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Green roof retrofits offer a promising avenue to increase greenspace and thus biodiversity in the city. The successful colonisation and establishment of plants and animals on green roofs is limited by the location and context of the green roof. Here we use a before, after, control, impact (BACI) design to monitor the colonisation of a new retrofit roof in Melbourne CBD. We find that colonisation for some taxa occurs rapidly, with honeybees (Apis mellifera) arriving four days after flowers had been planted. Other insect taxa, such as native bees, did not colonise the impact roof but were present on the green roof reference site, which was lower in height and planted with native plants. Invertebrate abundance increased for all sites after the retrofit was built, however this sampling period was in late spring when more insects are expected to be active. Bird richness and abundance didn't change in response to roof greening, but invertebrate richness was significantly higher on reference sites and increased after the green roof retrofit was built for the impact site. There was an apparent 'spill over' effect onto the nearby green roof control, which also increased in invertebrate diversity. Overall, invertebrate composition across roof types was driven by floral density. Floral density and richness increased on the impact roof as vegetation matured, subsequently correlating with higher abundance and richness of invertebrates. We use these findings to discuss colonisation of green roofs and argue for strategic placement of new green roof retrofits to maximise their biodiversity potential. [ABSTRACT FROM AUTHOR]