Plumbing
Sewer gas odor diagnosis and repair field guide for plumbers
Find where the barrier failed: check the dry trap first, then the wax ring and the vent, then smoke test for the hidden leak, and tell sewer gas from drain biofilm.
Direct answer
Sewer gas odor is the rotten-egg or sewage smell that gets into a building when the barrier between the drainage system and the room fails. The most common cause is a dry trap, so run water down every fixture first. Confirm the source, then smoke test for a hidden leak. Verify any repair against the adopted code.
Key takeaways
- A dry trap is the most common cause of sewer gas odor; pour water into every fixture first, and if the smell stops the trap was dry.
- Trap seals run commonly 2 in to 4 in deep and evaporate about an inch per week in unused fixtures, faster in dry heated air.
- Hydrogen sulfide deadens the sense of smell as it climbs, so a strong odor fading in a closed space means ventilate and leave, not relief.
- Biofilm smells worse when water runs while a trap problem improves with running water; clean the drain and overflow before blaming the sewer.
- Smoke test the DWV to locate hidden pipe breaks before opening walls, and notify occupants and the fire department first.
Sewer gas odor and how you hunt it
Sewer gas odor is the foul, rotten-egg or raw-sewage smell that reaches the living space when the barrier between the drainage system and the room has failed somewhere. The drainage pipes are full of gas by design, kept moving by the vents and held back by the water seal in every trap. When you smell sewer gas, that barrier has a break in it, and the whole job is finding where.
The break is almost always one of a short list, and they rank by how often they bite. A trap seal that went dry or got siphoned out. A wax ring under a toilet that failed. A vent that is blocked, broken, or terminating in the wrong place. A cracked drain pipe in a wall or under the slab. A loose cleanout plug. The smell is the only symptom most of these share, so you do not chase the smell. You hunt the break in order, from cheapest to find to hardest.
What a trap is and how its seal works is covered in the plumbing traps and trap seal guide, and how the vents protect that seal is in the DWV and venting guide. This one is the diagnosis: how to read the smell, where to look first, and how to prove the source before you open a wall.
What sewer gas is and the hazard
Sewer gas is not one gas. It is the mix that builds in any drainage system as waste breaks down: hydrogen sulfide, methane, carbon dioxide, ammonia, and assorted other products of decomposition. The rotten-egg smell is hydrogen sulfide, and it is the one that does real harm.
At low concentration hydrogen sulfide stinks and gives people headaches, nausea, and burning eyes and throat. The dangerous part is what it does higher up. It deadens the sense of smell, so the warning fades exactly as the level climbs, and at high concentration it is toxic and can drop a person fast in a confined space. A smell that suddenly goes away in a closed room is not good news.
Methane is the other half of the hazard. It is flammable and lighter than air, so it collects high in a closed space and turns a drainage leak into an ignition risk nobody is watching for. Treat a strong, persistent sewer smell in a basement, a pit, or any tight unventilated space as more than a nuisance. The smell is the part you can detect. The toxicity and the flammability are the parts you cannot.
The broken-barrier idea
Every fixture in a building has a trap, and the whole drainage system is meant to be sealed against the room except where it vents to the outside air above the roof. That is the model to carry in your head: a closed system with one set of intentional openings, the vents, all of them outside. Sewer gas inside the building means an unintended opening, a broken barrier, somewhere in that system.
The barrier fails in two ways. A water seal is lost, the few inches of water in a trap that block the gas, which the traps and trap seal guide covers. Or a physical seal is lost: a joint, a gasket, a wax ring, a cap, or the pipe wall itself. Both come down to the same thing. A path now runs from the gas side to the room side that should not be there.
This framing is the diagnosis. Do not ask what smells. Ask where the barrier broke. Once you think of it that way, the search has an order, because some barriers fail far more often than others, and the cheap, common ones get checked before you go looking for a cracked pipe behind drywall.
The number one cause: a dry trap
The single most common source of a sewer smell is a dry trap, and it is the first thing to check on every call, before anything else comes off the truck. A trap holds its seal only as long as it has water in the bend. A fixture that does not get used loses that water to evaporation, an inch or so a week, faster in a heated building with dry winter air, until the bend is empty and the gas comes straight up an open drain.
