The smell in a washroom almost never comes from the ceramic the cleaner has just wiped. It comes from the drain throat underneath. Urine scale builds up there and holds urine residue against the pipe wall. Urea in that residue hydrolyses to ammonia, and ammonia is what the room smells of. That is why a washroom can smell ten minutes after cleaning: what gets cleaned is what can be seen, and the source is a few centimetres deeper.
The second source is the same one that affects floor drains — a dry trap. In a urinal that goes unused for weeks, the water evaporates and sewer gas has a clear path.
What urine scale is, and why you cannot see it
Fresh urine is close to sterile and slightly acidic. As soon as it meets air and bacteria, urease hydrolyses the urea into ammonia. The pH rises, and dissolved calcium and phosphates start to crystallise. That deposit — struvite and calcium phosphate — grips the walls of the drain throat and the trap.
The layer grows by a few tenths of a millimetre a month. For the first month or two there is nothing to see. By six months the throat is visibly narrower, and the smell no longer goes away between cleans.
The key point: scale is not dirt. It cannot be cleaned off the surface because it is not on the surface.
Why standard products make it worse
Acidic cleaners — hydrochloric, phosphoric or sulphamic acid — dissolve part of the scale while etching the glaze and attacking rubber seals. On a rougher glaze the next deposit forms faster, so the interval between cleans keeps shortening.
Chlorine products kill bacteria indiscriminately, including the ones that would break uric acid down before it crystallises. After bleach the drain starts again from nothing, and it rebuilds faster than before.
Blocks and fragranced inserts cover the ammonia rather than removing it. Staff replace them weekly, the cost adds up and the cause is untouched.
| Approach | Acts on | After six months |
|---|---|---|
| Acidic cleaner | Surface scale within reach | Etched glaze, faster deposition |
| Chlorine cleaner | All bacteria, the useful ones included | No breakdown of uric acid |
| Fragranced block | Nothing — it covers the smell | Scale grows unchecked |
| Mechanical descaling | Deposit already formed | Effective, but needs a plumber |
| Bio-enzymatic tablet | Uric acid before it deposits | Throat stays clear, odour does not return |
How the tablet works
GD Uri-Tabs™ is a pressed tablet carrying over three billion active bacterial cultures. It sits in a ventilated cage in the urinal throat. Each flush releases some of the cultures, which break down uric acid and protein before they can crystallise into scale.
No acid, no chlorine and no fragrance covering another smell. The formulation is biodegradable, and the cultures do not disturb the biology of a septic system.
- One tablet lasts 30–45 days at typical traffic
- The cage holds the tablet in position and stops it washing away
- Works in waterless urinals, where scale is the biggest problem
- Does not damage ceramic, seals or pipework
If it smells when the throat is clean
Then the trap is the cause. A urinal that goes unused — an office over the Christmas shutdown, a school over the summer, a hotel wing out of season — loses its water seal in two to four weeks. The fix is the same as for a floor drain: a passive silicone seal that closes the drain mechanically, without water.
In practice the two problems usually arrive together. The tablet handles the scale and the ammonia; the seal handles sewer gas once the trap runs dry.
Quick diagnosis for facilities managers
- Smell immediately after cleaning → scale in the throat, not the surface
- Smell after a shutdown, easing once the building is back in use → dry trap
- Slow drainage → scale has already narrowed the throat; descale, then prevent
- Smell from one urinal out of six → check the stack venting
Full guide: drain smell — causes and a permanent fix — why traps dry out, why bleach and vinegar do not help, and what is done in homes, hotels, schools and hospitals.