Green roofs have been implemented on new buildings as a tool to mitigate the loss of post-industrial or brownfield land. For this to be successful, the roofs must be designed appropriately; that is with the right growing media, suitable substrate depth, similar vegetation and with a comparable soil microbial community for a healthy rhizosphere. This study compared soil microbial communities (determined using phospholipid fatty acid or PLFA analysis) of two extensive green roofs and two post-industrial sites in Greater London. It was found that green roof rootzones constructed using engineered growing media are not depauperate, but can have an abundant soil microbial community that in some cases may be more diverse and numerous than communities found in brownfield areas. In this preliminary study, one green roof supported abundant soil microbial communities that were dominated by gram negative and aerobic bacteria, whilst fungal abundance was similar across all sites analysed. Furthermore, ratios of fungal: bacterial PLFA’s were larger from post-industrial sites but overall were consistent with bacterial dominated soils typical of early successional habitats.