The ratio between precipitation and the volume of discharged water during a certain period gives us the so-calledrunoff coefficient (C). The lower the runoff coefficient (C), the less water, in relation to the amount of precipitation, that is discharged from the Green Roof System during precipitation. Any volume that exceeds the absorption capacity of the Green Roof is discharged with a time lag at a lower flow rate over an extended period of time. The value is determined in accordance with specific protocol which is pre defined and can vary from country to country.
In Germany the short-term runoff coefficient is defined by the FLL protocol in such a way that the Green Roof System (installed with a pitch of 2%) is fully soaked with water. The system is then left for 24 hours; after that time, 27 liters of water per m2 are poured over the surface within 15 minutes. During that time, the volume of water discharged from the system is measured, and the ratio between the added and discharged water is calculated to obtain the value of the C coefficient.
The lab testing protocol are good example how to do it, however these kind of lab tests do have some short comings as they do not consider some important elements influencing the performance of green roof during 24 hour “waiting” period as there are many climate factors affecting the water evaporation from fully saturated green roof like temperature, winds, air humidity…
Also important is the long-term runoff coefficient. It can be monthly, seasonal or annual. The long-term runoff coefficients are more complex to be calculated as they need to consider the real climate conditions of specific area where green roofs have been installed. The long-term runoff coefficients consider not only the climate conditions, but also the real performance of specific type of green roof and are actually determined how the green roof will handle the rain-water through the years.