Dr. Nigel Kaye and former PhD student, Dr. Will Martin and current lecturer in General Engineering received the 2021 Outstanding Robert Alfred Carr best paper awarded. The research focused on the ability of green roof systems to impact stormwater runoff from buildings. This research has been covered in many studies, however, a lot of these studies looked at long-term retention, with much less work focused on how to model a green roof’s response to larger design storms. Work that has examined flow routing for individual rainfall events has focused on empirical routing models that are tuned to the specific roof being modeled. This paper presents a new physics-based model for flow routing based on the green roof module geometry and soil properties, which requires only a single discharge coefficient to be measured. The results of this model were compared with the results of a series of experiments to quantify a modular green roof system’s hydraulic response to drawdown and steady rainfall.