The usual suspects are the fixtures nobody runs. The floor drain in a basement or mechanical room. The guest bathroom used twice a year. The tub or shower in a spare bath. A bar sink, a laundry standpipe, a seasonal cabin closed up for months. A house shut for a season comes back smelling of sewer from every dry trap at once, and the owner is sure the plumbing failed when it only needed water.
The tell is simple and it doubles as the fix: pour water into the suspect fixture and see if the smell stops. If it does, the trap was dry. This is why you check it first. It costs a bucket of water and two minutes, and it clears the most common cause before you spend an afternoon hunting a leak that was never there.
Fixing a dry trap so it stays sealed
Running water refills a dry trap, but if the fixture is going to keep sitting unused, the seal will evaporate again, and pouring a bucket every few weeks is not a fix anyone keeps up. The permanent answer depends on how the drain is served.
A trap primer is the standard fix for a floor drain in service. It is a small valve that feeds a little water to the trap on a schedule, off the potable supply or off the discharge of a nearby fixture, so the seal never gets a chance to dry out. The primer types and their listings are in the traps and trap seal guide. The catch is that a primer needs a water line run to the trap, which is easy at rough-in and a real job to add in a finished building.
For a drain in a finished space where running a primer line is impractical, a barrier-type trap seal device drops into the drain itself and seals against gas with no water supply at all. And for a trap that will sit idle a long stretch, an old trick still works: float a little mineral oil on the water in the trap. The oil floats on top and slows the evaporation, so the water under it lasts longer. Verify which method the adopted code accepts for the fixture.
The siphoned trap: a seal that empties when the fixture drains
A trap can lose its seal even on a fixture used every day, if the trap is being siphoned. Instead of evaporating slowly, the seal gets pulled out down the drain the moment the fixture, or another fixture nearby, drains hard. The fixture works fine, drains fine, and still smells, because the water that should sit in the bend is gone the instant it is needed.
The cause is almost always a venting problem. An S-trap siphons its own seal by design, dropping vertically into the drain with no vent to break the vacuum. A trap arm run too long or pitched too steep does the same thing by accident. A failed air admittance valve that should be letting air in stays shut, so the draining fixture pulls its air through the trap instead. The mechanics of all of this are in the DWV and venting guide.
The tell is a gurgle. A fixture that gurgles when it drains, or when another fixture drains, is pulling air through a trap seal because it cannot get it from a vent, and that gurgle is the seal being dragged toward the drain. Pour water in and the smell stops for a day, then comes back the next time the fixture runs. When pouring water gives only a temporary fix, stop looking at the trap and start looking at the vent.
Why does the smell come from the base of the toilet?
A sewer smell that is strongest right at the base of a toilet, not at the bowl, points at the wax ring. The wax ring is the seal between the toilet's outlet and the closet flange in the floor, and it does the same job a trap does: keep gas in the pipe and out of the room. When it fails, gas leaks out around the base, and often a little water with it.
Wax rings fail a few ways. The bolts that hold the toilet to the flange work loose and the toilet rocks, which breaks the wax seal a little more with every use. The flange sits too low after a floor was tiled over, so the wax never compressed right to begin with. Or the wax simply aged out and lost its squeeze. The signs travel together: a smell at the base, a toilet that rocks when you lean on it, and staining or softness in the floor around it.
The fix is to pull the toilet, scrape the old wax, check that the flange is sound and at the right height, and set a new ring with the toilet snugged down evenly, not cranked until the china cracks. If the flange is broken or too low, fix the flange first, because a new ring on a bad flange fails again. A rocking toilet is not a comfort problem. It is a broken seal getting worse.
Vent problems: blocked, disconnected, or open inside
The vents let air into the drainage system and carry the gas up and out above the roof. When a vent fails, two things go wrong at once. Traps start siphoning because they cannot get air, and in some cases the gas finds its way into the building directly through a broken vent pipe.
A blocked vent is common and seasonal. Leaves, a bird or wasp nest, a ball, or frost closing the opening in hard-freeze country all choke the vent at the roof. The fixtures below it gurgle and lose their seals, and the smell shows up at several traps at once rather than just one. A disconnected or cracked vent pipe inside a wall or attic is worse, because now sewer gas vents straight into a wall cavity, a ceiling, or an attic and seeps into the rooms from there with no fixture involved at all. That one smells in odd places, away from any drain.
How vents are sized, run, and terminated is in the DWV and venting guide. For diagnosis, the pattern matters. A single dry or siphoned trap smells at one fixture, a blocked vent makes several fixtures gurgle and smell together, and a broken vent in a cavity smells where there is no plumbing. Read which pattern you have before you climb on the roof.
