Although a vegetated roof cannot fully mimic natural catchment because of its limited soil profile and diversity of vegetation by slowing the stormwater release, it can nevertheless encourage infiltration and minimize surface runoff, which contributes positively to stormwater management in both quantity and quality. The extent to which it helps depends on the roof’s storage capacity, as well as the rainfall pattern where it is located. There is no one-size-fits-all solution—the system buildup must be designed and optimized for the local climate. The storage capacity comes from various components in the assembly.

Vegetation
Plants take up water from the roots and release it to the atmosphere from their leaves. They provide runoff mitigation by removing water from the growing medium and releasing it back to the atmosphere, thus ‘recharging’ the roof assembly’s water-storage capacity for the next rain.

While most terrestrial plants take up and release water during daytime, succulent plants such as sedums can store water within the tissues (e.g. leaves, stems, and roots), releasing it in the cooler time in the night, making them more heat and drought-tolerant. These characteristics are particularly suited for their survival on rooftops.

Growing medium
Typical green roof media are higher in mineral aggregates and lower in organic matters (i.e. less than 25 per cent) compared to regular garden soils to maintain soil structure and therefore long-term performance. Water is stored in the cavities of porous mineral particles (e.g. lava, expanded clay), small capillary pores between particles, and the organic matter fraction. Water stored in the growing medium is taken up by plants or returned to the atmosphere via evaporation.

Water-retention layer
Absorptive materials such as synthetic fleece and horticultural mineral wool store water in the space between fibres. They are highly effective in storing water compared to growing medium on a per-unit-weight basis. At the same time, they are also permeable so they do not lead to a waterlogged substrate or promote root rot.

Drainage
In addition to drainage function, some geocomposites consist of a three-dimensional drainage core bonded to a water-retention fleece, which stores water and releases it to the growing medium through capillary action. Some drainage panels are moulded with ‘cups’ to act as reservoirs to provide both drainage and retention capabilities. Open-pore aggregates, such as expanded clay, can also provide drainage and water storage, but incur considerable load on the roof structure.

Designing for stormwater management
How effective is a prescriptive-based green roof policy that mandates a minimum growing medium depth? Growing medium are not made equal. The water storage capacity of the growing medium depends on many factors such as composition, particle size distribution, and organic content.

Porous mineral such as lava and expanded clays can hold considerable water in their pores. Different particle size grading changes the capillary pores and thus the water-holding capacity. Adjusting the silt, clay, and organic content can affect the water retention in the growing medium, and so can the addition of water-absorbent additives.

Figure 2 shows the water-retention capacity of several typical growing media designed for extensive green roof systems, normalized to 25-mm (1-in.) thickness for ease of comparison. Each bar represents the thickness (thus, volume) that is composed of the dry component (red) and the stored water (blue). The capacity varies from 40 to 65 per cent by volume.