Green roofs have become an accepted alternative for stormwater management. The number of green roof installations across North America has greatly increased over the past decade. The annual market survey conducted by Green Roofs for Healthy Cities shows that the North American green roof industry grew by 14% in 2013, the tenth consecutive year of double-digit growth, with 6.4 million square feet (595,000 square meters) of new green roofs.
Many North American municipalities promote green roof implementation through public policies and incentive programs, primarily as a stormwater management strategy. Cities offering financially supportive policies consistently show the largest numbers of installed green roofs and area coverage.
Most incentive policies are prescriptive, often mandating standard depths of green roof growing medium. However, performance-based policies, which use actual stormwater management objectives instead of prescriptive green roof assembly requirements, are more likely to maximize the benefits of green roofs as a best management practice (BMP).
LID and Stormwater Management
In natural watersheds, surface runoff is minimal, because most rainfall infiltrates the pervious landscape. Soil and vegetation absorb some of the rainfall, while the rest recharges groundwater tables. The natural hydrological cycle returns stored water to the atmosphere through evaporation and transpiration.
Urban landscapes feature expansive, contiguous areas of impervious surfaces such as roadways, parking lots, sidewalks, and rooftops that increase surface runoff and reduce infiltration. Disruptions in natural water budgets lead to economic and environmental consequences such as flash flooding, waterway erosion, reduced channel stability, and degradation of stream habitats. In cities with storm sewers connected to sanitary sewers, heavy rain events cause combined sewage overflow (CSO) events that discharge untreated sewage wastes directly into surface waterways.
Conventional stormwater management focuses on mitigation and flood control. The objective is efficient collection and rapid conveyance of runoff from residential and commercial developments via extensive and expensive stormwater infrastructure. However, this conventional approach may not protect aquatic resources from the adverse effects of development.
Low-impact development (LID) is a stormwater management and land development strategy applied at a subwatershed scale that emphasizes natural features integrated with engineered hydrological controls to mimic predevelopment hydrological functions. This is primarily a source control strategy, focused on evaporation, transpiration, and infiltration of stormwater onsite through vegetation and soil. LID stormwater management strategies include EPA-recognized BMPs such as pervious pavements, rain gardens, and green roofs.
Green Roof Fundamentals
Green roofs–also known as vegetated roofs, living roofs, eco roofs, or garden roofs–provide multiple environmental, ecological, and economic benefits for densely urbanized areas. Green roofs help cities manage stormwater runoff and mitigate the urban heat island effect. They provide habitat for birds and beneficial insects and can serve as a platform for urban agriculture. Protective vegetated coverage prolongs the service life of roof materials by buffering daily temperature extremes and blocking destructive ultraviolet radiation.
The principal components of green roofs include vegetation, the living plant element; growing medium, which provides water, nutrients, and anchorage for plant roots; filter layer/retention fabric, which prevents small particles of growing medium from clogging the drainage layer and provides water storage capacity; drainage layer, which channels excess water to roof drains; and root barrier, which prevents plant roots from damaging the roof membrane (see illustration).
Green roofs are categorized as intensive or extensive based on system profile depth. Intensive systems, commonly known as landscaped roofs or rooftop gardens, feature growing medium soil depths of 6 inches (15 centimeters) or more. They can support a wide variety of plant types including turf grasses, flowering ornamentals, shrubs, and even trees. Similar to at-grade landscapes, intensive green roofs require regular maintenance regimens, including regular irrigation, weed removal, mowing or pruning, and plant litter removal. Because of their weight, which can range from about 40 to 300 pounds per square foot (psf) (200 to 1,500 kilograms per square meter) saturated weight, intensive green roofs impose high loadbearing requirements. Therefore, they are most appropriate for new construction.
Extensive green roofs are lightweight, low-profile systems with less than 6 inches (15 centimeters) growing medium soil depth. They support a more limited range of plants, including succulents, such as the commonly used sedum, and shallow-rooted, dry-climate herbs, grasses, and wildflowers. Extensive green roofs are less expensive than intensive systems and require minimal maintenance. With saturated weights ranging from 10 to 40 psf (50 to 200 kg/m2), extensive systems are practical for large-span structures or retrofit projects without the need for costly structural reinforcement.
