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Bathroom Smells Like Sewer? Check the Floor Drain Trap First

A dry drain trap lets sewer gas rise into the room. Refill it with water and oil, then add a trap seal so it never dries out again.

By Jamilur Rahman Updated November 5, 2026 · 6 min read

Time
15 min
Cost
Under $20
Difficulty
Easy
A small metal floor drain cover set in a tiled bathroom floor, looking dry and dusty
In this guide
  1. Quick answer
  2. Free vs. permanent fix
  3. What you'll need
  4. Why it happens
  5. Try this first (free)
  6. When free fixes aren't enough
  7. Buy the fix
  8. Bathroom smells like sewage or sulfur? Match the clue to the cause
  9. The pro option: a trap primer
  10. What to skip
  11. FAQ
  12. Related problems

Quick answer

If your bathroom smells like sewer, check the floor drain trap first. A dry trap cannot block sewer gas. Pour a quart of water down every drain, floor drain included, then add a spoonful of mineral oil to slow evaporation. A floor drain trap seal keeps the smell out even if the trap dries. If the smell stays or you feel ill, call a plumber.

Free fix vs. permanent fix

Free fix · 5 minutes

Refill the traps with water

  • Pour a quart of water down the floor drain
  • Run water in the sink, tub and shower for a minute each
  • Add a spoonful of mineral oil to the floor drain
  • Clean the strainer and the overflow hole

How long it lasts: Weeks to months. In a drain you rarely use, the water evaporates again.

Most readers choose Permanent fix
Floor drain trap seal (one-way silicone insert) (illustration)
Illustration

A floor drain trap seal

A trap seal is a one-way silicone insert or cover that closes the drain opening between uses. Water still flows out, but gas cannot come up, whether or not the trap holds water. It is the easy fix for a drain that dries out from disuse.

What to get: Floor drain trap seal (one-way silicone insert)

How long it lasts: Years. It blocks gas even when the water in the trap is gone.

Check price on Amazon

As an Amazon Associate I earn from qualifying purchases.

What you'll need

  • Pitcher or bucket

    For refilling the trap

    You have it

  • Mineral oil

    A spoonful floats on the water and slows evaporation. Cooking oil can go rancid.

    You have it

  • Flashlight

    To look down the drain

    You have it
  • Enzyme drain cleaner (illustration)
    Illustration

    Enzyme drain cleaner

    Breaks down slime that gives off a sulfur smell

    Check price
  • Our pick
    Floor drain trap seal (illustration)
    Illustration

    Floor drain trap seal

    Measure the drain pipe before you buy

    Check price

Why it happens

1

A dry trap in a rarely used drain

Every drain has a U-shaped trap that holds water to block gas. In a floor drain, a guest shower or an unused tub, that water evaporates within weeks.

2

Sludge in the drain or overflow

Hair, soap and bacteria coat the pipe wall. Bacteria in that slime give off a rotten-egg smell that is easy to mistake for sewer gas.

3

A failing wax ring at the toilet

If the seal under the toilet fails, gas leaks out around the base. The smell is strongest near the floor beside the toilet.

4

A blocked or damaged vent

Drain pipes need air from a roof vent. A blocked vent can pull water out of traps, so gas gets in, and drains gurgle.

5

A cracked pipe or a main line problem

A damaged drain pipe or a backed-up main line sends smell into several rooms at once and needs a plumber.

Try this first (free)

Do these in order, starting with the simplest.

  1. Find the source by nose

    Tool used: Flashlight

    Put your nose near each drain: floor drain, sink, shower, tub overflow and toilet base. Whichever smells strongest is where to start.

  2. Refill the floor drain trap

    Tool used: Pitcher or bucket

    Lift off the drain cover, which is held by a screw or just lifts out. Pour about a quart of water slowly down the drain until you can see or hear it standing.

  3. Run water through every other drain

    Run the sink, shower and tub for 30 seconds each. Pour a cup of water down each overflow hole too.

  4. Seal the water with oil

    Tool used: Mineral oil

    Add about a tablespoon of mineral oil to the floor drain. It floats on the water and slows evaporation, so the trap stays full longer.

  5. Clean out the sludge

    Tool used: Enzyme drain cleaner

    Pull hair and gunk from the drain basket with gloves. An enzyme drain cleaner can break down slime overnight. Do not pour bleach together with other cleaners.

