Sewer gas has a particular smell, rotten eggs with something musty behind it, and once it is in a house everyone wants it gone. The drain system is designed to keep it out with water-filled traps at every fixture and a vent to carry it above the roof, so a smell means a seal has failed or a pipe is open somewhere. Most of the causes are cheap and visible and need no camera at all. A few are under a floor or inside a wall, and for those the camera is the only way to find the open pipe without demolition.
Quick answer: check the dry traps first: floor drains, unused showers, basement sinks and any fixture that has not run in weeks. Then the toilet’s wax ring, the washing machine standpipe, cleanout caps and the roof vent. If all of those are sound and the smell persists, especially from under a floor or from a crawlspace, a camera inspection of the building drain finds the broken joint, the cracked pipe or the abandoned line that is venting gas into the structure. The camera cannot smell; it finds the opening.
The cheap causes, in order
A dry trap: a floor drain, a basement shower or a guest sink that has not run long enough for its water seal to evaporate. Pour water in; if the smell fades over a day, that was it. A toilet wax ring that has failed, letting gas out around the base; the toilet rocks or the floor is stained. A washing machine standpipe with no trap or a dried one. A cleanout cap that is loose or missing. A vent on the roof blocked by leaves or a nest, so gas cannot escape above the roof and comes out the traps instead. A biofilm-coated overflow or trap in a bathroom, which smells without any gas escaping; the biofilm guide covers that one. None of these needs a camera and all of them should be eliminated first.
When the camera is the tool
A smell that comes from a floor, a wall or a crawlspace rather than from a fixture. A smell in a room with no plumbing. A smell that is worse when the house is closed up or when the furnace runs, which means gas is being drawn from a cavity. A smell that persists after every trap has been filled and every fixture checked. These point at an open pipe inside the structure: a cracked cast iron drain under the slab, a separated joint in the crawlspace, an abandoned branch that was cut and never capped, or a vent that was disconnected in a wall. The camera from the main cleanout or the stack base inspects the building drain and its branches for the opening. The under-slab cast iron guide describes the most common finding.
What the camera shows
A crack or a hole in the pipe wall, usually on the top or side where gas escapes rather than the invert where water does. A separated joint with the gap open to the structure. A branch opening that leads nowhere, which is an abandoned line. A vent connection that ends in darkness. Soil or insulation visible through a gap. Dim the LEDs; cracks on the crown show as shadow lines at reduced power. Record the distance of every opening and, with a sonde, locate it from the floor above, because the smell will be strongest near the surface mark. The abandoned line guide covers the dead-end case.
What the camera cannot do
It cannot see a vent that was cut inside a wall, because the vent is above the drain and the camera is in the drain. It cannot find a failed wax ring, which is outside the pipe. It cannot detect gas. And it cannot inspect a branch too small for the head. For those, a smoke test is the tool: smoke blown into the drain system escapes wherever gas does, and the escape points show the leak. Sewer camera vs smoke test compares the two, and in a persistent smell case they are often used together.
A sensible sequence
Fill every trap and wait a day. Check the toilet base, the standpipe and the cleanout caps. Look at the roof vent. If the smell persists, note where it is strongest and when. Then a camera inspection of the building drain and the lateral, looking for openings. Then, if the camera finds nothing, a smoke test. Then, with the opening located, the repair. Most sewer smell cases end at the first step, and the camera is for the ones that do not. Sewer gas safety applies to anyone working in a space where it has collected.
Common questions
Can a sewer camera find a sewer smell?
It can find the open pipe the gas is escaping from, if the opening is in the drain system. It cannot find a dry trap or a failed wax ring.
Why does the smell come and go?
Wind over the roof vent, the furnace drawing air, a trap drying and refilling, or a main surcharging in rain all change the pressure in the system.
Is sewer gas dangerous?
In a house at the concentrations that cause a smell, it is unpleasant rather than acutely dangerous. In a confined space such as a pit or a crawlspace it can be, and should be treated with care.
Related guides
See drain flies and the camera, vent stack problems, and what a sewer camera can detect.