A cracked or broken DWV pipe
A cracked or broken drainage pipe leaks sewer gas at the break, and where the break is hidden, in a wall, above a ceiling, or under a slab, the smell shows up with no obvious source. This is the one that sends people in circles, because there is no fixture to check and no trap to refill. The gas is coming out of the pipe itself.
Old cast iron is the usual culprit. It rusts from the inside and the bottom of a horizontal run goes first, opening a channel that leaks gas and eventually waste. A failed joint, a cracked hub, a pipe a remodel cut into and never sealed, a drain abandoned and capped badly, all do the same. Under a slab, a broken drain can vent gas up through the soil and into the building through cracks in the concrete or around penetrations, which is why a slab-on-grade house can smell of sewer with every visible fixture working fine.
You do not find a hidden pipe break by smell alone, and you do not open a wall on a guess. This is what the smoke test is for, covered further down. When the easy checks come up clean and the smell has no fixture behind it, a broken pipe in the structure moves to the top of the list, and the smoke test is how you locate it before anything gets opened.
The cleanout plug and a failed air admittance valve
Two small parts leak sewer gas often enough to check early, because both are quick to find and quick to fix. The first is a cleanout plug. A cleanout is the capped access fitting on a drain line, and if the cap is missing, cracked, or left loose after a snaking, that opening is a direct hole into the drain. A cleanout inside the building, in a basement, a closet, or a floor, with a bad plug smells exactly like the open pipe it is. Hand-loose is not sealed. The plug needs to be in and snug.
The second is an air admittance valve. An AAV is the one-way valve found under island sinks and other spots where a vent to atmosphere is impractical, accepted under the IPC and restricted under the UPC, as the DWV and venting guide covers. It opens to let air in and is supposed to fall shut and seal against gas when flow stops. When the diaphragm fails or sticks open, it stops sealing and leaks sewer gas into the cabinet whenever the system is under pressure.
Open the cabinet under a smelly island sink and check the AAV before you tear into anything. A failed AAV is a five-minute swap once you find it, and a loose cleanout plug is a thirty-second one. Both get ruled out early for the same reason: they are cheap to check and they fail often.
The shower and floor drain
A shower or floor drain that smells gets misdiagnosed constantly, because two completely different problems present the same way, and the fix for one does nothing for the other. The first is a dry or partly dry trap, the same evaporation problem as any seldom-flooded drain, fixed by running water and, if it recurs, a primer or a barrier device.
The second is not sewer gas at all. It is biofilm, the slick black gunk of soap scum, skin, hair, and bacteria that builds up inside the drain, on the strainer, and under the cover. It smells foul, sour, and sewage-like, but it is rotting from the drain itself, not gas coming up from the sewer. Pour water and the smell stays, because the trap was never the problem.
The way to tell them apart is to clean the drain. Pull the cover, pull the strainer, and scrub the inside of the drain body and the top of the trap with a brush and a degreaser or an enzyme cleaner. If the smell goes with the gunk, it was biofilm. If it survives a real cleaning and a full trap, the source is downstream and you keep diagnosing. Clean first on a shower drain, because more often than not the smell is in the drain you can reach, not the sewer beyond it.
Biofilm that smells like sewage but is not sewer gas
This one earns its own warning, because it is the most common false alarm on a sewer-smell call and the easiest to waste a day on. A drain that has not been cleaned grows a film of bacteria feeding on everything that goes down it, and that film gives off a sour, rotten, sewage-like smell that people swear is coming from the sewer. It is not. It is the drain.
It shows up worst at the overflow openings on a lavatory or a tub, which never get cleaned and hold a thick mat of organic gunk that smells the moment warm water runs past it. It shows up in garbage disposals, in shower drains, in any fixture that handles food, soap, or skin and does not get scrubbed. The smell is stronger when water runs, the opposite of a trap problem, where running water fixes it.
The fix is mechanical cleaning, not masking. Brush the drain and the overflow, flush with an enzyme drain cleaner that eats the film rather than a fragrance that hides it, and clean the disposal. Skip the chemical pour that perfumes the problem for a day. If a genuine cleaning kills the smell, you have saved the customer a needless leak hunt. If it does not, the barrier really is broken somewhere, and now you know to keep looking.
