That smell is sewer gas, and it is getting into your bathroom through a path that is supposed to be sealed. The good news: the most common cause takes thirty seconds and costs nothing to fix. The bad news: a few of the causes are serious, and sewer gas is not something to live with.
Here is how to tell them apart.
The quick answer
If the smell is in a bathroom you do not use often — a guest bath, an upstairs bathroom, a spare shower — go run the water in every drain for about 30 seconds. Then wait a day.
That fixes it most of the time. Here is why.
1. A dry P-trap — by far the most common cause
Look under your sink. That U-shaped bend in the pipe is a P-trap, and it is not there by accident. It holds a small amount of water permanently, and that little plug of water is the only thing standing between your bathroom and the open sewer.
Water blocks gas. That is the whole mechanism. It is beautifully simple, and it has exactly one weakness: water evaporates.
In a drain nobody uses, the trap slowly dries out. Once it is empty, there is nothing left to block anything, and the sewer comes up through the pipe and into the room.
The fix is free. Run the tap for 30 seconds. Pour a couple of cups of water down any floor drain or unused shower. The trap refills, the seal comes back, the smell goes away.
Leaving town for a few weeks? Run every faucet before you go, and pour a tablespoon of cooking oil into rarely-used drains. It floats on the water and slows evaporation.
2. A blocked or damaged vent stack
Your plumbing has to breathe. A pipe runs up through your roof — the vent stack — and it does two jobs: it lets sewer gas escape upward and outside, and it lets air in behind draining water so the drain does not siphon your P-traps dry.
Block that pipe and both jobs fail at once. The gas has nowhere to go but back into the house, and draining water starts pulling the water out of your traps.
Vents get blocked by bird nests, leaves, a stray tennis ball, or ice in a hard freeze.
Tell-tale sign: your drains gurgle. If the toilet burps when the washing machine empties, or a sink glugs when you drain the tub, your plumbing is struggling to breathe. Treat that as a vent problem until proven otherwise.
This one is not a DIY job — it is on your roof, and diagnosing it properly means looking at the whole system.
3. A failed toilet wax ring
Your toilet sits on a wax seal pressed against the floor flange. When that wax ring fails — and eventually they do — you get a small, permanent gap between your toilet and the sewer pipe.
Tell-tale signs: the smell is strongest right at the toilet. The toilet rocks slightly when you sit down. The floor around the base feels soft or looks discolored.
Do not ignore a rocking toilet. Every rock breaks the seal a little more, and eventually you are not just smelling sewage — you are leaking it under your floor.
4. A cracked or broken sewer line
If the pipe itself is damaged, gas escapes wherever the crack is and works its way up through the ground, the slab, or a wall cavity.
Tell-tale signs: the smell is not confined to the bathroom. You smell it in the yard, or near an exterior wall, or it comes and goes with the weather. Drains across the whole house are slow — not just one.
In older North Valley homes this is common enough that we look for it early. Houses built before roughly 1975 often have clay sewer laterals, and clay cracks. Tree roots find the cracks, force them wider, and now you have a blockage and an escape route for gas. In Chico’s older neighborhoods, where the street trees are beautiful and enormous, this is one of the most common calls we run.
A camera down the line settles it in about ten minutes.
5. Biofilm buildup in the drain
Sometimes it is not sewer gas at all. It is the drain itself.
Hair, soap scum, toothpaste and skin cells build a slimy layer — biofilm — on the inside of the pipe just below the drain opening. Bacteria live in it, and what they give off smells rotten, sulfurous, sometimes genuinely sewage-like.
Tell-tale sign: the smell is strongest right at the drain opening, and gets worse when you run hot water.
Do not reach for chemical drain cleaner. It is caustic, it does a poor job on biofilm, and over time it damages your pipes. We pull out plenty of lines that were killed by the stuff people poured in to save them. Scrub what you can reach, flush with hot water, and if it keeps coming back the pipe needs a proper cleaning.
6. A blocked or slow main drain
When the main line is partly blocked, waste sits in the pipe instead of moving out — and sitting waste produces gas that pushes back up toward the house.
Tell-tale signs: multiple fixtures are slow at once. The lowest drains in the house back up first — a downstairs shower, a floor drain. You hear gurgling in one fixture when you use another.
Multiple slow drains means the problem is not under your sink. It is downstream, in the line every fixture shares.
7. A forgotten floor drain
Same dry-trap problem as #1, but in a drain people forget exists: a laundry-room floor drain, a garage drain, an old drain hidden under a box in the corner.
Pour a bucket of water down every drain you can find. It is the cheapest thing you will do all year.
Is sewer gas actually dangerous?
Sewer gas is mostly hydrogen sulfide, methane, ammonia and carbon dioxide.
At the small concentrations you get from a dried-out P-trap, it is unpleasant rather than dangerous — headaches, nausea, irritated eyes.
At higher concentrations it is genuinely hazardous, and methane is flammable. Treat a strong, persistent sewer smell as something to fix now, not later. If the smell is strong and you feel dizzy or nauseated, open the windows and get out of the house.
One thing worth knowing: hydrogen sulfide deadens your sense of smell. So “it seems less bad now” is not reliable evidence that it is getting better.
What to do, in order
- Run every faucet and pour water down every drain. Wait a day. This solves it more often than anything else, and it is free.
- Listen for gurgling. Gurgling points at a vent or main-line problem — not a trap.
- Check whether the toilet rocks. If it does, the wax ring is going.
- Notice where the smell actually is. At the drain? Biofilm. At the toilet? Wax ring. All over the house or out in the yard? The line itself.
- Still there? Call a plumber. By that point you have ruled out everything free.
When to call us
We are not going to pretend a dry P-trap needs a professional. It does not. Go run the tap, and we hope that is the end of it.
But if you have run the water and the smell came back — or your drains gurgle, or your toilet rocks, or you can smell it out in the yard — that is a real problem, and it will not fix itself.
We will put a camera down the line and show you what is actually happening under your house, on the screen, while you stand there. Then we quote the repair you actually need, not the one we guessed at.
Gallagher’s has kept the North Valley comfortable since 1989. If your house smells like a sewer, let’s fix it.
Frequently asked questions
Why does my bathroom smell like sewer only sometimes?
Usually a partially dry P-trap, or a venting problem that only shows up when another fixture drains. Intermittent smells often track with the wind, the weather, or which appliances are running.
Why does my bathroom smell like sewer at night?
Houses cool and air pressure shifts overnight, which can pull gas in through a weak trap seal. It is the same underlying problem — the smell just becomes noticeable when the house is quiet.
Can a dry P-trap really cause a strong smell?
Yes. The trap is the only barrier there is. Once it is empty you have an open pipe to the sewer, and a single unused guest bathroom can make an entire hallway smell.
Will bleach fix a sewer smell?
No. It masks the odor briefly without fixing the cause, and it does nothing at all for a dry trap or a cracked line. Never mix it with other drain cleaners.
How much does it cost to fix a sewer smell?
If it is a dry trap: nothing. If it is a wax ring, it is a modest repair. If it is a cracked or root-filled sewer line, it depends entirely on what the camera finds — which is exactly why we scope it before we quote it.
Do I need a camera inspection?
Only if the free fixes did not work. If they did not, then yes. Guessing at a sewer line is how people end up paying for the wrong repair.
