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Plumbing guide
Sewer Smell in the House: Causes and What to Do
A rotten-egg or sewage smell indoors is sewer gas coming through a path it should not. The cause is often simple, and sometimes it is a damaged line outside.
By the Urban Comfort Plumbing team ·
What sewer gas is, and when it is a danger
Sewer gas is a mix of gases produced as waste breaks down. It includes hydrogen sulfide, which smells like rotten eggs, and methane. Plumbing is designed to keep it out of the house with traps, which hold a plug of water, and with vents that carry gas up and out through the roof.
A faint smell is a nuisance and a sign of a plumbing fault. A strong smell, especially in a closed room, is more serious. Hydrogen sulfide in high levels is dangerous, and methane is flammable. If the smell is strong, dizziness or headaches show up, or you cannot find the source, open windows, leave the area and call for help.
Dry traps
The most common cause is a trap that has dried out. Every drain has a curved section of pipe that holds water as a seal. If a sink, tub, shower or floor drain is not used for a few weeks, the water evaporates and the seal is gone. Guest bathrooms and basement or garage floor drains are the usual victims.
The fix is to run water. Pour a quart of water down the drain, or run the tap for a minute. In a floor drain that is rarely used, a little mineral oil on top of the water slows evaporation. If the smell goes away within an hour or so of running water, the dry trap was the cause.
Toilets and wax seals
A toilet sits on a ring of wax or a rubber seal that connects it to the drain. If the seal fails, sewer gas can leak around the base, and the toilet may rock a little or show water at the base. A smell strongest near one toilet, especially with a loose or wobbly toilet, points to the seal.
Replacing the seal means pulling the toilet, cleaning the flange and setting the toilet on a new seal. If the flange itself is broken or sits too low, it needs repair. See toilet repair for what this involves.
Vents and floor drains
Plumbing vents let air in so that drains flow, and they carry gas out. A vent can clog with leaves, a bird nest or debris, or it can crack inside a wall. A blocked vent can make drains gurgle and push gas into the house through a trap. A smell with gurgling is a sign.
Floor drains deserve a look. A drain with a missing or cracked cover, or a trap that has lost its water, can smell. Check the cleanout caps too. A cap that is missing or loose on a cleanout lets gas into a crawlspace or the yard.
Damaged sewer lines
If the smell is strongest near the cleanout in the yard, in a garage or at a lowest-level drain, and especially if drains are slow in more than one fixture, the sewer line may be damaged. Cracks, a collapsed section or an offset joint let waste and gas escape underground. A wet or soggy patch in the lawn along the line is another sign.
A camera inspection shows the condition of the pipe. See sewer camera inspection for how it works, and sewer line repair for the repair options. Repeat clogs along with the smell are common, and drain cleaning is the first step for those.
What a plumber does to find the source
A plumber starts by asking where the smell is strongest and when it began. Then each trap is checked, the toilet seals are inspected and the vents are looked at from the roof if needed. Smoke testing pushes harmless, visible smoke through the drain system. Smoke that rises from a spot in the yard or the roof shows a break or a vent that is open where it should not be.
If the in-house checks come up empty, a camera run through the lateral shows cracks, sags and offsets. That is the step that separates a trap problem from a line problem, and it avoids guessing.
Smells that come and go
A smell that comes only after rain may point to a vent or a trap that is disturbed by pressure changes, or to a lateral that takes in groundwater. A smell that comes only when the dishwasher or washing machine drains points to a trap or a drain connection near that appliance. A smell that shows up only in one season may be tied to dry traps in rooms that stop being used.
Keep notes of when it appears and where it is strongest. Those details help a plumber far more than a general description.
What is different in St. Petersburg
In St. Petersburg, the owner is responsible for the private sewer lateral inside the property line under the city code, and a repair needs a licensed plumbing contractor and a permit. The city has offered a rebate program for private lateral work, and its terms change, so check with the city before you start.
The city has tied past sewage overflows to heavy rain seeping into cracked pipes. If the smell and slow drains follow heavy rain, mention it to the plumber, since it can point to a cracked lateral that lets groundwater in.
What you can do today
Run water in every drain in the house, including the ones you rarely use. Check that cleanout caps are on. Clean the drains with a brush and a flush of hot water. Open a window and use a fan if the smell lingers.
If the smell returns within a day or two, or you cannot find the source, call. A plumber can smoke-test the system, which pushes harmless smoke through the pipes and shows where it escapes, or run a camera through the line. Do not mask the smell with air fresheners, since it hides a problem that can get worse.
Common questions
Sewer smell questions
A dry trap is the most common cause. Run water in the sink, tub and shower, and check the toilet base for a failing seal.
A faint smell is mainly a nuisance. Strong or lingering odor can be harmful, so ventilate and call for help.
Yes, especially when drains are slow and the smell is strongest near the cleanout or in the yard. A camera inspection can confirm it.
Under the city code, the owner is responsible for the private lateral inside the property line. Ask the city about any rebate program.
Related services
Sewer Line Repair
Fix a cracked, rooted or sagging sewer pipe, from spot repair to a longer section.
Drain Cleaning & Unclogging
Clear kitchen, bath, laundry and main-line clogs and find out why they came back.
Sewer smell that will not go away?
Call and describe where it is strongest. The dispatcher will connect you with a local contractor.