Snapshot: NYC Water tanks southwest of Union Square. These rooftop tanks have been a familiar sight since the 19th Century, and are required on buildings over 6 stories to regulate water pressure via gravity (okay, I don’t really understand how it works). The water most NYC apartment dwellers use for drinking, cooking, and bathing are stored in such tanks. There are approx. 10,000 – 15,000 wooden water tanks throughout the city.