    Enzyme drain cleaner (illustration)
    Illustration

    Tool for this step

    Enzyme drain cleaner · Breaks down slime that gives off a sulfur smell

    Check price on Amazon

  6. Fit a trap seal

    Tool used: Floor drain trap seal

    If the smell returns within weeks, remove the cover, measure the pipe and press a trap seal into the floor drain. Check it fits snugly before putting the cover back.

    Floor drain trap seal (illustration)
    Illustration

    Tool for this step

    Floor drain trap seal · Measure the drain pipe before you buy

    Check price on Amazon

When free fixes aren't enough

If the smell is back within days of refilling, or it appears in several rooms, the trap is not the only problem. A blocked vent, a cracked pipe or a toilet seal failure needs a plumber. Call one right away if the smell is strong, you feel dizzy or sick, or drains gurgle or back up. Sewer gas contains hydrogen sulfide and methane, so open windows and leave the room if it is strong. If you smell rotten eggs and cannot trace it to a drain, treat it as a possible natural gas leak. Go outside, call your gas company from there and do not touch light switches.

Buy the fix

Still stuck after the free fixes? These are the products we would look at.

Best pick
Floor drain trap seal (one-way silicone insert) (illustration)
Illustration

Floor drain trap seal (one-way silicone insert)

Fixes: sewer gas rising from a drain whose trap has dried out

Look for: Fits your drain pipe diameter, so measure before you buy; Flexible silicone that opens for water and closes against gas; Removable so you can clean underneath.

Who it's for
Floor drains in basements, laundry rooms and bathrooms that you rarely use.
Honest downside
The smell comes from the toilet base, a vent or a broken pipe. A trap seal only blocks gas at the drain it sits in. Also skip it for hair-heavy shower drains, where a hair catcher fits better.
Check price on Amazon

As an Amazon Associate I earn from qualifying purchases. We don't show prices because they change, so use the button to see today's price.

How we picked: How we picked: this page is based on plumbing guides on drain traps and sewer gas, public safety advice on gas smells, and the fixes people report in reviews of trap seals. We do not test products by hand.

Bathroom smells like sewage or sulfur? Match the clue to the cause

  • Strongest at the floor drain: a dry trap. Refill it.
  • Strongest at the toilet base: a worn wax ring.
  • Only when you use hot water: the water heater.
  • Only at one sink: slime in the overflow.
  • Gurgling drains: a vent problem. Call a plumber.

The pro option: a trap primer

A trap primer is a small plumbing fitting that tops up a floor drain trap with a little water each time a nearby fixture runs. A plumber installs it. A trap seal is the do-it-yourself version of the same idea.

What to skip

Air fresheners and bleach down the drain

Sprays and plug-in scents cover the smell but leave the dry trap. Bleach does not refill a trap, and if it meets other cleaners in the pipe it can release dangerous gas.

Frequently asked questions

Why does my bathroom smell like sewer or sewage, and keep coming back?
The usual reasons are a dry trap in a drain you rarely use, slime in a drain or overflow, or a failed seal at the toilet base. If water refills do not work, suspect the vent or a damaged pipe.
Why does my bathroom smell like rotten eggs or sulfur?
Bacteria in a slimy drain give off a sulfur smell, and so can a water heater when only the hot water smells. If you cannot place it, treat it as a possible gas leak: go outside and call your gas company.
Why does my house smell like sewer or sewage?
Check basement and laundry floor drains first, then any bathroom you rarely use. A dry trap anywhere in the house can send gas up. If every room smells, or the main drain gurgles, call a plumber.
Why does my apartment smell like sewage?
A guest bathroom nobody uses is a common source, because its trap dries out. Refill every drain. If it persists, tell your landlord, since the cause may be a shared vent or stack.
Why does my bathroom smell like poop when the toilet is clean?
That is usually sewer gas, not waste. Look at the toilet base for a loose seal, then the floor drain and the sink overflow. A dry trap is the easiest thing to rule out first.

As an Amazon Associate I earn from qualifying purchases. Affiliate disclosure

Floor drain trap seal (one-way silicone insert) (illustration)
Illustration

Our pick: Floor drain trap seal (one-way silicone insert)

Check price