A urinal does not block overnight. Urine scale grows by a few tenths of a millimetre a month and narrows the drain throat for a year or more before water stops draining. The blockage is the last step in a process that was visible the whole time — through slower drainage and odour returning within an hour of cleaning.
How the deposit forms
Fresh urine is slightly acidic and holds calcium and phosphates in solution. Bacterial urease hydrolyses the urea into ammonia, the pH climbs past 8 and the dissolved salts begin to crystallise as struvite and calcium phosphate. The crystals grip microscopic imperfections in the glaze and the rubber seal below the throat.
Hard water speeds this up by adding calcium to the system. In hard-water areas the deposit grows noticeably faster.
| Period | State of the throat | What is noticed |
|---|---|---|
| 0–2 months | Thin film on the walls | Nothing |
| 2–6 months | Layer of about a millimetre | Odour returns soon after cleaning |
| 6–12 months | Bore narrowed by roughly a third | Visibly slower drainage |
| 12–18 months | Bore narrowed by two thirds | Standing water, constant odour |
| 18–24 months | Complete blockage | Urinal out of service |
What it costs when it runs to the end
The cost is not only the plumber. In a hotel or a restaurant a closed washroom is something guests notice. In a food production facility under HACCP, a blocked washroom drain goes into the audit record.
Mechanical descaling clears the deposit that is there. If nothing else changes, the same process starts again the following day.
Prevention instead of remediation
The only way to stop the deposit forming is to break uric acid down before it crystallises. That is what bio-enzymatic tablets do: bacterial cultures released into the throat on every flush, working on protein and uric acid while both are still in solution.
Sequence for a urinal that already has a problem:
- Descale mechanically first — the tablet does not dissolve existing deposit
- Fit a cage with a tablet in the throat
- Stop using acidic and chlorine products — they kill the cultures
- Replace the tablet every 30–45 days
For buildings with several urinals the bulk pack works out cheaper: a 5 kg tub holding roughly 65 tablets, used to refill existing cages.
When scale is not the whole story
If a urinal drains slowly and the throat is clean, the restriction is further down the stack and needs a plumber. If there is odour but drainage is normal, the cause is most likely a dry trap — covered in why a urinal smells.
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.