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What causes a sewer smell inside a house?
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Short answer
Usually a dried out drain trap in a room nobody uses, a failed toilet seal, or a blocked vent that stops the drains breathing. Less often it's a cracked drain line under the house. Start with the rooms you use least, because that's where traps dry out.
Every drain holds a small plug of water that blocks sewer gas. In a guest bathroom, a laundry standpipe or a floor drain that never gets used, that water evaporates in a few weeks. The gas then comes straight up the pipe.
Points to this if: Smell in one unused room, comes and goes, worse in warm weather
The wax ring under the toilet seals the pan to the floor flange. When it fails, gas escapes at the base and water can seep into the subfloor. A toilet that rocks slightly when you sit on it's the usual tell.
Points to this if: Smell strongest at floor level near one toilet, possibly a soft floor
Drain lines need air from a vent stack through the roof. When that vent blocks with debris or a nest, draining water pulls the seal out of nearby traps instead, and you get gurgling and smell together.
Points to this if: Gurgling drains, smell after using a fixture, several rooms affected
A cracked cast iron stack inside a wall, or a damaged line under the slab, lets gas escape into the structure. This is the least common and the most expensive, and it needs a camera to confirm.
Points to this if: Persistent smell with no obvious source, sometimes damp or staining
Local context
Southern California works against trap seals. There is no long heating season to notice a smell early, windows stay open, and evaporation in a warm dry house empties an unused trap faster than it would in a cold climate. A guest bathroom in a Fountain Valley or Tustin house can go dry over a summer with nobody in it.
The other local factor is pipe age. Older Santa Ana houses often still have original cast iron drain stacks inside the walls. Cast iron corrodes from the inside and eventually perforates, usually low down where it has carried the most water. A persistent smell in an older house with no dry trap and no toilet problem is worth a camera.
Rental and multi-family properties are worth calling out separately, because Santa Ana has a lot of them. Vacant units between tenancies are where traps dry out, and the smell often turns up in the occupied unit next door rather than the empty one.
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Call (714) 555-0142At the levels a dry trap produces it's mostly unpleasant. At higher concentrations, hydrogen sulphide is genuinely hazardous. Treat a strong persistent smell as something to fix promptly rather than live with, and ventilate the room meanwhile.
That pattern almost always means a trap that keeps drying out. Find the fixture nobody uses and pour water down it monthly. A trap primer or a small amount of mineral oil on top of the water slows evaporation.
A rotten egg smell in hot water only is usually a reaction between the anode rod and bacteria in the tank, not a drain problem. That's a water heater question and it's often solved by replacing the anode.
Rarely. Dry traps and toilet seals account for most of these calls. Broken lines do cause it, which is why a smell that survives the simple checks deserves a camera inspection rather than more guessing.
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