On the rooftop of a hospital in the Dutch city of Rotterdam, patients can visit a small orchard filled with fruit trees. A neighboring art museum has a rooftop forest planted with birch trees that were raised to survive at a slightly higher altitude. Nearby, a nearly 4,000-foot-long building is topped with a park that has vegetable gardens, picnickers, and grazing sheep. On other roofs in the city, pilot projects are testing the potential of tiny homes.

The city is a pioneer in finding new uses for a part of urban space that’s often ignored. In a new book called Rooftop Catalogue, Rotterdam-based architecture firm MVRDV and the organization behind an annual “rooftop day” festival in the city explore more potential ways to transform roofs and how the whole rooftop landscape could change.

“Rooftop use could make a huge contribution to the densification of the city—and it could also prevent us from building more on the outskirts of our cities,” Winy Maas, cofounder of MVRDV, said in an interview included in the book. In Rotterdam, the team calculated that if 10% of the city’s roofs were used for housing, it could create 15,000 comfortably sized small homes. Like backyard houses, small rooftop homes can help add new residential space to neighborhoods that are already built up. Roofs could also be used to extend space from existing apartments or for adding new coworking spaces to apartment buildings.

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