Most of the time, an urban rooftop is not a pretty place—a tarpaper or concrete griddle in summer , a windswept waste in winter. But if a new study in the journal Sustainable Cities and Society is correct, city roofs—along with vacant lots and urban greenspaces—could become something else entirely: farms . Deployed and reclaimed just right, the researchers found, urban gardens could meet up to 28% of the fruit and vegetable demands of 190 million people across the continent of Europe. The study, led by Stepan Svintsov, a researcher at the Leibniz Institute of Ecological Urban and Regional Development in Dresden, Germany, employed a surveying and modeling program known as geographical information systems (GIS) to catalogue available space on rooftops and the ground in 840 cities across 30 European countries. The smallest city in terms of population was Melun, France, with 9,000 people; the largest was Paris, with 7 million. The smallest city in terms of geographical reach was Mislata, Spain, at 0.2 sq.…