If your drains look clear but the house smells like sewage, the problem is almost always one of four things: a dry P-trap, a cracked or blocked vent stack, a partial blockage fermenting gas, or a failed wax ring. None of those cause visible backup, which is why the smell feels like a mystery. It isn’t.
The Most Likely Culprit: Dry P-Traps
Every drain has a P-trap, that curved section of pipe under the fixture. It holds a small plug of water that physically blocks sewer gas from rising into the room. When a fixture sits unused for a few weeks (a guest bathroom, a basement floor drain, a bar sink), that water evaporates. The seal is gone and you get a direct line from the sewer into your living space.
This is the easiest cause to rule out before calling anyone. Run water through every fixture you haven’t used recently, including floor drains. Pour about a quart down each one and wait 10 minutes. If the smell fades, that was it. For floor drains that dry out repeatedly, a plumber can install a trap primer or a mechanical trap seal so it doesn’t keep happening.
Vent Stack Problems
Your drain system vents through a pipe that runs up through the roof. It does two things: lets sewer gas escape outside, and lets air into the drainpipes so water flows freely. When that vent is cracked, corroded (common in older homes with cast iron), or clogged with debris (leaves, a bird nest, ice in winter), gas that should vent outside gets pushed back into the house instead.
A cracked vent is hard to spot because the crack is often inside a wall or up on the roof. A clogged vent sometimes shows as slow drains throughout the house, not just in one spot, because restricted airflow affects the whole system. Diagnosing and fixing a vent problem requires a licensed plumber. It’s not a homeowner repair.
Partial Blockages and Trapped Organic Matter
A full blockage backs up water. A partial one is subtler. Grease, hair, and soap can build up in a pipe to the point where organic matter starts decomposing inside it and producing hydrogen sulfide, the rotten-egg component of sewer smell. The drain still runs, just slower than it should.
If you notice any drains running slower than usual, that’s worth mentioning when you call a plumber. They can clear partial blockages with a drain snake or hydro-jetting, depending on where the buildup is.
Wax Ring Under a Toilet
The wax ring seals the base of each toilet to the drain flange. It fails over time, or if the toilet has any rocking movement, which slowly destroys the seal. A failed wax ring lets sewer gas seep up around the toilet base. The toilet still flushes fine. There’s no backup. The smell is often worse right after flushing because the flush pressure pushes gas through the gap.
You can check for this: push gently on the sides of the toilet base. Any rocking is a sign the wax ring is compromised. Tell your plumber that when you call. Replacing a wax ring is a standard repair.
What a Plumber Checks
When they arrive, they’ll typically start with a walkthrough: checking all fixtures for water in the traps, testing drain flow, and looking for any visible cracks or loose connections under sinks. For vent stack issues, they use a smoke test, pumping non-toxic smoke into the drain system and watching for where it escapes inside the house. That reveals cracks, failed seals, or open cleanouts that aren’t obvious otherwise.
If they suspect a partial blockage or a crack deeper in the system, they’ll send a camera down through a cleanout access point to see inside the pipe.
What You Can Do Before Calling
Running water through dry traps is safe and worth trying first. Beyond that, note a few things to share with the plumber:
- Whether any drains run slower than they used to.
- Which rooms smell worse and when (right after a flush? only in the morning? all day?).
- Whether any toilet base has even slight rocking.
What to skip: pouring chemical drain cleaners into a slow drain (they don’t fix root causes and can damage pipes), opening cleanout caps yourself, or getting on the roof to probe the vent stack.
When to Call a Licensed Plumber
If running water through all fixtures doesn’t clear the smell within a day, call a licensed plumber. Same goes for slow drains anywhere in the house, multiple rooms smelling, or a toilet that rocks. Sewer gas contains hydrogen sulfide, which causes health problems at lower concentrations and is toxic at high ones. Methane is also present and is flammable. This isn’t something to sit on.
Before anyone starts work, verify their license at cslb.ca.gov. California requires a contractor’s license for all plumbing work. Any contractor who hesitates when you ask for their number is a red flag.
The causes here are all fixable. A competent plumber can usually identify which one it is in a single visit.