A luxury Fifth Avenue apartment tower’s strange fake rooftop

Facing it head-on from the sidewalk, 1001 Fifth Avenue is impressive enough—a 23-story residential building clad in a limestone facade topped by what looks like a mansard roof that stretches into the sky. Viewed from the side, however, and this mansard roof is just a massive limestone slab supported by steel buttresses, creating no extra living space the way a mansard roof should. It’s a faux mansard, or perhaps a very plain pediment; an empty limestone billboard with nothing behind it that serves no functional purpose. Was it a playful feature designed by an architect with a sense of humor, or an attempt to make the building—a latecomer on the block, completed in 1979—as impressive in height as some of its neighbors without actually spending the money to construct more floors? Whatever the thinking behind it, the limestone slab has a complicated history. The story of 1001 Fifth Avenue’s faux mansard roof begins in the 1970s with developer Peter Kalikow, who hired architect Philip…