell is in several rooms, or you also see water or sewage coming up from a basement drain, skip the DIY steps. See what to do right now if sewage backs up in your basement.

What you can safely do yourself

These are low-risk steps for a homeowner. Stop if they do not work.

Refill the trap

For a suspected dry trap, pour water down the drain. Make It Right’s advice is about a gallon, while the County of Brant suggests at least a quart into sinks, showers, tubs and floor drains. Pour slowly, then run the tap for a bit. Do this to every drain in the bathroom.

If the drain tends to dry out again, such as a floor drain or a spare-room shower, a small amount of cooking oil poured on top of the water can slow evaporation. Then run water in it every few weeks, or add a reminder to your calendar.

Clean the drain

If the smell is biofilm, lift out the stopper or hair catcher and clear what you can reach. Then pour in baking soda, follow with white vinegar, wait, and rinse with hot water. This helps break down organic buildup. For a step-by-step walkthrough of getting rid of drain smell, including how to keep it from returning, there is a clear guide to follow.

We avoid harsh chemical drain openers. They can damage older pipes and fittings, and they are unsafe for whoever works on the drain next.

Replace the toilet seal, if you are handy

A wax ring swap is a job a seasoned DIYer can do: shut off the water, remove the toilet, replace the ring and set the toilet back securely. It is messy and heavy. If the flange is cracked, the floor is soft or the toilet still rocks afterwards, stop and call a plumber, because fixing the wrong layer hides the damage.

What not to do

Do not run rotating cleaning equipment or high-pressure water jets into a main sewer line without knowing what is underground. Manitoba Hydro warns about cross bores, where a natural gas line has been installed through a sewer service line. Clearing that sewer with the wrong tool can break the gas line, with a risk of leak, fire or explosion. The same hazard applies in any province where trenchless gas installation was used, which is one reason mainline clearing belongs with a professional.

Why you smell it and others do not

If your partner swears there is no smell, you may not be imagining it. Sensitivity varies between people. Alberta’s government notes that odour perception is personal, and that people adapt to a smell over time and become less aware of it. The New York State Department of Health adds that age, sex, smoking and prolonged exposure can affect how well someone detects an odour.

Two practical points follow. First, if you live with a faint smell for weeks, you may stop noticing it, while visitors still do. Second, an intermittent smell, like one that only appears after the shower runs or when a fan is on, is still real and still worth tracing.

Sewer gas, your health and natural gas

A faint smell now and then from a dry trap is mostly a nuisance. Persistent or strong smells deserve more care.

Sewer gas exposure

Sewer gas contains hydrogen sulfide among other gases. Health Canada says that at high levels, hydrogen sulfide can damage the nervous system or other organs and tissues. If you are concerned you have been exposed, talk to a health care provider. If you feel a headache, nausea or dizziness while you smell it, open windows, leave the building and get fresh air first. Then call a plumber, and a doctor if symptoms continue. Anyone with breathing conditions, young children and pets deserve extra caution.

Rotten eggs: sewer gas or natural gas?

Both can smell like sulphur. British Columbia’s Office of the Fire Commissioner explains that natural gas is odorized with mercaptan, giving it a rotten-egg smell, and that a sudden rotten-egg odour or a hissing sound could mean a gas leak.

Use this rule: if the smell is strongest at a drain and the toilet or a trap is suspect, it is likely sewer gas. If it is in the air near the furnace, water heater, stove or fireplace, or there is a hissing sound, treat it as natural gas.

If you suspect natural gas indoors, the same provincial guidance says to stop what you are doing and not use a cellphone or landline, smoke, light matches, or operate electrical switches. Go outside first, then call from a safe distance. Our gas fitters handle this around the clock, and our guide to gas fireplace smells explains what is normal and what is not.

When you are not sure which gas it is, treat it as natural gas. Leaving the house costs you nothing.

Rotten eggs: sewer gas or natural gas?

When the smell means you need a plumber

Call a licensed plumber if any of these are true:

  • The smell comes back within days of refilling traps and cleaning drains.
  • The toilet rocks, or the floor around it is stained or soft.
  • You hear gurgling from drains or see water levels in the toilet bowl rise and fall.
  • Several drains smell, or the smell is in more than one room.
  • Sewage or water comes up from a floor drain, tub or toilet.
  • You suspect a vent problem on the roof or in the walls.
  • The home is older and has not had its drain lines inspected.

A plumber can trace the source with a camera inspection of the drain line or a smoke test, which pushes harmless smoke through the system so you can see where it escapes. Our drain cleaning service covers clearing blocked lines. For seal, vent and pipe repairs, see our plumbing services.

Our view on sewer smells

Most calls about a sewer smell start the same way: the homeowner has already tried a jug of water, some vinegar, maybe a candle. Those are good first steps, and if they work, great.

What we see is the other group. The smell keeps coming back because the cause is behind the wall or under the floor: a vent that was cut short in a renovation, a toilet flange that was never set right, a cracked line. Spraying air freshener over that costs you months of living with gas in the house.

When I come out, I look at the whole path of the smell, not just the drain it seems to come from. I tell you what it is, what it needs and what it costs before any work starts. If it turns out to be a dry trap, I will say so and you will not be pushed into a bigger job. If it is a failed seal or a venting fault, I fix it properly, once.

And if it is the smell of natural gas, do not wait to phone anyone. Get out first. We pick up the phone around the clock for exactly that reason.

Jon

Need it checked? We come to you

Paradigm Services and Installations is an owner-led plumbing and gas fitting company in Port Coquitlam, serving the Tri-Cities and Metro Vancouver 24 hours a day, 7 days a week. Jon is the technician on site, we give on-site quotes, and our call-out fee is $60, waived if the work goes ahead.

Paradigm Services and Installations

If a persistent smell, a gurgling drain or a rocking toilet has you worried, book our plumbing team. If sewage is coming up or water is spreading, use our 24/7 emergency plumbing line instead. You can also start at paradigmservices.ca.

FAQ

What causes a sewer smell in my bathroom?

The common causes are a dry P-trap, a failed toilet wax ring, a blocked or damaged vent, and biofilm buildup in the drain. Less often, a cracked pipe is leaking gas inside a wall or under the floor. Start with the drain you use least and the base of the toilet.

Why does my shower drain smell like sewer?

If the shower is rarely used, the trap has probably dried out. If you use it daily, hair and soap residue are likely feeding bacteria in the pipe. Run water, clean the drain and flush it with hot water. If it keeps returning, a partial clog or venting issue may be the cause.

Is sewer gas in a bathroom dangerous?

A faint smell from a dry trap is usually low-risk, but it should still be fixed. Strong or lingering smells are different: Health Canada says high levels of hydrogen sulfide can harm the nervous system. If you get a headache, nausea or dizziness, leave the building, get fresh air and see a health care provider if symptoms continue.

Why can I smell sewage when no one else can?

People differ in how sensitive they are to odours, and sensitivity changes with exposure time. Someone who has been in the home for weeks may adapt, while another person notices it. Trust your nose and check the drains anyway.

How do I know if it is sewer gas or natural gas?

Sewer gas tends to be strongest at a drain or toilet. Natural gas is odorized to smell like rotten eggs and may come with a hissing sound near an appliance or gas line. If you are unsure, leave the house, avoid switches and phones inside, and call from outside.

Sources