What Causes a Sewer Smell in the Basement? (And Why Ignoring It Is a Mistake)

What Causes a Sewer Smell in the Basement? (And Why Ignoring It Is a Mistake)

You walk into your basement…And something smells off.

Not just musty. Not just damp.

That sharp, rotten egg, sewage-like odor that immediately makes you wrinkle your nose and wonder:

“Is this dangerous?”“Is something leaking?” “Why does it smell like a sewer down here?”

You’re not alone — and no, this is not normal.

Let’s break down exactly what causes sewer smells in basements, how to identify the source, and what you should (and shouldn’t) do next.

First: What That Sewer Smell Actually Is

That odor is usually sewer gas.

Sewer gas is a mixture of gases produced by decomposing waste in your sewer system. It commonly includes:

• Hydrogen sulfide (that rotten egg smell)

• Methane

• Ammonia

While small amounts can escape briefly without harm, persistent sewer smells are a warning sign that something in your plumbing system is failing.

And the basement is usually the first place it shows up.

Why?

Because that’s where your home’s main drains, traps, and sewer connections live.

The Most Common Causes of Sewer Smell in the Basement

Let’s walk through the real reasons — not guesses, not myths — actual causes we find in homes every week.

1. A Dry or Empty Floor Drain Trap

This is the #1 cause of basement sewer smells.

Every drain has a P-trap — a curved section of pipe designed to hold water and block sewer gas from coming back up.

But in basements?

Floor drains often go unused for months or years.

When the water evaporates, the trap dries out — and sewer gas has a straight path into your home.

Signs this is the problem:

• Smell is strongest near a floor drain

• Odor comes and goes

• Basement drain rarely gets water

Quick test:

Pour a gallon of water into the drain.If the smell disappears within minutes — you found your issue.

2. Cracked, Broken, or Missing Drain Pipes

Older homes — especially in Pittsburgh — often have:

• Cast iron drains

• Clay sewer lines

• Rusted or deteriorated piping

Over time, these materials crack, flake, or collapse, allowing sewer gas to leak directly into the basement air.

Sometimes the pipe isn’t leaking water — just gas — which makes this harder to spot without experience.

Red flags include:

• Rust stains on pipes

• Flaking metal

• Damp spots near drains

• Smell that never fully goes away

3. A Failed Wax Ring on a Basement Toilet

Yes — even a toilet you “never use” can be the culprit.

The wax ring seals the toilet to the drain below. If it dries out, cracks, or shifts, sewer gas escapes every time pressure changes in the system.

Common clues:

• Odor near the base of the toilet

• Smell after flushing another fixture upstairs

• Toilet rocks slightly when touched

4. Sewer Line Partial Blockage or Backup

When your main sewer line starts to clog — grease, roots, scale buildup — sewer gas gets trapped and redirected.

Instead of venting safely through the roof, it looks for the lowest exit point.

That’s usually your basement.

You might notice:

• Gurgling sounds

• Slow drains

• Floor drain bubbling

• Smell that worsens after laundry or showers
  

This is often the early warning sign before a full backup.

5. Improper or Broken Venting

Your plumbing system relies on vent pipes to release sewer gas safely outside.

If a vent is:

• Broken

• Disconnected

• Improperly installed

• Blocked by debris or animals

Gas can get forced back into the home instead.

This issue is especially common after renovations or DIY plumbing work.

6. Sewer Gas Leaking From Behind Walls or Under the Slab

This is the one homeowners never expect.

A cracked underground pipe, broken house trap, or failed joint under the concrete slab can leak gas long before it leaks water.

Because gas rises, it collects in basements first.

This is where professional testing becomes critical.

Is Sewer Gas Dangerous?

Short answer: Yes — it can be.

While occasional odors aren’t an emergency, long-term exposure can cause:

• Headaches

• Nausea

• Eye and throat irritation

• Dizziness

• Fatigue

And methane is flammable in high concentrations.

If the smell is persistent, strong, or worsening — it should never be ignored.

Why DIY Fixes Often Don’t Work

Homeowners often try:

• Air fresheners

• Dehumidifiers

• Fans

• Drain cleaners

These don’t fix the source.

At best, they mask the smell.At worst, they delay a real repair until damage becomes expensive.

Sewer odors are a plumbing system problem, not an air problem.

How Professionals Actually Find the Source

At Revelation Plumbing, we don’t guess.

We use proven diagnostic methods like:

smoke pouring out of a compromised drain line

• Smoke testing to force non-toxic smoke through the system and reveal leaks

• Camera inspections to see inside sewer lines

• Trap and vent evaluations

• Pressure testing when needed

This allows us to pinpoint the problem — not replace parts blindly.

How We Fix Sewer Smells for Good

Depending on the cause, solutions may include:

• Refilling or repairing traps

• Replacing damaged drain sections

• Sealing failed joints

• Repairing or replacing the house trap

• Clearing blockages with hydro jetting

• Correcting venting issues

• Trenchless sewer repairs when possible

Our goal is always the same:

Fix it once.

Fix it right.

Never smell it again.

Should You Call a Professional When You Smell Sewer Gas In Your Basement?

You should call immediately if:

• The smell lasts more than a day

• It keeps coming back

• You notice gurgling or slow drains

• The odor worsens after water use

• Anyone in the home feels sick

Sewer smells are your plumbing system asking for help.

Your Basement Shouldn’t Smell Like a Sewer

A sewer smell is never “just part of owning a house.”

It’s a warning.

A signal.

A problem that gets worse — not better — with time.

If your basement smells like sewage, rotten eggs, or sulfur, there is a reason.

And once you find it?

Relief is immediate.