Sewer Smell in the House: The Six Causes, Easiest to Hardest
Your drain system is supposed to be sealed. If you can smell it, something has stopped sealing — and two of the six causes you can fix yourself in a minute.
August 2, 2026 · 7 min read
A properly functioning drain system is sealed from the living space at every point. Traps hold water, seals hold at the floor, and vents carry gas up through the roof. If you can smell sewer inside your house, one of those has stopped doing its job.
The good news is that the two most common causes cost nothing to fix and take about a minute. Work through these in order — there is no sense calling anyone before you have ruled out the easy ones.
1. A dried-out P-trap
Every fixture has a U-shaped bend below it that holds a small amount of water. That water is the seal — it physically blocks gas from coming up the drain. If a fixture goes unused for long enough, the water evaporates and the seal is gone.
This is the single most common cause of a mystery sewer smell, and it shows up in predictable places: a guest bathroom nobody uses, a basement or garage floor drain, a laundry standpipe, a wet bar sink, a shower in a spare room. In our summers, evaporation happens faster than people expect.
The fix: run water down every drain in the house for thirty seconds, including the ones you never use. Pour a bucket of water into any floor drain. For fixtures that will stay unused, a small amount of mineral oil on top of the trap water slows evaporation considerably.
2. Biofilm in the drain
Not technically sewer gas, but it smells like it and it accounts for a large share of the calls. Organic material coats the inside of the drain and the underside of the stopper, particularly in bathroom sinks and kitchen drains, and the bacterial growth on it produces a genuinely foul smell.
The tell is that the smell is worse right at one fixture and improves as you move away. The fix is mechanical: pull the stopper out entirely and clean it — the underside is usually the culprit and it is worse than most people expect. Then clean inside the drain opening as far as you can reach with a brush. Enzyme drain treatments help maintain it afterward. Bleach mostly does not, because it runs straight past the film rather than removing it.
3. A failed toilet wax seal
The toilet sits on a flange in the floor with a wax ring between them making the seal. When that ring fails — from age, or because the toilet has been rocking — gas escapes at floor level and water works into the subfloor.
- The smell is strongest at floor level near the toilet, and often noticeable when you first walk into the room.
- The toilet rocks even slightly when you shift weight on the seat.
- There is discolouration, softness or staining on the floor around the base.
- Caulk around the base has cracked or separated.
Do not just shim it and re-caulk. Sealing around the base traps the water underneath rather than stopping it. The toilet needs to come up so the flange and the subfloor beneath can be inspected — and if the flange is broken or the wood has softened, that is the actual repair.
4. A blocked or improperly terminated vent
Your drain system needs air. Vent pipes running up through the roof let air in behind draining water and let gas out above the house. When a vent blocks — leaves, a bird nest, a wasp nest, or frost in an unusual cold snap — the system starts pulling air from wherever it can, and that means siphoning water out of your traps.
The signature symptom is gurgling. Flush a toilet and hear a nearby shower gurgle, or run a sink and hear a burble from the tub. That is air being pulled through a trap, and once the trap loses its water the smell follows. Drains that are slow across the whole house with no obvious blockage point the same direction.
5. A cracked or disconnected drain line
A break in a drain line inside a wall, under the slab, or in a crawl space lets gas escape directly into the structure. The smell tends to be persistent, hard to localise, and often stronger in one room or in a closet backing onto plumbing.
In older North Texas homes this is frequently cast iron that has corroded through along the bottom — the part you cannot see from a cleanout. A camera inspection finds it. Guessing does not, and neither does more cleaning.
6. A problem in the main line
A partially blocked, cracked or root-invaded main sewer line can push gas back up through the system. If the smell is accompanied by multiple slow drains, gurgling toilets, or anything backing up into a low fixture, this is where to look — and it is the one on the list with the most urgency behind it.
Work it in this order
- Run water down every drain in the house, including unused fixtures and floor drains. Wait a day.
- If it persists, pull and clean the stoppers on the fixtures nearest the smell.
- Check whether any toilet rocks, and look closely at the floor around each base.
- Listen for gurgling when you flush or drain. Gurgling points at the vents.
- If the smell is persistent, unlocalised, or paired with slow drains anywhere, stop guessing and get a camera in the line.
Most of these calls end at step one or two, and we would rather tell you that than come out and charge you for running a tap. If you have worked through the list and it is still there, we can smoke test the system to find exactly where it is escaping. Call (469) 338-5533.
Frequently asked
Why do I only smell sewer in the morning or when the AC runs?
Both are about air pressure. Overnight, with no water use, traps sit undisturbed and any weak seal has hours to let gas seep in. And a running air handler can slightly depressurise parts of a house, which pulls air — and anything in it — from wall cavities and drain penetrations into living space. A smell that appears when the HVAC kicks on often points at a leak in a wall rather than at a fixture.
Can I pour bleach down the drain to fix the smell?
It rarely works and it is not a great idea. Bleach runs straight through and past the biofilm coating the pipe wall without removing it, so the smell returns within days. It also produces hazardous fumes if it meets other cleaning products in the trap. Mechanical cleaning — pulling the stopper and scrubbing — plus an enzyme treatment is what actually addresses it.
What is a smoke test?
A diagnostic where non-toxic smoke is introduced into the drain system under slight pressure. Because the system should be sealed, the smoke escapes only where there is a fault — a cracked line, a failed seal, a bad vent connection — and it shows visibly. It is the fastest way to find a leak inside a wall or under a slab without opening anything up.
Is a sewer smell outside the house also a problem?
Sometimes, sometimes not. A faint smell near a roof vent on a still day is normal — that is the system doing its job. A persistent smell in the yard, especially over the line running from the house to the street, or paired with soggy ground or a stripe of unusually green grass, points to a break in the buried lateral. That one is worth a camera inspection.