Green Roof Stormwater Management
Green roofs mimic natural catchments by infiltration and reduced surface runoff. However, there are two critical differences. First, the limited vertical dimension of growing medium profiles, even in intensive systems, does not allow deep infiltration compared to the soil in natural catchments. Second, the vegetation is less diverse than in natural catchments, which affects interception, vertical absorption, and evapotranspiration processes. Nonetheless, research has demonstrated that green roofs effectively delay and reduce roof peak flow and runoff volume, which during heavy rain events can help prevent flash flooding and the frequency and intensity of CSO episodes.
Stormwater management potential is influenced by the individual and combined holding capacity of several green roof system assembly components.
Vegetation. Plants contribute to green roof stormwater retention capacity via two alternate pathways. Most terrestrial plants take up water from the root zone with simultaneous release during the daytime. Succulents draw water from the growing medium into their leaves and stems for storage during the day with release into the atmosphere during cooler nighttime conditions. Both approaches provide runoff mitigation by removing water from the rooting layers and releasing it back into the atmosphere, which “recharges” green roof system storage capacity for the next rain event.
Because they hold and conserve water during the hottest part of the day, succulents, such as sedum, are drought tolerant and can endure the hot, dry, windy conditions that generally prevail on rooftops. They are also shallow rooted. These factors make succulents well suited for the plant palettes of extensive, low-profile green roofs, which account for 75% of the total installed green roof area in North America.
Growing Medium. Green roof growing medium “soils” are high in mineral aggregate content (65 to 80% by volume) and low in organic matter (10 to 25% by volume). Water is retained in the porous mineral particles (such as lava or expanded clay), the capillary pores between particles, and in the organic matter fraction. Typical green roof growing medium has dry weight of 3.5 to 4 psf per 1-inch depth (17.5 to 20 2.0 kg/m2) and maximum saturated weight of 5.5 to 7 psf per 1-inch depth (27.5 to 35 kg/m2), which represents a water-holding capacity that approximates a half-inch (12-millimeter) rainfall event.
Water Retention Layer. Absorptive materials, such as synthetic fleece and horticultural rockwool, are highly effective in storing water compared to growing medium on a relative unit weight basis. For example, a 0.4-psf dry weight (2.0 kg/m2) water retention fleece has a saturated weight of 3 psf (15 kg/m2), which is a holding capacity equivalent to a 0.54-inch (13-millimeter) rain event. To retain the same amount of water, 1 inch (25 millimeters) of growing medium with 4-psf dry weight (20 kg/m2) and a saturated weight of up to 7 psf (34 kg/m2) would be required. Thus, for equivalent rainwater storage capacity, retention textiles can offer approximately a 4-psf (20 kg/m2) weight reduction (57%), versus growing medium. The weight advantage of retention textiles is a particularly important consideration for green roofs on retrofits or other buildings with limited structural loadbearing capacity.
Drainage Layer. The green roof drainage layer provides a flow zone to convey excess water to roof drains. Some assemblies utilize a fully open geotextile layer with minimal stormwater retention capability. Some drainage products are molded plastic sheets with “cups” that act as water reservoirs and provide both drainage and water retention capabilities. Alternatively, the drainage layer may be composed of a bed course of aggregate media, such as expanded shale or clay, which may provide both drainage and water storage. However, an aggregate drainage layer adds a significant load burden relative to geotextile drain products, similar to the weight tradeoff between fleece or rockwool and growing media.
Municipal Policy Incentives
Since 2000, numerous North American cities and municipal jurisdictions have enacted programs (Table 1) to facilitate adoption of green roofs on newly constructed and re-roofed properties to mitigate stormwater runoff and provide ancillary benefits. Most policies provide financial incentives, such as direct cash grants or reduction to or exemption from property taxes and stormwater fees. Some policies specifically target implementation of green roofs, while other incentive programs include green roofs as part of a suite of BMPs. The variety of incentives includes accelerated building permit review (Chicago), low-interest loans for green roof projects (Cincinnati), and outright grants (Milwaukee). While municipal stormwater/green roof programs generally rely on “carrot” incentives, others utilize a “stick.” For example, Toronto penalizes noncompliance with its green roof mandate with a one-time $200-per-square-meter charge.
