Three observations
What says the problemsits further out.
Three observations that put the problem past the sink in front of you.
01
The sink is the last opening
The sink that stopped is the end of the route, not the problem. Hot water carries grease past it before settling further out.
02
It returns somewhere else
Note where the water stopped each time. Sink, then branch, then outside line means the last clearance shifted the mass, not removed it.
03
In flats the line is shared
In a block every kitchen shares one line. A ground floor backup is not necessarily its own, since it sits at the lowest opening.
After the sink
The route towhere it cools.
Grease does not end at the trap. It travels through the wall branch, the building line, and a buried run under the interlock to the chamber outside, cooling as it goes.
It cools where the pipe runs steady and low: a bend, a junction, a weak fall. There it stiffens and binds, building inward for months until an ordinary wash stops the sink outright.
The repeat
Why it returnsfurther out.
Boiling water and shop bottles do not remove grease, they move it along. They open a path through the mass, pushing what came loose until it settles somewhere colder and further out.
So the next blockage sits somewhere new: sink, then branch, then line outside, each wider and harder to reach. In flats every floor shares one line, so the lowest opening floods first.
Breaking the cycle
What breaksthe cycle.
The cycle breaks at two ends. First, oil never enters the pipe at all: cooled in a sealed container for the bin, plates wiped before washing, the strainer emptied by hand.
Second, the whole layer must come off the wall along its length, not a narrow path through the middle. What is left stays rough, catching everything and rebuilding the blockage.
On the grease route
Asked about the greaseroute.
Because the last clearance moved the mass instead of removing it. It settled further out in a colder pipe, so the next opening to back up sits above that new spot.
No. Hot water only carries the oil further before both cool and it binds in a colder pipe. Let it cool in a sealed container and bin it instead.
Because the kitchens share one line. Water with nowhere to go rises through the lowest opening, usually the ground floor, even when the blockage sits above it.