Ten Design and Engineering Strategies for Managing Stormwater on Green Roofs
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With summer temperatures comes summer thunderstorms and as predicted by climate scientists, these storms have become much more intense as a result of the climate crisis. Almost everyone now has stories of how the one in a hundred year storm has appeared several times already, or how our existing gray stormwater infrastructure is not up to the task of dealing with the impact of increasing amounts of stormwater. The results of these storms are often disastrous, with billions of dollars in damages to property and infrastructure, severe ecosystem impairment and even loss of life. Moreover, areas of the world that hardly ever get much rain, like Dubai for example, suddenly find themselves under water. In April 2024 more than 10 inches of rain fell in the desert region over 24 hours.
Intense rainfall is occurring in all the wrong places: U.A.E. in April after 10 inches of rain in 24 hrs. Courtesy: The National News.
Flooding damages are one of the most expensive impacts of the climate crisis thus far. Flood waters affect everything in their path: buildings, possessions, automobiles, infrastructure, crops, landscapes, wildlife and humans. Stormwater management in the face of ever increasing storm intensity requires that we use all of the tools in our tool basket - gray and green. Redesigning our communities to become “sponge cities” that soak up and adapt to increasing amounts of stormwater is a priority for many jurisdictions. This involves implementing a suite of strategies, (a treatment train), ranging from facilitating the infiltration of stormwater into the ground with permeable pavers and then back to aquifers, to temporary water storage in open spaces, parks, ponds and on trails, to the capture and slow release of water on our rooftops.
This article presents ten Design/Engineering Strategies, in no particular order, for improving the water management capacity of green roof systems, and by incorporating detention through the use of blue roofs, which capture and temporarily store water prior to its controlled release.
Extremely heavy rains in early June 2024 (+20 inches) have recently created flooding in southern Florida where flooding was not common.
More is Better - Part 1
Covering a larger roof surface area with a green roof will increase its ability to capture and retain water. In some jurisdictions, there is a troubling and somewhat ironic practice of trying to minimize the green roof area as much as possible by looking for and exploiting loopholes in regulations. This often results in postage stamp sized green roofs that are both more costly to build per square foot and significantly reduce public and private benefits like stormwater retention, urban heat island reduction, biodiversity, energy savings and membrane durability. Since green roofs can typically capture more than 70 to 80 percent of annual precipitation in most North American cities, (depending on rainfall patterns and green roof designs), jurisdictions should consider requiring a minimum green roof area on new buildings in order to avoid postage stamp sized green roofs.
Another approach for municipalities to take, is to prescribe a minimum amount of retention capacity on a rooftop and then allow the creativity of the manufacturers, engineers and designers to adjust the assembly accordingly to meet the performance requirements. Examples of these requirements range from zero runoff for a hundred year storm at one end of the spectrum, to the first inch of rainfall at the other. Meeting stormwater volume retention requirements would likely involve combining a number of different strategies, described in more detail below.
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More is Better - Part 2
Leaf Area Index is a measure of the density of leaf surface area. The greater the density and diversity of plant species the more rain will be intercepted and the faster the green roof will replenish its retention capacity through the process of evapotranspiration (the transpiration of the plants during photosynthesis and evaporation of moisture from their leaves). Diverse and dense plantings also result in greater root mass build up which also enhances stormwater retention by slowing down water molecules on their way to drainage systems.
Plant composition can also influence the water holding capacity of a green roof because different kinds of plant forms (e.g. succulents, forbs, bulbs, grasses) have different rates of evapotranspiration. Evapotranspiration plays a major role in the retention performance of green roofs due to removal of retained water from the system and the corresponding increase in volume available for storage in the green roof medium for subsequent precipitation events. Plants which utilize a lot of water may also be subject to wilting more quickly than drought tolerant plants and may therefore require supplemental irrigation during periods of drought. Grasses and wildflowers, for example, can process more water than sedums, (which tend to hold onto it to help survive drought conditions), but may require irrigation in warm and dry climates.
Understanding the performance of the plants you are using is key. Beware of plant performance comparisons conducted by researchers because they are often hampered by short study durations, and end up taking stormwater retention measurements on green roofs with immature plants. While succulent based green roofs can soak up a lot of water quickly, meadow-based green roofs tend to soak up more when fully established. They also contribute additional biodiversity and carbon sequestration related benefits. As always, there are tradeoffs that have to be made when selecting green roof plant species and developing maintenance plans.
