Sourced from the Living Architecture Monitor
The ability of a green roof to retain stormwater depends on many factors including plant species, substrate depth and composition, and the pre-existing moisture present before a given rain event. All of these factors can be influenced by irrigation practices which in turn may have a positive or negative influence on the roof and its ability to retain stormwater.
Plant species can influence stormwater retention. Plant photosynthetic metabolism has a major effect on transpiration (see my 2016 discussion comparing C3, C4, and CAM plants, Living Architecture Monitor 18(1):24-29). In addition, those plants with more biomass have a greater ability to intercept rainwater with their foliage and allow it to evaporate before reaching the substrate surface. Nagase and Dunnett (2012, Landscape and Urban Planning 104:356-363) showed that plant types such as forbs and grasses that possessed greater shoot and root biomass were more effective in reducing water runoff than those with less biomass such as sedum.
Tied to plant species is substrate depth and composition. Deeper substrates are able to hold more water and therefore are able to support plants with greater biomass. Changes in composition can also influence water holding capacity and capillary movement. As aerated pore space decreases, water holding capacity increases and vice versa.
In addition, pre-existing substrate moisture present before a given rain event is a major factor. To achieve maximum stormwater retention we want the roof to be dried out before each rain event. However, a minimum moisture level must be maintained if one wishes to keep alive the herbaceous perennials and grasses that possess this greater biomass. Because of their higher transpiration rates many of these plants may die during periods of drought. In addition, in some cases a shallower depth supporting sedum may actually be able to hold more rainwater since this shallow depth may be completely dry, whereas a deeper depth may still contain pre-existing moisture.