Sewer Smell in the House? Causes, From Dry Traps to Broken Drains

Few things are as unpleasant, or as worrying, as a sewer smell inside your home. Sometimes it is a simple fix: a dry drain trap or a loose toilet. Sometimes it points to something bigger, like a broken drain under the slab. This guide walks through the common causes from simplest to most serious, how to check each one yourself, and when to call for a drain test.

Why sewer smells get into homes

Your drain system is designed to keep sewer gases out. Every fixture has a trap, a bend in the pipe that holds water and blocks gases. Vent pipes through the roof let gases escape outdoors and keep air pressure balanced so traps do not get siphoned dry. Toilets seal to the drain with a wax ring or similar seal. When any part of that system fails, sewer gas can enter the house.

Cause 1: Dry traps

Fixtures that are rarely used, such as guest bathroom sinks and showers, floor drains, and laundry standpipes, can lose the water in their traps through evaporation, especially in hot Texas summers. Without water, the trap cannot block gases.

Check: Run water in every sink, tub, shower and floor drain for a minute. If the smell fades within a day, a dry trap was likely the cause. For floor drains you rarely use, pour in water every few weeks.

Cause 2: Toilet seal problems

If the wax ring or seal under a toilet fails, sewer gas can escape around the base. Signs include a smell near the toilet, a toilet that rocks, staining around the base or water seeping when flushed.

Check: Gently try to rock the toilet. Look for gaps or staining around the base. A plumber can reset the toilet with a new seal.

Cause 3: Vent problems

Plumbing vents can be blocked by debris, bird nests or, in rare cases, poorly installed fixtures. A blocked vent can cause gurgling, slow drains and siphoned traps that let gas in.

Check: Listen for gurgling when other fixtures drain. A plumber can inspect and clear vents.

Cause 4: Cracked or loose pipes in walls

A loose fitting or cracked drain pipe inside a wall can leak gas and, sometimes, water. The smell is often concentrated near one wall.

Cause 5: Drain leak under the slab

A broken drain under your slab can let sewer gas rise through cracks in the slab, around plumbing penetrations, and up through walls and flooring. It also lets wastewater soak the soil under your home.

Signs that point to a drain leak under the slab include:

• The smell is at floor level and does not go away after refilling traps and checking toilets.
• It is worse near bathrooms or in closets next to them.
• Slow drains or gurgling in several fixtures.
• Damp flooring or a musty smell near bathrooms, with a normal water meter test.
• Drain flies or roaches around floor drains or plumbing.
• Older home, especially one built before about 1980 with cast-iron drains.

Cause 6: Main sewer line problems

Problems in the main line leaving the house, such as roots, a collapse or a backup from the city side, can cause odors, slow drains throughout the house and backups in the lowest fixtures.

A step-by-step approach

1. Refill all traps, including floor drains and rarely used fixtures.
2. Check every toilet for rocking, gaps and staining.
3. Note where the smell is strongest and when, such as after showers or laundry.
4. Check for slow drains and gurgling.
5. Look for damp areas near bathrooms and the kitchen.
6. If the smell persists, call a plumber about a drain evaluation.

How drain leaks under the slab are found

The standard test is a hydrostatic test: the main drain is sealed at the cleanout with a test ball, the drain system is filled with water to slab level, and the level is watched. If it drops, a drain under the slab is leaking. Isolation testing finds which branch, and a camera inspection shows the damage and lets the plumber mark the spot on the floor.

Our slab leak detection overview explains how drain testing fits with supply-line testing, and our guide on drain leak signs covers what to watch for.

Track when and where

Smells that come and go are hard to pin down. Keep a short log for a week: when you notice the smell, which room, what had just happened (a shower, laundry, rain, the AC turning on), and how strong it was. Patterns help. A smell that appears after the washing machine drains points to the laundry standpipe or its branch. A smell strongest in the morning in a guest bath points to a dry trap. A smell that is always there at floor level near the same wall points to a drain under the slab or inside that wall. Share the log with your plumber.

Is sewer gas dangerous?

Sewer gas is a mixture that can include hydrogen sulfide, which causes the rotten-egg smell, methane and other gases. At the low levels typically found from a dry trap or minor leak, the main effects are odor and possibly headaches or nausea for sensitive people. Higher concentrations can be harmful. If the smell is strong, ventilate the home, and if anyone feels unwell, get fresh air and seek medical advice. If you ever smell gas that could be natural gas, which utilities add a rotten-egg odorant to, leave the house and call your gas utility from outside.

Rarely used bathrooms

Guest bathrooms and showers that sit unused for weeks are the most common source of mystery sewer smells. Make it a habit to run water in every fixture at least once a week, and pour a little water into any floor drains in laundry rooms or garages. A small amount of mineral oil on top of the water in a floor drain trap can slow evaporation.

What not to do

• Do not mask the smell with air fresheners and ignore it.
• Do not pour chemical drain cleaners into a drain you suspect is broken.
• Do not seal over floor drains or cleanouts.

Repair costs and options

Fixes range from free (refilling a trap) to modest (resetting a toilet) to more significant (repairing a drain under the slab). For under-slab drain leaks, a camera inspection helps decide between a spot repair, replacing a section, tunneling or lining. A licensed plumber can explain options once the problem is located.

The rotten-egg smell from hot water

Not every rotten-egg smell is sewer gas. If the smell appears only when you run hot water, the cause may be the water heater: a reaction between bacteria in the water and the tank's anode rod can produce hydrogen sulfide. This is common with well water and some softened water. A plumber can flush the heater and suggest a different anode type. Run cold water at the same faucet; if it smells fine, the heater is the likely source, not the drains.

Smells after rain or flooding

Some homeowners notice sewer smells after heavy rain. Rain can overwhelm municipal sewers, push air back up through drains, or saturate soil around a cracked drain so odors rise more readily. If the smell appears only during or after storms and goes away, it may be a municipal or venting issue. If it persists, test the drains.

A sewer smell is a common first sign of a cracked drain in older homes in Richardson, Temple and the established neighborhoods of Lewisville.

The bottom line

A sewer smell inside your home usually has a simple cause, like a dry trap or a failing toilet seal, and those are worth checking first. If the smell persists at floor level near bathrooms, especially in an older home or with slow drains and damp floors, a broken drain under the slab is a real possibility. A hydrostatic test and camera inspection find it. When you are ready, find your city to book a licensed Texas plumber, or browse our slab leak FAQs.