Why does my house smell like sewer sometimes, then fine other times?
Quick Answer
A healthy drain system is airtight, so any sewer smell inside your home means gas is escaping somewhere it should not. Smells that come and go usually track with water use, weather, and pressure changes, which is why the problem seems to vanish right when the plumber shows up.
Where the smell comes from
The drain system under your home carries sewer gas as well as water, and it is designed to keep that gas sealed inside the pipes and vented above your roof. When cast iron corrodes, cracks open up at joints and thin spots in the pipe wall. Gas escapes through those openings into the soil under your slab and finds its way up into the house.
The on-again, off-again pattern is typical. Running water can temporarily seal small openings, dry weather and wet weather change how the soil breathes, and daily pressure swings in the system push gas out at some times and not others. An intermittent smell is not a false alarm. It is a leak that has not become constant yet.
A note on the simple causes first
Not every sewer smell is a failing pipe. A rarely used shower or floor drain can have a dried-out trap that lets gas straight in, and running water in it for a minute fixes that for free. If the smell persists or moves around the house, the trap is not your problem, and a camera inspection is the honest next step.
The Homeowner Takeaway
A sewer smell you notice only sometimes is still a sealed system leaking. Rule out a dry trap first, then get the pipes inspected before intermittent becomes constant.