How do you find the source of a sewer smell?
Finding the source is a process of elimination worked cheapest first, not a guess at the most likely culprit. Start by pinning down the smell. Which room, which fixture, and when, because a smell that is constant points differently than one that only shows when a fixture drains or when the wind blows a certain way. Constant and everywhere suggests a vent or a hidden pipe. At one fixture suggests that fixture's trap or seal.
Then check the easy, common breaks in order. Run water in every trap, especially the seldom-used ones, since a dry trap is the most likely cause and the cheapest to clear. Check the toilets for a smell at the base and a rock that points at a wax ring. Check accessible cleanout plugs and any AAVs. Clean the drains and overflows that could be biofilm. Most calls end here, because most sewer smells are one of these.
If the easy checks come up clean, move to the vents: look for blockage at the roof terminal and for the gurgle pattern that says a trap is siphoning. Only when the smell has no fixture, no trap, and no visible source behind it do you reach for the smoke test to find a hidden leak. The order matters because each step is cheaper than the next, and jumping to the expensive test on a problem a bucket of water would have solved is how you lose money on a call.
The smoke test and the peppermint test
When the easy checks fail and the smell has no obvious fixture behind it, the smoke test is how you find a hidden leak without opening walls. A blower pushes a dense, visible, non-toxic smoke into the DWV system through a cleanout or a roof vent and pressurizes it. The smoke takes the same path the sewer gas does, so it puffs out wherever the barrier is broken: a cracked pipe in a wall, a failed joint, a dry trap, a bad wax ring, a disconnected vent. You watch for where the smoke shows and you have your leak. It is the quickest and cheapest way to locate a breach, which is why it is the standard tool for the smell that will not give up its source.
A few cautions go with it. Tell the occupants and, where it applies, the fire department or alarm company before you start, so a wisp of smoke from a vanity does not turn into a panic. Prime or cap the floor drains you are not testing so they do not just dump smoke as false positives. And read the smoke at the roof terminal too, because smoke that should rise freely from the vent and instead backs up into the building tells you the vent is blocked.
The peppermint test is the low-tech substitute where a smoke test cannot be run. One person pours a couple of ounces of peppermint oil into a roof vent and follows it with hot water, then stays outside, because the scent on their clothes will fool the test if they come back in. A second person works through the building sniffing for peppermint at traps, joints, and fixtures. Peppermint detected at any point means a leak there. It is cruder than smoke and slower, but it finds the same kind of hidden breach with nothing but oil and two people.
Where the vent lets out: terminals near windows and intakes
Sometimes the barrier is intact and the gas is still getting in, because the vent is doing its job in the wrong place. A vent terminal that ends too close to an openable window, a door, a fresh-air intake, or a rooftop air conditioner dumps its sewer gas right where the building draws air back in. The system is venting correctly. The problem is the gas going out one opening and being pulled straight back in another.
This shows up as a smell that comes and goes with the wind or with the HVAC running, and that gets blamed on the drains when the drains are fine. A vent added during a remodel without thought to where it lands, a window or a mini-split installed later near an existing vent, or a low vent on an addition next to a second-story window are the common setups. The code keeps vent terminals a set distance from openings and intakes for exactly this reason, and those clearances are in the DWV and venting guide.
The fix is to move the offending end: extend the vent up or over to clear the opening, or relocate the intake. Before you cut into any drain, check whether the smell tracks the wind and the air handler, because a termination problem dressed up as a leak will survive every smoke test you run.
Sewer gas safety
Most sewer-smell calls are a nuisance, a dry trap or some biofilm, and the gas is at a low concentration that stinks more than it threatens. The job is knowing when it is not that. Hydrogen sulfide is toxic at high concentration and deadens your sense of smell as it climbs, so a strong smell that suddenly weakens in a closed space is a reason to get out and ventilate, not a sign it is clearing. Methane in the same space is flammable and pools high.
The danger concentrates in confined, unventilated spaces: a basement, a crawlspace, a pit, a tank, a sump. Treat any of those with a strong, persistent sewer smell as a space that needs ventilation and, on a commercial or industrial job, gas monitoring before anyone works in it. A hydrogen sulfide or multi-gas detector is cheap next to the alternative, and confined-space rules apply where the space qualifies.
The blunt version: in a house with a faint smell from a guest bath, pour water and move on. In a tight space with a strong smell and no airflow, ventilate first and verify the air before you put your head in it. The smell is the warning. Respect it where it is strong.
