Green Roofs (GRs) are a type of Blue Green Infrastructure, growing in popularity due to their ability to provide numerous benefits. Designers and modelers of GRs typically make the assumption that GRs are homogeneous, which means that for different roof areas and the same structural factors such as substrate depth and composition, they have the same runoff per unit area. Based on this assumption, the measured data from small scale GRs in the laboratory can be applied to large GRs in the field. However, this assumption of homogeneity has not been sufficiently verified. To investigate this gap, an outdoor experiment was carried out from April 25 to December 1, 2021. Thirty-six GRs were constructed following an orthogonal experimental design with four primary structural factors investigated (i.e., substrate type (ST), substrate depth (SD), plant richness (PR), and roof area (RA)). Rain and runoff data were recorded and N-way ANOVA was used to evaluate the impact of the structural factors. Results showed that there were no statistically significant interactions between the structural factors that affected the GR runoff. The order of influence of the individual parameters, determined by the F value statistical metric was: SD > RA > ST > PR. However, substrate depth was the only structural factor which significantly affected the runoff per unit area of GR. Plant richness and substrate types showed no statistically significant effect on the GR runoff. The results of this study indicate that the implicit assumption: GRs are homogeneous, is acceptable.