Subaerial biofilms play a central role in the ecology and biodeterioration of many outdoor monuments and pieces of art. It is well established that microorganisms can face a broad range of stress by living in these subaerial environments, but their origin, taxa determinants and physiological traits are debated. Here, we hypothesized that the bacteria forming these biofilms originate from the surrounding air and soil and that the selective pressure of a life on rocks shapes the community. To verify this hypothesis, we studied the microbial communities of nine tombstones of the Monumental Cemetery of Milano, by collecting samples in three seasons. We analyzed the structure of these subaerial biofilms, compared them with the bacteria identified in the surrounding air and soil and found that only few rare taxa are shared among the three compartments and have been selected by the stone environment. In addition, we considered which parameters - among temperature, humidity, light, season and lithotype - concur to structure the microbial community.