Prescriptive Policies. Most municipal green roof programs are prescriptive. One common prescription is mandating a specific minimum growing medium depth.
Prescriptive approaches assume targeted stormwater storage volume or runoff retention. However, specific stormwater reduction criteria are not generally stated or indicated as essential for eligibility. For instance, policies requiring a growing media depth of 4 inches (10 centimeters), featured in several municipalities’ policies, imply an expectation for a water retention coefficient equivalent to the maximum water-holding capacity of the media layer, which would be approximately a 2-inch (50-millimeter) rain event. However, retention of runoff from such a rain event is not an explicit policy goal.
Some prescriptive policies require that at least a minimum of the rooftop footprint be covered by green roof vegetation, in most cases 50% of roof surface area, such as New York City’s 2013 program. Cincinnati requires the green roof area to be at least as large as 50% of the ground floor area, accounting for the permeable ground level surface displaced by the building footprint, rather than just the roof top surface area. In addition to green roof percentage area, some policies require the green area to be larger than a designated minimum size.
What all the prescriptive policies have in common is the assumption that the mandated green roof features will have a positive effect on reducing runoff. What they lack is a specific reduction goal.
Unfortunately, prescriptive policies, particularly those focused on required growing media depth, do not automatically or consistently promote effective stormwater management. Whereas virtually any type of vegetated roof assembly outperforms conventional roof surfaces for runoff reduction, use of media depth as an implied proxy for runoff management is likely to be ineffective–or worse, counterproductive–as a means to achieve optimal stormwater management.
As noted, growing medium depth is not the only relevant factor for stormwater retention in green roof assemblies. Given their weight advantage, the holding capacity of lightweight water retention materials, as a supplement to or substitute for deeper and heavier amounts of growing media, should be taken into account.
Moreover, commercially available green roof medium formulations are not sufficiently alike to have predictably similar water-holding capacities. Generic, depth-based growing medium mandates rely on the marketplace to supply a standardized product. However, media components have significant differences in hydrological properties, due to wide variation in composition, aggregate particle size, porosity, dry-wet weight ratios, and purity. Individual designers and installers may opt to use alternative or unproven materials–such as crushed bricks, pea gravel, recycled rubber tire material, and other base materials or additives–with wide ranges of water-holding capacities.
Generic requirements of 4-inch depths (for example, Nashville, Portland, and Toronto) represent a 28 psf (137 kg/m2) saturated weight for the growing medium only, before factoring in the additional load of other green roof assembly components. Arbitrary depth-based prescriptive policies can be counterproductive because they exclude green roof applications on large-span structures, older building retrofits, or any new construction with limited roof load capacity.
Excluding green roofs with less than 4 inches of growing medium is imprudent policy, because such green roofs can achieve significant runoff reduction. For example, the large-span 454,000-square-foot (42,193-square-meter) green roof atop the Ford truck assembly plant in Dearborn, MI, was constructed with a total 25-psf (122 kg/m2) loadbearing capacity limit. The green roof system utilized includes only 1.25 inches (32 millimeters) of growing medium and has a saturated loading weight of only 8 psf (39 kg/m2). Yet, it retains more than 4.3 million gallons of annual rainfall.
The Silver City Townhomes green roofs in Milwaukee provide another example. In this urban infill project, there was little space at ground level for stormwater features. Therefore, the project plan included five green roofs, funded in part by a grant from the Metropolitan Milwaukee Sewerage District (MMSD). Each measures 2,315 square feet (215 square meters). Because of the building’s wood frame construction, common in many new affordable multi-housing developments, light weight was a key consideration in selecting a green roof system to minimize the structural requirements and associated costs to accommodate the load from the added weight. The 9.5-psf (46 kg/m2) green roof system used for the project contains only 1.25 inches (32 millimeters) of medium. To optimize stormwater retention performance, the assemblies feature two supplemental layers of lightweight 1/4-inch (6-millimeter) water retention/filter fleece.
