A typical home requires water pressures of 50 to 100 psi. City water systems use pumps to maintain that pressure in the pipes. Vertical pipes are needed to supply the upper floors with water. Each foot of height reduces the pressure by 0.443 psi. Auxiliary pumps at various floors can provide the needed increase in pressure. An alternative is to put a large storage tank on the roof and use pumps to fill it. It then supplies the building with water under pressure due to the height of the tank. It also allows the pumps to be run to fill it at night when electricity rates may be cheaper. In addition, it provides a backup source of water in case of fire.

Small towns, which often use wells as a water source, use water towers to store water in case there is an interruption in electrical service. It also allows the town to use smaller pumps because the tower can supply the pressure during peak water demands. A typical daily water use is 500 gallons per minute, but this can rise to 2,000 gallons a minute in peak times. A tower typically stores one day’s worth of water.