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They’ve been an integral part of New York City’s skyline for more than a century: thousands of wooden water tanks, banded with steel and capped by conical plywood roofs, perched alone or in clusters on buildings throughout the borough (1). Fed by the city’s aquifers, they serve as fire-code–required reservoirs and keep the apartments and offices below supplied with gravity-fed tap water.

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Most of the rooftop tanks were built by the Rosenwach Tank Co., a five-generation family-run business that dates back to the late 1800s. Tank components are made of decay-resistant Western red and yellow cedar; fabrication takes roughly a week per tank (2). An average domestic tank measures 10 to 12 feet in both diameter and height and holds around 10,000 gallons (3). Some units, though, are much bigger. According to company president Andy Rosenwach, buildings can have sprinkler-reserve requirements of up to 50,000 gallons, which calls for a container 24 feet in diameter and 20 feet high. When filled, such a behemoth weighs upwards of 245 tons.

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Wood tanks are typically replaced every 35 years. A tightly orchestrated six- to 10-man crew can remove the old tank and assemble the new one — then fill it up with water — in about 10 hours (4, 5).

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Why wood? As a natural insulator, wood keeps water cool in summer and protects it from freezing in winter. Wood tanks are faster to assemble and easier to clean and maintain than steel ones, and they last longer and deliver better-tasting water. And besides, Rosenwach says, “Wood tanks make people happy.”

Watch a video about tank fabrication and assembly — a segment from TV’s “Dirty Jobs” — at rosenwach.com.