Query: In the 19th century, New York city required water towers in the roofs of buildings that were more than how many stories tall?
Context: A distinctive feature of many of the city's buildings is the wooden roof-mounted water towers. In the 1800s, the city required their installation on buildings higher than six stories to prevent the need for excessively high water pressures at lower elevations, which could break municipal water pipes. Garden apartments became popular during the 1920s in outlying areas, such as Jackson Heights.