Commercial buildings, multi-fixture sites, and data centers
On a commercial building the sewer-smell problem scales up and changes character. A large building has dozens of floor drains, floor sinks, mop basins, and mechanical-room drains that almost never see water, so dry traps are the default cause of a back-of-house or mechanical-room smell, and managing them is a maintenance program, not a one-off bucket. Every infrequently-used drain wants a trap primer at rough-in or a barrier device, and the ones that slip through get put on a route with someone pouring water on a schedule.
Commercial kitchens add grease. A grease interceptor overdue for cleaning, or a fixture upstream of it with a lost trap, smells like sewage from the fats, oils, and grease breaking down, and the fix is the cleaning schedule and intact traps, not air freshener. The grease interceptor itself is not a gas seal, as the traps and trap seal guide notes.
A data center or other critical facility raises the stakes without changing the rule. A dry trap in a room next to live equipment is a sewer-gas path into a space people work in, and hydrogen sulfide carries corrosion that is unwelcome around electronics. Keep the floor-drain trap-primer program documented and on a schedule, because on a critical building a forgotten dry trap becomes a callback that never should have happened.
What to document
A sewer-smell call that nobody wrote up is a call that comes back, because the next person starts the diagnosis over from zero. Record where the smell was, what you found, and what fixed it, so a recurrence has a history behind it instead of a fresh guess.
Capture the location of the smell and when it appeared, the cause you traced it to, the fixtures whose traps you refilled or primed, any wax ring, cleanout, AAV, or pipe you repaired, and whether a smoke or peppermint test was run and what it showed. On a commercial building, record the floor-drain locations and the primer or barrier device on each, with the maintenance schedule, so the dry-trap program has a route behind it.
| Smell location | Likely cause | What to check |
|---|---|---|
| At one seldom-used drain or fixture | Dry trap, evaporated seal | Pour water; if the smell stops, the trap was dry |
| At the base of a toilet | Failed wax ring or loose flange | Check for a rock and floor staining; pull and reset |
| At several fixtures, with gurgling | Blocked or undersized vent | Check the roof vent for blockage; look for siphoning |
| Where there is no plumbing | Broken vent or drain pipe in a cavity | Smoke test to locate the hidden break |
| Stronger when water runs | Drain biofilm, not sewer gas | Clean the drain and overflow, then recheck |
| Comes and goes with wind or HVAC | Vent terminating near a window or intake | Check the terminal clearance; extend or relocate |
| Constant, with no fixture source | Hidden pipe leak or loose cleanout plug | Check cleanouts; smoke test the system |
Common mistakes
- Not checking the dry trap first, the most common cause and the cheapest to clear, before hunting a leak.
- Confusing drain biofilm, which smells worse when water runs, with sewer gas coming up a trap.
- Overlooking a failed wax ring or a loose cleanout plug because the fixture itself drains fine.
- Masking the smell with bleach, gels, or fragrance instead of finding the broken barrier.
- Skipping the smoke or peppermint test and opening a wall on a guess at a hidden leak.
- Ignoring a blocked or badly terminated vent that siphons traps or feeds gas back into the building.
- Treating a strong smell in a confined space as a nuisance instead of ventilating and checking the air first.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
The framework lives in the two model plumbing codes, and the adopted edition with local amendments controls. The IPC, published by the ICC, and the UPC, published by IAPMO, both require every fixture to be trapped, every trap to be vented and protected against the loss of its seal, and every drain and vent to be sealed and tested. The trap-seal range, the trap-arm and venting rules that keep a seal from siphoning, the vent-termination clearances from windows and intakes, and the cleanout requirements all sit in those chapters. The section and table numbers shift between editions, so confirm them against the edition the jurisdiction enforces before citing one.
The smoke test and the peppermint test are long-standing methods for finding leaks and odor sources in a DWV system, and many jurisdictions recognize one or both for that purpose. The peppermint test is the common substitute where a smoke test cannot be run. Trap primers and barrier-type seal devices carry their own ASSE product standards, covered in the traps and trap seal guide, and the venting that protects every trap seal is in the DWV and venting guide.
Three things carry across every call regardless of code. Check the dry trap first, because it is the most common broken barrier and the cheapest to confirm. Hold the broken-barrier idea in your head, so you hunt the break instead of chasing the smell. And when the source is hidden, smoke test before you open anything. The standard that controls a given repair is the one the AHJ has adopted and enforces, so verify against that code and confirm with the inspector where it matters.
