City industrial zones are densely built and lack large green spaces. The plants in such zones can develop without human interference and are able to re-establish within a foreseeable period of time. This is known as spontaneous vegetation. The aim of the paper is to characterize flora established on the roofs of several abandoned buildings in the main industrial areas of Warsaw, Poland. The field studies were performed in 2006−2008 and 2009−2010. There were 26 plants recorded in 11 syntaxonomic classes, with companion species in the herb layer. Spontaneous plants belong not only to syn-anthropogenic communities but also to natural and semi-natural ones. Economic factors currently play a major role in restoring and designing new urban green areas. Restoration of abandoned and new industrial and commercial buildings via spontaneous vegetation has been proposed as a cheap alternative to extensive technical reclamation.