What is happening
Drain smells come from one of two places: a trap that has dried out, or material sitting in the line and decomposing.
Drains report problems indirectly. The fixture showing the symptom is often not where the trouble is, which is why the useful first question is not what is blocked but which fixtures are affected and when.
What usually causes it
In this area, four explanations cover most cases:
- A dried-out floor drain trap
- Build-up in the line holding waste
- A venting fault pulling gas back into the house
- A cracked line under the slab
Which one applies changes the fix entirely, and they are not distinguishable from inside the house. We would want to see it before saying anything for certain.
What you can check yourself
Run the fixtures one at a time and note which ones react to each other. Two drains that respond to one another share a pipe, and that narrows things considerably.
Note whether it is getting worse gradually or started suddenly. Gradual usually means the pipe has been closing up for a long time. Sudden usually means something specific is in the way.
Narrowing it down
Pour a bucket of water into the floor drain and wait a day. If the smell goes, it was a dry trap and you have fixed it yourself for nothing. If it comes back, something in the line is holding waste.
What guessing costs
Smell is the symptom people tolerate longest, because it is unpleasant rather than disruptive. It is also one of the few that occasionally indicates a cracked line under the slab, which is worth ruling out.
When it is worth calling
If it has happened before, it is worth finding the cause rather than clearing it again. A drain that has been opened twice and returned twice will return a third time.
On this particular pattern, sewer camera inspection is the service that most often resolves it, though we would confirm that by looking rather than by assuming.
