Rainwater Volume Calculation

There is so much variation in green roof design—I doubt that a single reference figure would necessarily have any relevance. Climate and storm frequency is also highly relevant—if the growing medium is saturated by one storm, how quickly does the next storm come along? It might have 100% runoff if it comes quickly.I would consult with your landscape architect.

Learning from our experience, I suggest you simply calculate the rain water harvesting potential using the run-off co-efficient of 0.3 (30%) to 0.1(10%) depending on the thickness of the green roof. So the final calculation will be area (mts) * Avg annual rainfall (mm) * co-efficient for surface run off. While Tristan has made a valid point above you will find that there are some mild rainy days where the run off is zero and there are some days where the storm is continuous hence the run off can be up to 95% . So looking at rain water harvesting potential annually the values I have provided above are reasonably accurate as it balances out by the end of the year. This has been tested by us here in India. Green Roof 100 mm - 200 mm = 0.30 run off co-efficient Green Roof >= 500 mm = 0.10 run off co-efficient Hope this helps.