Designing green roofs for stormwater management

XF301 system Courtesy of Xeroflor
Images courtesy Xeroflor Canada

By Karen Liu, PhD
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.

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