The particle size distribution in a growing media is a key factor that determines its porosity and water retaining capacity. Photo: Steven Peck
More is Better - Part 3
But… Engineered growing media depth is often used as a proxy for stormwater retention capacity. This is true to some extent, but stormwater retention capacity is more a function of the composition of the growing media than its depth. Generally speaking, the higher the volume of coarse aggregates in the growing media mix, the less stormwater retention capacity. Particle size distribution in the growing medium is a key factor in determining water holding capacity. Having a range of smaller particles in the mix increases the pore space between them which facilitates water holding. In addition, the use of porous aggregates, like expanded shale, perlite and volcanic rock which have enormous surface areas from their many pores greatly improves water storage capacity. Despite their small size, the surfaces of their internal pores significantly increase their water holding capacity. The flow rates through various types of engineered growing media are another factor to consider. Generally speaking, growing media has multiple performance functions, so it is very important to use high performance growing media as tested by ASTM standard testing protocols such as E 2399 which addresses saturated weights.
For some time, manufacturers have been working to develop growing media products designed to meet multiple design objectives. Omni Ecosystems for example, has developed a growing medium that has very high porosity which is being used to retain stormwater on roofs and also for land based green infrastructure applications, like bioswales. Rooflite has numerous blends of growing media for all types of green roofing, including nutrient rich blends for agricultural applications. See rooflite and Joe DiNorscia’s article in this LAM. Naturcycle has multiple growing media products to meet a variety of green roof design requirements. American Hydrotech has developed a wide array of growing media blends that are based on physical, nutritional and biological requirements of the different type of plants used in extensive, intensive, lawn and urban agriculture assemblies. All of these firms have in house expertise at your disposal.
For a more comprehensive review of the research on the subject of growing media and water retention please see Kazemi et al.
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Close up of the internal porosity of this oversized piece of aggregate. Growing media aggregates are an important factor in determining porosity and water retention. Photo: Steven Peck
Drainage Layers Can Do More!
Drainage is very important in green roof assemblies. There are many different types of drainage layers on the market. Their main job is facilitating run off from the roof when the layers above become fully saturated with rainfall. This avoids water ponding which can kill plants, and also reduces the weight load on the roof during heavy storms. Yet there are also drainage layer products that capture and retain rainfall before releasing it, often including the ability to wick the moisture back up into the growing media to support plant growth. Most modular green roof systems have some degree of additional water storage built into them. Liveroof has recently introduced a new modular system called ‘Retain’ that holds additional stormwater to be used for irrigation of the green roof above. Research has shown that the reusing of water on a roof in this way can help plants thrive and also reduce the frequency of irrigation, saving money.
Moisture Retention Fleeces
In addition to drainage layers, there are a number of moisture/water retention fleeces on the market. They are a high loft, non-woven geosynthetic consisting of 100 percent recycled high tensile polyester fibers. Available in half inch and inch depths, these UV stable products provide additional water retention and porosity for root anchorage. The water holding capacity of fleeces can vary considerably. Water retention fleeces are sometimes used as a replacement for growing media in very lightweight green roof applications. Some degree of growing media is required to sustain the nutrient demands of the plants, or the assembly will require hydroponics to supply the plants with nutrients through the irrigation system.
Roof Slopes
The slope of a roof has a significant impact on the drainage rate, with greater slopes resulting in faster migration of water through the plants, growing medium and other layers to the drains. The sheer verticality of living walls results in a rapid loss of water from these green wall systems due to gravity, and is one of the reasons they must be irrigated regularly and carefully maintained. Flat roofs, or roofs with only a slight slope, are best for blue-green roof applications, since water can be detained on the roof membrane, below the green roof layers. A pitch under 10 degrees is fairly typical for most green roofs, facilitating runoff into drains and allowing for easy maintenance access. Sempergreen’s “Purple Roof” product is designed to slow the movement of water off a roof, even on sloped roofs, where conventional blue roof water storage is not possible. There are also new types of pavers on the market that have enhanced stormwater management capabilities, see LiveRoof.
For a detailed research paper on the effect of slope on drainage please see here.
