Sewage smell in the house: finding the source
Sewer gas in a building is never just an odour problem. Sometimes it is a trap that dried out over a holiday. Sometimes it is a cracked line under the slab. The difference matters, so work through it in order.
Published 2026-08-30 by Floriano Plumbing Service · Serving all of Connecticut · (888) 809-6756
If this is happening right now and water is moving, stop reading and call (888) 809-6756. Our line is answered 24 hours a day, 7 days a week across Connecticut. We will tell you what to shut off while a truck is on the way.
Start with the cheapest cause
A dry P-trap. Every drain has a U-bend that holds a plug of water to block sewer gas. Guest bathrooms, basement floor drains, laundry standpipes and unused showers evaporate over a few weeks and let gas straight into the room.
The fix costs nothing: pour a couple of litres of water down every drain you do not use often, plus a splash of cooking oil in floor drains to slow evaporation. If the smell goes within an hour, that was it.
The other five causes
- A failed toilet wax ring. Smell strongest near one toilet, sometimes with slight rocking of the bowl.
- A blocked or improperly terminated vent stack. Nests, leaves or ice cap the roof vent, so the system pulls air through traps instead. Often paired with gurgling drains.
- Biofilm in the drain. Kitchen and bathroom sinks build a layer of organic gunk just inside the tailpiece. Smells sour rather than sulphurous, and cleaning the drain walls fixes it.
- A cracked or disconnected drain line inside a wall, under a floor or beneath the slab. Smell without a visible source, often worse in one room.
- A sewer line failure outside. Smell in the yard, soggy or unusually green patches over the line, and often slow drains inside too. This is the one that needs a camera in the line.
When to stop investigating and call
- The smell is constant and you have already filled every trap
- It is worse after heavy rain, which points at the exterior line
- Drains gurgle or run slow at the same time
- There is a soggy or sunken patch in the yard along the sewer run
- The smell is in a room with no plumbing fixture in it at all
Frequently asked questions
Is sewer gas in a house dangerous?
Sewer gas contains hydrogen sulphide and methane. At the concentrations most homes see it is mainly an irritant, but it should never be treated as a permanent feature of a building. Ventilate and find the source.
Why does the smell get worse when it rains?
Rain raises groundwater and can push gas back through a cracked or disconnected exterior line. It is a strong hint that the problem is outside rather than at a fixture.
Can a blocked vent cause sewer smell?
Yes. If the roof vent is capped by a nest or ice, the system draws air through the nearest trap instead, pulling gas into the room.