A crew from Rosenwach Tank Co. puts a new tank together. PHOTO: Michael Axley

he principles behind water towers are as simple as their appearance.

New York City water pressure at street level runs at about 55-60 pounds per square inch. According to Valentine Lehr, a partner and consulting engineer at Lehr Associates in Manhattan, one pound of pressure per square inch will push a column of water about 2.3 feet vertically.

Most apartment plumbing fixtures, said Lehr, require 20 pounds per square inch to function properly. Therefore, the street level water mains will provide enough pressure to operate faucets up to a height of about 80 feet—the height of a five or six story building.

Tightening the hoops. PHOTO: Michael Axley

In the late part of the 19th century, when high-rises first began to appear in New York City, hydraulic engineering had not advanced to a point allowing production of variable flow pumps. Water usage, however fluctuates throughout the day. People get up, take showers and cook breakfast—water use peaks. Another peak occurs at lunchtime, another in the evening. The available technology was not equipped to handle these variations in flow. Further, as buildings rose over five stories, some mechanism was required to generate water pressure on the upper stories.

Inside the tank. PHOTO: Michael Axley

The elegant solution to these problems was the rooftop water tower. The towers are filled by pumps that generate enough pressure for water to reach the roof. When water in the tank is depleted to a certain level, float valves are activated, turning on the pumps. Thus, water tanks solve two problems at once: they maintain water pressure on the upper levels of tall buildings, and they serve as reservoirs, acting as buffers against variations in peak flow.