Designing green roofs for stormwater management, By Karen Liu

XF301 system Courtesy of Xeroflor
Courtesy Next Level Stormwater Management

The number of vegetated roofs has consistently experienced double-digit growth over the past decade. An annual market survey conducted by Green Roofs for Healthy Cities (GRHC) shows the North American industry grew by 10 per cent in 2013 over the previous year, when a total of 519,151 m2(6.4 million sf) were installed over 950 projects.

Green roofs offer multiple environmental, ecological, and economic benefits to urban areas; many municipalities are encouraging their implementation through bylaws and incentive programs. There are currently 33 cities in North America with dedicated policies, incentives, or guidelines to promote green roof implementation. These strategies are successful—seven out of the 10 North American metro regions with the highest green roof area installed in 2013 have supportive policies or programs.

These policies come in many forms. Some programs offer financial incentives directly through grants and rebates (New York City) or indirectly through reduction in stormwater fees and property taxes (Washington, D.C.). Others involve an accelerated building permit process (Chicago), low-interest loans (Cincinnati), or providing density bonuses for additional floor space based on green roof coverage (Portland, Ore.). While most North American policies generally involve incentives that encourage green roof implementation, there are also those like the one found in Toronto, which penalizes non-compliance.

Prescriptive versus objective-based policies
Many of the vegetated roof policies and programs focus on stormwater management benefits. Current green roof policies tend to be prescriptive-based, mandating a minimum depth and/or composition of green roof growing medium. For example, Portland’s Density Bonus Program and Nashville’s Green Roof Credit Program require a minimum depth of 100 mm (4 in.). Others are objective-based that require the green roof to achieve a specific stormwater performance. For example, New York City’s Green Infrastructure Grant program requires the vegetated roof to manage at least a 25-mm (1-in.) rainfall event, which also applies to Washington D.C.’s RiverSmart Rooftops program.

While both prescriptive and objective-based policies are meant to promote green roof implementation and mitigate stormwater in the urban areas, sometimes the good intention can become a disincentive or even hinder implementation of green roofs.

It is important to understand how green roofs work to mitigate runoff. The policy-makers can then implement requirements that ensure effectiveness for the municipalities, while the architects and landscape architects can design green roofs to maximize the stormwater management potentials.

fig 1 edit
Figure 1: Principal components of a vegetated roof system—compare this with the assembly specified for Toronto’s Sherway Gardens mall.

How green roofs manage stormwater
The principle components of a green roof
system include:

  • vegetation or the living component;
  • growing medium that provides water, nutrients, and anchorage to the plants;
  • a filter/retention layer that works to prevent small particles from clogging the drainage and
    also serves to store water;
  • a drainage layer that will channel excess water away; and
  • a root barrier that prevents plant roots from damaging the root membrane (Figure 1).

These assemblies are generally categorized as ‘intensive’ or ‘extensive,’ based on weight.

Rain is intercepted by the plants before it reaches the growing medium on a green roof. Much of the ‘incident rain’ is infiltrated, absorbed, and adsorbed by the growing medium and plants. Excess rain travels through the growing medium, exits the filter layer into the drainage layer, and flows along the roof membrane to the roof drains. The stored water is either taken up by the plants or returned to the atmosphere through evaporation.

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.

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.

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.

Figure 2: Water-retention capacity of typical growing media.

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.

When an incentive policy mandates a green roof to have a minimum of 100-mm (4-in.) growing medium, it will achieve a water-retention capacity of 65 mm (2 9/16 in.) when GM#1 is used but only 40 mm (1 9/16 in.) if GM#2 is employed. While both growing media will meet the prescriptive requirements of this green roof policy, GM#1 can retain 25 per cent more water on a volume basis compared to GM#2. While it is easy to specify a minimum growing medium depth, prescriptive policy does not necessarily achieve specific stormwater performance.

Additionally, soil is heavy when wet. Typical green roof growing medium weighs 24 to 34 kg/m2 (5 to 7 psf) per 25-mm (1-in.) depth when fully saturated. Prescriptive green roof incentive programs in North America generally require a minimum growing medium depth of 75 to 100 mm (3 to 4 in.), which translate to an additional loading of 72 to 136 kg/m2 (15 to 28 psf). This extra weight often prevents lightweight construction such as factories and warehouses from adopting vegetative roofs. Unfortunately, these buildings often have large footprints, which exert a considerable burden on the municipality’s stormwater infrastructure.

Water-retention fleece and horticultural mineral wool offer lightweight alternatives to growing media to achieve water retention in green roof systems. Figure 3 compares the saturated weights of typical growing media and water-retention materials (again, normalized to 25 mm for ease of comparison). Each bar represents the saturated weight composed of the dry component (red) and the stored water (blue).

The saturated weight of the water-retention layers is an average of about 25 per cent lighter than typical growing media. Additionally, only 9.8 kg/m2 (2 psf) out of the saturated weight of 38.8 kg/m2 (7.9 psf) or 25 per cent by weight, for the lava/pumice/dolomite growing medium is water.

Figure 3: Water-retention capacity of different green roof components.

In comparison, a high fraction of the saturated weight for the retention layers comes from the stored water as evident from the high blue fraction compared to the small red fraction in each bar. For example, out of the 24 kg/m2 (4.9 psf) saturated weight of the mineral wool, 23.4 kg/m2 (4.8 psf) or 98 per cent of which is water. It is clear the water-retention-to-weight ratio for the water-retention materials is significantly higher than growing media. Replacing all or part of the growing medium in a green roof with materials such as fleece and mineral wool can achieve equal or better water-storage capacity while keeping the system weight low.

A prescriptive-based policy mandating a minimum growing medium depth can impose too much loading on the roof structure and prevent buildings with limited structural capacity (e.g. factories and warehouses) and the existing building stock from adopting green roofs. However, an objective-based policy that requires specific stormwater performance (e.g. water-retention capacity) enables these buildings to adopt green roof systems based on lightweight water-retention layers instead of growing media and contribute positively to the overall stormwater management goal in the municipality. (For more, see “Sherway [Rooftop] Gardens”)