A sewer smell and a natural gas smell are two different problems, and only one of them means you leave the house. Natural gas is given a rotten-egg smell on purpose, so if you smell that, or you aren't sure which smell you have, go outside first, then call 911 and your gas utility.
If it's a sewer smell and not gas, a common cause in a bathroom is a drain that has dried out. Run water in any drain you rarely use and see if the smell clears. If it doesn't, or it comes back, the cause is in the vents or the lines, and that's a job for a licensed plumber.
What if it smells like gas, not sewage?
Utilities add that smell to natural gas so people notice a leak. Many people describe it as rotten eggs, which is exactly why it gets mistaken for sewer gas.
If you smell gas, leave the house right away. Do not flip switches, use a lighter, or use a phone inside. From outside, call 911 and your gas utility. Do not look for the leak yourself.
Your gas bill shows who your utility is. In the Valley that is often Southwest Gas, and in parts of Mesa it is the City of Mesa's own gas utility. Once the utility says the house is safe, a licensed plumber can check the appliance or gas line. Never repair gas lines yourself.
If you can't tell which smell you have, treat it as gas and go outside first. Stepping out costs you a few minutes. Staying in to figure it out isn't worth the risk. Our plumbing emergency guide covers what else can't wait.
Could a dry P-trap cause the smell?
Yes, and it's the first thing to check. Every sink, tub, shower, and floor drain has a P-trap, a curved section of pipe that holds a small amount of water. That water is a seal. It keeps gas from the sewer line out of your house.
If a drain sits unused, the water in the trap can dry up. That can happen in a guest bathroom, a laundry room floor drain, or a spare shower nobody uses. With the seal gone, the smell comes up. Traps in drains you rarely use are the ones most likely to dry out, so start with those.
The one safe fix is simple:
- Run water in every unused drain in the house, including floor drains and guest baths.
- Let it run long enough to refill the trap.
- Wait a bit and see if the smell fades.
That's as far as you should take it. Don't take pipes apart, and don't mix cleaning products to cover the smell. Mixing cleaners can make dangerous fumes.
Is sewer gas dangerous?
It can be. Sewer gas is a mix of gases that comes from waste breaking down. The CDC's Agency for Toxic Substances and Disease Registry (ATSDR) and OSHA both list hydrogen sulfide, the rotten-egg part of the smell, as a hazard, and OSHA notes that it can dull your sense of smell at higher levels. So your nose isn't a reliable alarm.
We're plumbers, not doctors, so we'll keep this practical. Don't shrug off a faint smell that keeps coming back. And a strong smell, or a smell in more than one room, isn't something to live with while you wait and see.
If anyone in the house feels sick or dizzy while the smell is strong, go outside to fresh air and get medical help. Then call a plumber.
Could a blocked roof vent or a bad line cause it?
Yes. If the trap is full and the smell stays, these are the usual suspects. Every one of them needs a licensed plumber.
- A blocked or damaged plumbing vent. Your drains connect to vent pipes that go through the roof. They let air in so water can drain, and they let sewer gas out above the house. When a vent is blocked, drains can gurgle, and traps can lose their seal. Please don't climb on the roof to check. Falls are a real danger, and the fix isn't a DIY job.
- A leaking toilet seal. A smell right at the base of one toilet, sometimes with a toilet that rocks, can point to the seal under it.
- A partly blocked or damaged sewer line. Slow drains in more than one room, gurgling, or a smell in the yard as well as the house point to the main line. A camera inspection shows what's going on inside the pipe. Our sewer line service page has more.
- A buildup in a drain. Grime on the walls of a drain can smell without any gas at all. A plumber can clear the line. Our drain cleaning page explains what that involves.
Clues help the plumber. Note which room smells, when it started, and whether it's worse after you run water. Write down whether it's strongest after a shower or first thing in the morning, and tell the plumber if anyone has felt sick near the smell.
When to call a licensed plumber
Call when the smell doesn't clear after you run water in unused drains, when it's strong, when it's in more than one room, or when drains gurgle or back up. Sewage backing up inside the house is an emergency, so stop using water and call right away.
A plumber will track down where the smell gets into the house and tell you what the fix involves. Our line is answered 24/7, and a real person usually picks up. For anything urgent, use our emergency plumber line. And if the smell might be gas, the order never changes: outside first, the gas utility second, then us.
Common Questions
Yes, it is a common cause. A P-trap is the curved pipe under a drain that holds water to block gas. If a drain goes unused, the water can dry out and let the smell up.
It can be. The CDC's ATSDR and OSHA both list hydrogen sulfide, the rotten-egg part of the smell, as a hazard. A strong smell that does not clear up needs a licensed plumber.
Leave the house right away and call 911 and your gas utility from outside. Do not flip switches, use a lighter, or look for the leak yourself.
Yes, a blocked or damaged plumbing vent can let gas into the house or cause drains to gurgle. Vents are on the roof, so leave this to a licensed plumber.
