Water reaches a rooftop tank because a pump in the building pushes it upward against gravity, and once the tank is full, the water then comes back down to taps by gravity. For a 3-floor building, the pump makes up for the pressure loss that would otherwise prevent water from rising that high reliably.

What happens when the pump is off

If the pump is turned off, the roof tank stops refilling, so the building starts using the water already stored in the tank. Water can still flow to the floors for a while, but only until the tank level drops enough that gravity- fed pressure becomes too weak.

Why the tank still helps

The rooftop tank acts like a buffer, so short pump shutdowns do not always cause an immediate outage. That is why many buildings keep upper-floor supply working for some time even when the pump is not running.

Simple way to picture it

Think of the pump as the part that “fills the bucket,” and the tank as the bucket on the roof. When the pump is on, it refills the bucket; when it is off, water still comes out until the bucket runs low.