A drain is not a hole. It is a hole with a water lock on it
Under every drain there is a U-bend — the trap — and after the last flush it keeps a small reservoir of water. That standing water is called the water seal, normally 4–5 cm deep. It is the only thing between your room and the shared sewer line:
- Water present — gas pushing up from the sewer hits a liquid plug and stops.
- Water gone — the pipe is now a straight, open tube into the sewer. Structurally, the drain has no lid.
So the order of checks is fixed: is the seal there, then is the pipe clean, then is the vent clear. Skipping to the third is how people end up pouring chemicals into a drain that only needed a litre of water.
Three smells, three causes
- Sewer / rotten eggs, in a drain you rarely use — the seal evaporated. About half of all cases. A litre and a half of water and it is over.
- Musty and sour, in a drain you use every day — biofilm: a living mat of bacteria on the pipe wall living off hair, soap and grease. The water seal is fine; the smell is coming up through it from the film above the waterline.
- Sewer smell with a full trap, plus gurgling — siphonage. A blocked roof vent means a flush elsewhere creates negative pressure and sucks the water out of your trap. Refilling buys you a day, not a fix.
That gurgle is the single best field test in the whole job: a bubbling, gulping sound from a drain when another fixture runs means the water is being pulled out, and no amount of pouring will hold the seal until the vent is cleared.
Why bleach is the wrong instinct
Bleach and caustic drain openers kill the surface of the biofilm and leave the mat underneath intact — the smell is back within a couple of days, and you have added a corrosive that can attack rubber seals. A baking-soda-and-vinegar fizz lifts the film mechanically instead: half a cup of soda, then a cup of vinegar, let it foam for 15–30 minutes, then flush with hot water. Hot, not boiling — PVC deforms.
For a drain you rarely use, the refill alone will be dry again in a fortnight. Pour a tablespoon of mineral oil (unscented baby oil does the same job) on top of the restored water: oil floats, seals the surface from the air, and the seal survives for months. Check it every few weeks either way.