Odour from a floor drain almost always comes from a dried-out trap, not from dirt. The trap seals with water; once that water evaporates, nothing stands between the room and the sewer. In a drain that is not used every day the water evaporates within two to four weeks, and faster in summer. That is why cleaning, bicarbonate of soda and vinegar do not help — they do not address the cause, they simply top up or cover over water that is going to evaporate again.
The permanent solution is to replace the water barrier with a mechanical one: a one-way silicone seal that fits into the existing drain, lets water through downwards and closes when there is no flow. No water, no power, no maintenance.
How a trap protects you and why it fails
Underneath every drain there is a trap — a bend in the pipe holding water. That water is a plug that keeps sewer gases down. The problem starts when the water disappears:
- A rarely used drain — the bathroom floor drain, a guest bathroom, a drain in the cellar or utility room. The water evaporates within a few weeks and the trap runs dry.
- Negative pressure in the pipework — when someone in the building runs a lot of water, the suction can "pull" the water out of your trap and break the barrier. The smell then gets stronger after the neighbours have flushed.
- Biofilm in the shower drain — a slimy layer of bacteria, soap, hair and grease on the pipe wall. It smells in its own right and holds moisture, regardless of the trap.
Hydrogen sulphide (the rotten-egg smell), methane and ammonia — that is what you are smelling. A dry trap = an open route for those gases into the bathroom.
Why household tricks do not solve the problem
Bicarbonate of soda and vinegar, chemical unblockers, gels and air fresheners all work on the symptom:
- They mask the smell or partly break down deposits, but they do not restore the water barrier.
- As soon as the water evaporates again (or negative pressure empties the trap), the smell is back — often within a few days.
- Aggressive chemicals damage seals and pipes over time, and the problem remains.
Pouring water into the drain does help, but only temporarily and only if it is repeated regularly — which, with rarely used drains, nobody does.
| Approach | What it does | How long it lasts |
|---|---|---|
| Bicarbonate of soda and vinegar | Briefly breaks down some of the deposits in the drain throat | A few days; does not restore the water barrier |
| Chemical unblocker | Dissolves grease and hair in the pipe | Does not act on the cause; damages seals over time |
| Gel or air freshener | Covers the smell with a stronger one | As long as the gel lasts |
| Pouring water into the drain | Tops the water back up in the trap | 2–4 weeks, less in summer; has to be repeated |
| A cover or plug on the drain | Physically closes the opening | While it is in place; the drain cannot be used |
| Passive silicone seal | Replaces the water barrier with a mechanical one — water passes, gases do not | 5+ years, maintenance-free |
If the smell is there even when the trap is full of water, the cause lies elsewhere: a badly vented stack siphoning the trap through negative pressure, a damaged seal on an inspection point, or biofilm in the drain throat. The first two need a plumber, the third is dealt with by cleaning the throat — but all three are rarer than a dried-out trap.
The permanent solution: a passive silicone seal
Instead of relying on water that keeps evaporating, a passive silicone seal with a one-way valve (Green Drain™) is fitted into the drain. It lets water through downwards while it is flowing, and the moment the water stops it closes and blocks the gases, even when the trap is completely dry. It works mechanically, without water and without chemicals.
- Fitted with no tools, in about 30 seconds, with no work on the pipework
- 11 sizes (1.25″–6″) for floor drains and showers
- Medical-grade silicone that does not degrade — one installation, nothing to maintain afterwards
- Also solves odour from rarely used drains (cellar, guest bathroom)
Short guide: what to do
- Work out which drain smells and measure the diameter of the grate or drain.
- If the drain is blocked or full of biofilm, clean it mechanically first.
- Fit a silicone seal of the right size into the drain.
- Done — the barrier is now mechanical and no longer depends on water.