New York is known for its skyline jam-packed with skyscrapers straining toward the clouds. But it’s a sight rarely seen from above, looking down on the penthouses terraces, rooftop gardens, bars, basketball courts, and pools. To the aerial photographer Alex McLean , this unexplored territory is the city’s fifth façade, and it’s the subject of his new picture book, Up on the Roof (Princeton Architectural Press, $50), which serves as a visual document of how city dwellers have begun to cultivate their once-desolate roof spaces.

McLean was struck by the evolving roofscape while on assignment to photograph Brooklyn Bridge Park in 2010. “While flying over West Chelsea, I became aware of multiuse roof spaces that seemed to be everywhere. It was obvious that a recent shift in culture and in financial resources had led to the construction of these new outdoor spaces; it was as if the spirit of the High Line had spread.” The most striking element in these photos is the amount of greenery–lush landscapes that compensate for the lack of trees below. But perhaps the most striking feature is the vast swaths of white. Once uniformly covered in black asphalt, today’s buildings sport bright white caps to reflect the heat of the sun, allowing the interior to stay relatively cool. (Some studies suggest that white roofs, far from being a silver bullet for global warming, might actually increase temperatures.)

So apart from sparking jealousy of those who get to enjoy upstairs gardens while the rest of us jockey for space on the sidewalk below (Is that a putting green?), McLean’s pictures hint at a future solution to the problem of overcrowding cities. As more people migrate to urban areas, all of those vacant rooftops will hopefully be optimized with green roofs that will cut energy consumption, produce clean air, and provide oases of calm away from the bustling streets.