Question

Why is the storage water tank of a building kept on the rooftop?

Answer 1

Hint: We know that water flows from higher level to lower level without any external input. The main reasons why the water tanks are placed at the rooftop or usually at elevated height is as follows:
1. Water flows towards gravitational force.
2. Greater the height, greater is the pressure.

Complete step by step answer:
The water stored in a tank is distributed by gravity. The water tanks are always built as far as feasible at the highest location and further built on staging suitably high enough such that the water reaches all points by mere gravity.
Thus, the water tanks are placed at roof top so that gravity can push or pull the water out without using any external forces, such as using electric motors. Also, the tanks are built to use the principles of hydraulics, which basically state that pressure applied to any part of a confined fluid transmits to every other part with no loss.
Storage tank of a building is kept at top to store energy as potential energy and we know that potential energy is directly proportional to height.
Thus, storage tanks are usually kept at rooftop in order to have easy water flow and so that we do not need any external input to access water at low level.

Note: It is not mandatory to keep the storage tanks at the top level, we can even place them on the ground level but there will be a requirement of an external aid to make water flow through given pipelines.