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Blue-Green Roofs
They combine water detention practices of a blue roof, with the water retention abilities of green roofing. The continued use of amenity decks are ideal for including blue roof technologies to address very significant amounts of stormwater detention. A blue roof is a roof that is designed with the express purpose of detaining water followed by its controlled release, at a certain depth, or after a certain period of time. This can be accomplished through a variety of means: flow-control drain, cisterns, vaults, grid systems, drainage dams etc. All of these systems exist below the green roof plants, growing medium and drainage layers, or below pavers or stone ballast as well. The retention of water directly on a roof membrane provides engineers with greater certainty over the volumes of water that will be managed by the system. Some jurisdictions have regulations that govern the amount of water that may be stored on a roof so it’s important to check. Blue-green roofs require a high degree of expertise in terms of design detailing and roofing contracting to ensure the water tightness of the structure.
Stronger is Better
Structural loading capacity to hold the system assembly of a blue-green roof is essential. The saturated weight of the green roof, combined with the blue roof system and its total water holding capacity need to be carefully calculated to determine the minimum structural loads required. High quality, long lasting waterproofing systems designed for continuous water exposure and hydrostatic pressure are required. The waterproofing on the roof has to be carefully designed and installed in a manner that allows for water detention, under or over the various assemblies.
Blue-green roof cross section, protected membrane assembly. Courtesy American Hydrotech. See Blue Green Case Study by Richard Hayden of the Northwestern Biomedical Project in Living Architecture Monitor Fall 2023.
Drains
The drains on a blue roof are designed to include a standpipe in the center of the drain body. This standpipe includes an orifice at the deck level that is specifically sized to create water release in accordance to what the stormwater engineer requires. This orifice allows water to leave the roof in a restricted flow over a certain length of time as required by the stormwater engineer. In the event of a larger rain, the standpipe acts as a safety outlet and allows water to enter the drain unimpeded and unrestricted. This protects the roof from overloading.
Drainage dams can be remote controlled, so that the discharge can be timed to allow the roof to store more water during an upcoming storm event. Remote controlled systems require very careful monitoring. Other drains have fixed dams that will hold the water until it has evaporated, or released from the roof automatically at a certain depth. The design of the height of drains and water detention levels are both important considerations with blue-green roof assemblies.
Conclusion
While green roofs are known for their stormwater retention capabilities, the addition of blue roofs under a green roof enables an even greater degree of certainty related to the temporary detention of stormwater. While the practice of detaining water on roofs is by no means new, it requires careful consideration of structural loading, high quality drainage design, high quality waterproofing design and high quality roofing installation. By retaining and detaining stormwater onsite, blue-green roofs reduce the strain on infrastructure and can help to mitigate the impacts of flooding.
Green roofs are the first step in the stormwater treatment train idea, and can contribute to the evolution of ‘sponge cities’ to reduce or even eliminate flooding. When combined with other green infrastructure approaches, such as permeable pavers and rain gardens, it is possible to shift towards more natural hydrological systems in our cities and support plants that also contribute significantly to our ability to adapt to climate extremes and maintain human health. Fortunately, there are now lots of tools in the designer/engineer toolbox to maximize the stormwater retention benefits of green roofs and for policy makers to pass effective regulations to maximize the many public benefits provided by these technologies.
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Steven W. Peck, GRP, GRIMP, Honorary ASLA is the founder and president of Green Roofs for Healthy Cities and the editor of the Living Architecture Monitor magazine.
Special thanks to Kees Govers, GPR, Richard Hayden, GRP, Dennis Yanez, Tricia Billings and Bruce Dvorak for their input on this article.
For More Information
For an analysis of ASTM and International Product Testing Methods and Guidelines see the following article by Bruce Dvorak
The effect of slopes on drainage, see B. Rowe et al. in Ecological Engineering, 2011.
For a comparison of various strategies to retain stormwater, see B. Rowe et al. in Journal of Living Architecture.
American Hydrotech, Inc. - Garden Roof, Blue Roof, and Ultimate (paver) assemblies and components that can be combined in a nearly endless array of rooftop configurations.
Columbia Green - Blue Green Roof systems and moisture retention fleeces.
Growing Media Experts/Blenders - Naturecycle; Omni Ecosystems; Rooflite
The Blue-Green Roof Symposium proceedings are available on the internet through the Living Architecture Academy and feature engaging discussion and case studies from multiple experts.
The German industry association has developed detailed guidelines for green roofs. The FLL Guidelines contain considerable detail regarding the components and performance expectations of growing media and its drainage, moisture, nutrients, organic content, and structural stability over time.