With the additional lightweight fleece boosting stormwater retention, the green roofs have more than a 1-inch (25-millimeter) rainfall holding capacity. If MMSD had arbitrarily required 4 inches (10 centimeters) of growing medium, it would have foregone total annual runoff retention of more than 164,000 gallons.
In addition to excluding important individual installations, media depth-based criteria may restrict effective green roof applications in priority subwatershed districts. Critical subwatershed areas are commonly characterized by combinations of minimal permeable land area, large roofed commercial and industrial buildings, and a high relative percentage of older properties with limited loadbearing capacity. Depth-based criteria for green roof incentives can therefore deprive municipal governments and the communities they serve of the benefits of lightweight green roofs for runoff management in these priority areas.
Performance-Based Policies. Some municipal green roof incentive policies utilize performance-based stormwater management criteria as designated target requirements for eligibility. A performance criterion specifically states a targeted stormwater event quantity that the proposed green roof system must manage for eligibility.
Whereas previous New York City green roof incentives required a prescriptive 3-inch (76-millimeter) growing medium depth, the current program requires management of a 1-inch (25-millimeter) rain event for the entire property footprint using any combination of approved stormwater BMPs, including green roofs. Similarly, Washington DC’s RiverSmart Rooftops program requires retention of a 1-inch (25-millimeter) rainfall, with substantial per-unit-area bonuses for projects in highest priority subwatershed communities. These performance-based policies encourage dynamic combinations of treatment options, including use of structurally compatible green roof assemblies, without arbitrary medium depth mandates that could make otherwise effective and beneficial green roofs infeasible due to weight.
Most buildings can support lightweight green roof assemblies with minimal to no supplemental costs for structural reinforcement. In the face of prevailing incentive policies, these rooftops represent an underutilized resource for stormwater treatment and incentive funding benefits. Outcome-based performance incentives allow officials to consider and promote lighter weight green roof designs, which are also lower cost. Such green roofs can be especially important and valuable in highly urbanized subwatershed areas, such as older downtown and industrial sections of urban cores.
Selected References
Aquilina, D. 2013. “Case Study: Green Roofs Provide Signature Element for Affordable Townhomes.” Multi-Housing News. www.multihousingnews.com/in-focus/case-study-green-roofs-provide-signature-element-for-affordable-townhomes/1004077771.html. April 17, 2013.
Aquilina, D. 2013. “Downtown Stormwater Management.” Carolinas Roofing. November-December 2013.
Aquilina, D. 2013. “Ford Dearborn Truck Plant Green Roof at the Rouge Complex: Looking Back Ten Years.” Greenroofs.com. www.greenroofs.com/content/articles/109-Ford-Dearborn-Truck-Plant-Green-Roof-at-the-Rouge-Complex-Looking-Back-Ten-Years.htm. October 22, 2013.
Dreps, C., L. Hanson, and P. Raabe. 2014. Ellerbe Creek Green Infrastructure Partnership Technical Report. Ellerbe Creek Green Infrastructure Partnership. April 2014.
Getter, K. K., D. B. Rowe, and J. A. Andersen. 2007. “Quantifying the Effect of Slope on Extensive Green Roof Stormwater Retention.” Ecological Engineering 31: 225–31.
Green Roofs for Healthy Cities. 2014. 2013 Annual Green Roof Industry Survey. April 2014.
VanWoert, N., D. B. Rowe, J. A. Anderson, C. L. Rugh, et al. 2005. “Green Roof Stormwater Retention: Effects of Roof Surface, Slope, and Media Depth.” Journal of Environmental Quality 34(3):1036–44.
Acknowledgements
David Aquilina, strategic storyteller, Minneapolis, MN; Chris Dreps, executive director, Ellerbe Creek Watershed Association, Durham, NC; and Peter Raabe, North Carolina conversation director, American Rivers, Durham, NC.
