If a floor drain smells of sewage but still drains freely, the drain is almost certainly fine. What has failed is the water seal inside the trap: the small body of water that sits between your room and the sewer. When that water evaporates, nothing is left to stop sewer gas rising through the grate. Refilling the trap usually stops the smell within minutes. Keeping it filled is the part most buildings get wrong.
The water seal is the only thing between you and the sewer
A floor drain is not a hole in the floor. Under the grate sits a trap — a chamber shaped so that water collects in it and forms a barrier. Waste water flows over the top of that barrier and away. Gas cannot travel back through it while the barrier is intact.
That is the whole mechanism. No cartridge, no membrane, no chemical treatment. Just water.
This is why building codes treat seal depth as a safety dimension rather than a detail. EN 1253-1, the European standard covering floor drains, sets a minimum trap seal depth of 50 mm. A shallower seal evaporates faster and is more easily broken when pressure in the drainage stack changes.
Why the seal disappears
Evaporation is slow but relentless, and ordinary conditions speed it up.
The room is unused. A guest bathroom, a holiday property, a basement shower nobody touches for months. No water arrives to replace what evaporates.
The building is warm and dry. Underfloor heating, air conditioning and low winter humidity all pull water out of the trap faster than a cool, damp room would.
Extract ventilation runs continuously. A powerful extract fan creates negative pressure. Air is drawn through the drainage system, and it carries moisture out of the trap with it.
The drain was never meant to be the only trap. Washing machines and some showers discharge into a gully whose trap is shared. When the appliance is removed or the hose is pulled out, the trap can drain away.
The installation is wrong. A drain fitted without a proper trap, or with a shallow one, has no reserve at all. This turns up most often in older refurbishments where the floor build-up was too shallow to take a correct trap.
The property has stood empty. Anything longer than a few weeks, especially in summer, is enough for a small seal to disappear completely.
Thirty-second diagnosis
Before refilling anything, work out which of three problems you have, because the fix is different for each.
Observation
Likely cause
Fix
Smell strong at the grate, water drains normally
Dry trap
Refill the trap
Water drains slowly, gurgling after use
Partial blockage
Clear trap and branch
Smell returns within a day of refilling
Seal design or installation fault
Replace core or drain
Smell in the room but not at the drain
Another source
Check the list below
Smell that appears only when the extract fan runs, or only in windy weather, points to a seal being broken by pressure rather than simply drying out. Refilling helps for a while; the real answer is a trap that resists pressure.
How to refill a trap properly
Pour water slowly and directly into the drain, not across the surrounding tiles. The aim is to fill the trap chamber, and a fast pour mostly runs straight past it.
Watch the level. If it holds, the seal is restored. If the water drains away immediately and the smell persists, the trap is not holding water at all — that is a mechanical fault, not a dry one.
Repeat for every floor drain in the building, not only the one that smells. Air moves, and the drain you notice is often not the drain that is dry.
Making it stay sealed
Refilling is a fix, not a solution. In a room used every day the trap looks after itself. In a room that is not, one of three approaches is needed.
Build a refill routine. Once a week in unused rooms is enough for most buildings. Put it on the cleaning schedule instead of relying on memory, because a dry trap nobody notices for a month is a complaint waiting to happen.
Fit a mechanical anti-odor core. A gravity-flap or magnetic core closes when there is no flow and opens when water arrives. It does not depend on a water seal, so it cannot dry out. This is the practical answer for guest bathrooms, holiday lets and any drain that sits idle for weeks. Cores are sized by the drain body they fit into, so match the insert to the drain rather than guessing. We make several insert patterns — the B floor drain core and the F floor drain core are two of them — and the opening in the body decides which one fits.
Replace the drain. Where the trap is shallow, damaged or badly installed, a core will not rescue it. A drain with a proper deep trap, such as the BQ20157-Q brass floor drain, solves the problem at the source rather than treating the symptom.
What not to do
Do not pour oil down the drain. It coats the pipe, catches debris and eventually blocks it. It seals nothing.
Do not rely on bleach. It does not remove the cause, and repeated use attacks rubber seals and metal finishes.
Do not tape over the grate. It stops the smell and stops the drainage with it. The water then finds another way out, usually across the floor.
Do not cover it with a mat or a bucket. Trapped moisture and no airflow make things worse, and the cover hides the fact that the drain has stopped working.
When the smell is not the drain
If the trap is full and the smell continues, look elsewhere. A failed wax ring under a toilet, a sink overflow that has never been cleaned, a washing machine standpipe without a trap, or a stuck air admittance valve all produce the same complaint.
There is a simple test. Seal the drain temporarily and see whether the smell changes. If it does not change, the drain is innocent and the search moves indoors.
Questions buyers ask us
My drain only smells in summer. Why? Higher temperatures and lower humidity increase evaporation. A trap that survives winter can still dry out during a heatwave.
Will a core fit an old drain? Only if the body is sound and the insert matches. Measure the opening in the drain body before ordering, and check that the existing trap is not cracked.
How often should an empty property be refilled? Weekly during the letting season, and again immediately before guests arrive. If that is impractical, fit a mechanical core and stop depending on the schedule.
Does a dry trap let in more than smell? It lets in sewer gas, which is unpleasant at low concentration and a genuine health concern at higher ones. It is not a problem to leave until the next inspection.
Next step
If you are specifying drains for a project, send us the drain body dimensions, the pipe size and the rooms involved. We will tell you which core fits and which drains need a deeper trap.
If you are replacing a core on an existing drain, a photo of the open drain body with a ruler across the opening is enough for us to identify the size. Send it through the contact form on our site and we will come back with the matching part.
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