This document discusses using green roofs to manage stormwater runoff and reduce storage tank volumes. It summarizes previous research on green roof water retention and models stormwater performance in Micro Drainage's WinDes software. Field tests at the University of Sheffield calibrated the model, showing a green roof can retain the first 5mm of rainfall and reduce required storage from 450 cubic meters to 300 cubic meters for a 2 hectare warehouse roof. The document advocates the exponential method for modeling green roof attenuation storage and calls for further research calibrating the approach under different roof types and locations.