Three early signals
Hear it, smell it,watch it.
Each of the three comes from a different cause in the run.
01
The gurgling sound
A gurgle from the gully or the pan means air is struggling through the run, pulled back through the nearest trap.
02
A smell with nothing blocked
A smell that comes and goes with normal drainage means one water seal has gone, nothing blocked.
03
Slower than it used to be
Compare a fixture's drain today with before. Worsening week by week is a narrowing still growing, not one heavy day.
The gurgle
A gurgle is air,not water.
A gurgle comes from water pulling air through a trap's standing water as it drains. You hear it at a different opening, the floor gully answering when you flush the pan.
That means air could not reach the vent, blocked at the roof or shut off by standing water below. A one-off gurgle means nothing; one that repeats every time is worth watching.
Smell, nothing blocked
A smell means aseal is gone.
The smell is air escaping once a trap has lost its seal. The easiest cause is evaporation, an unused opening drying out. Pour water in; if the smell returns, it isn't that.
A smell with a gurgle is different: the gurgle empties the trap on every flush, so the seal drains and refills, and the smell escapes while it sits briefly empty.
Which one is urgent
The slow drain isthe urgent one.
The slow drain alone shows the bore has narrowed, not a vent or seal. Worsening slowness is usually grease building patiently; a sudden stop is solid matter gone down at once.
Worsening slowness comes first, it ends in an overflow; the gurgle next, the run is under strain. One case cannot wait: every fixture slowing together means the main line, so stop and call.
About these signs
Asked before itblocks.
Because air takes the nearest opening it can find. Water going down one line pulls air through the closest trap.
Hot water shifts grease at the surface, but what melts cools and sticks further along. Boiling water damages plastic pipework.
If the same signs show in more than one flat, it is the shared stack or building vent. Speak to building management.