Units and terms
Sewer-gas work borrows its vocabulary from traps and venting, and the same part shows up under different names across a code book, a service ticket, and a supply-house counter.
The trap seal is measured in inches of depth. Gas concentration, where it gets measured on a commercial or confined-space job, is read in parts per million on a detector. The terms below are the ones a plumber and an inspector use to talk about the same parts without crossing wires.
- Sewer gas
- The mix of gases in a drainage system, including hydrogen sulfide, methane, and ammonia, that smells foul and is toxic and flammable at high concentration
- Hydrogen sulfide (H2S)
- The rotten-egg gas in sewer gas, toxic at high concentration and deadens the sense of smell as it climbs
- Trap seal
- The plug of water in a trap bend, commonly 2 in to 4 in deep, that blocks sewer gas from the room
- Dry trap
- A trap whose seal has evaporated or been siphoned away, leaving an open path for sewer gas
- Siphonage
- The loss of a trap seal pulled down the drain when a fixture drains without adequate venting
- Wax ring
- The seal between a toilet outlet and the closet flange; a failure leaks gas and water at the base
- Cleanout
- A capped access fitting on a drain line; a missing or loose plug is a direct opening for sewer gas
- AAV
- Air admittance valve, a one-way vent valve that should seal against gas when flow stops; a failed one leaks
- Biofilm
- The bacterial film of soap, skin, and food in a drain that smells like sewage but is the drain, not the sewer
- Smoke test
- A diagnostic that pressurizes the DWV system with visible smoke to reveal where the barrier is broken
FAQ
Why does my house smell like sewer gas?
A sewer smell means the barrier between the drainage system and the room has broken somewhere. The most common break is a dry trap on a seldom-used fixture, so pour water down every drain first. Other causes are a failed wax ring, a blocked or broken vent, drain biofilm, or a cracked pipe.
What is the most common cause of a sewer smell?
A dry trap is the most common cause. A fixture that sits unused loses its water seal to evaporation, leaving an open drain that passes sewer gas. Check it first by pouring water into the trap; if the smell stops, it was dry. Floor drains and guest baths are the usual offenders.
How do you find a sewer gas leak?
Work cheapest first. Pour water in every trap, check toilets for a base smell and a rock, check cleanouts and AAVs, and clean any biofilm. If the smell has no fixture behind it, smoke test the system: visible smoke pumped into the drains puffs out wherever the barrier is broken.
Is sewer gas dangerous?
Yes, at high concentration. The hydrogen sulfide in it is toxic and deadens your sense of smell as the level climbs, so a strong smell that fades in a closed room is a warning, not relief. The methane is flammable. In a basement, pit, or crawlspace, ventilate and check the air first.
Why does my toilet smell like sewer at the base?
A smell at the base of a toilet, rather than from the bowl, usually means a failed wax ring. The bolts loosen, the toilet rocks, and the seal breaks, leaking gas and often a little water. Pull the toilet, check the flange height, and set a new ring with the toilet snugged evenly.
Why does my floor drain or shower smell like sewage?
Two different problems look alike. The trap may have dried out, fixed by pouring water and, if it recurs, a primer. Or the drain is full of biofilm, the slick gunk of soap and hair that smells like sewage but is the drain itself. Clean the drain first; if the smell stays, keep diagnosing.
What is a plumbing smoke test?
A smoke test pumps dense, non-toxic visible smoke into the drain-waste-vent system and pressurizes it. The smoke escapes wherever sewer gas can, so it shows cracked pipes, failed joints, dry traps, and bad seals without opening walls. Tell occupants and the fire department first, and watch the roof vent for blockage.
Why does my drain smell worse when the water runs?
That points at biofilm, not sewer gas. A film of bacteria feeding on soap, skin, and food coats the drain and overflow and gives off the smell when warm water runs past it. A trap problem does the opposite, since running water fixes it. Scrub the drain and overflow and flush with an enzyme cleaner.
Can a blocked vent cause a sewer smell?
Yes. A vent blocked by leaves, a nest, or frost stops air from reaching the traps, so they gurgle and siphon dry, and several fixtures can smell at once. A broken vent inside a wall leaks gas straight into the cavity. Check the roof terminal and look for the gurgle pattern.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.