Video Tour of ASLA’s Green Roof

4 01 2010

Back in October 2009 in our post “Green Roofs Address D.C.’s Environmental Problems”, we covered the research the American Society of Landscape Architects was doing with the green roof on its National headquarters and the many benefits it provides. I recently came across this well produced video tour of ASLA’s green roof (see below). It does a wonderful job of showing off the space and the diverse habitat that has been created. I particularly love the areas that use the steel grating to span some of the green roof areas. Enjoy.

Cooling Our Cities: An Interview with Dr. David Sailor

23 12 2009

I had the pleasure of conducting an email interview with Dr. David Sailor, the director of the Green Building Research Laboratory (GBRL) at Portland State University (link to resume). He is a leading researcher in the effects of green roofs and energy use in buildings and the impact green infrastructure can have on cooling our cities. He and his colleagues have developed tools to help quantify these impacts.

Green Infrastructure Digest (GrID): I understand that this year you became the first Director of the Green Building Research Laboratory (GBRL) at Portland State University. What is the focus of the research you are conducting at GBRL? Why and how was GBRL started?

Dr. David Sailor: The Green Building Research Laboratory was essentially an outgrowth of the funded research agendas of myself and my GBRL colleagues. This group of Portland State University faculty included Graig Spolek in Mechanical and Materials Engineering, Loren Lutzenhiser r in Urban Studies, and Sergio Palleroni in Architecture. Over the years we had developed a number of collaborative projects and decided it was time to build upon this collaboration by creating a physical laboratory where we, our students, and our industry partners could work together on fundamental and applied research to benefit the green building industry.

We pitched the idea of a collaborative shared-user facility to Oregon BEST and to the PSU Center for Sustainable Processes and Practices, both of whom agreed to provide the initial funding for the lab. As a result, while the lab was initiated by four faculty members at Portland State, it really serves as an Oregon University System shared resource.

There really is a wide range of research activities going on in the lab. This includes several monitoring projects with local builders, property owners, and school districts with a focus on understanding the thermal and moisture performance of building envelopes as well as the indoor environmental quality of these buildings. We also continue to make advances in the monitoring and modeling of green roof performance. Personally I have been involved in a number of monitoring projects and in creating an energy modeling tool for evaluating the energy performance of green roof design decisions. My colleague Graig Spolek has been using some of the GBRL water quality testing equipment to better understand the chemical composition of green roof runoff. We are also using GBRL facilities to understand the interactions between buildings and the urban atmospheric environment. This involves both modeling and field measurements.

GrID: A lot of your research has been focused on green roofs and heat island mitigation through the use of green infrastructure. How significant of a role does green infrastructure have in addressing thermal heat gain within our built environment? It appears through the tools you have developed for the MIST program that you have been able to quantify these impacts? Can you explain to our readers, what the MIST program entails?

Dr. Sailor: Yes, my research career actually started with a focus on urban heat island mitigation through use of urban vegetation and highly reflective (high albedo) urban surfaces. The US EPA funded some of my early modeling efforts in an attempt to provide a quantitative assessment of how much potential there is for cities to cool their summertime air temperatures through city-wide modification of urban surface characteristics (vegetation and albedo). We used regional scale atmospheric models of about 20 cities across the US to create information on the potential impacts of such mitigation on summertime urban air temperatures, peak ozone concentrations, and energy consumption. The result of that was a fairly user-friendly urban heat island screening tool – the Mitigation Impact Screening Tool – or MIST. The tool uses fairly simple interpolation and extrapolation of our modeled results so that policy makers in any US city can easily estimate the order of magnitude of the impact that any particular mitigation strategy might have in their city. I like to emphasize the “S” in MIST – this is a Screening tool. Ultimately, any policy decisions that involve significant investment of public funds to mitigate the urban heat island ought to be based on a more thorough, city-specific analysis – which of course can start by running MIST.

Thermal Imaging Photo of Portland Buildings (Summer 2009)
Source: Green Building Research Laboratory (GBRL)

SP1=Typical Washed Rock Roof Membrane Cover (151.5F)
SP2=High Albedo (.75) White Roof (110.0F)
SP3=Low Albedo (.10est.) dark roof, NE Exposure (135.0F)
SP4=Low Albedo (.10est.) dark roof, SW Exposure (144.0F)
SP5=3-month Old Green Roof (Essentially Bare Soil) (100.0F)

GrID: Many of our readers would like to know if green roofs can reduce energy use in buildings and if so, by how much? What factors most influence the outcome? What light has your research been able to shed on this pressing question?

Dr. Sailor: The thermal performance of green roofs depends on a number of factors. Specifically, roof construction, depth and properties of the growing media – including soil moisture, plant characteristics and coverage, and local climate characteristics all affect heat transfer into the building. The role of the roof on building energy consumption also depends on internal building loads and schedules. An often overlooked point is that the performance of any alternative technology depends on the baseline that is used for the comparison. As a result, I hesitate to assign a specific level of savings that could be expected from green roof implementation. That said, the various simulations that we have conducted for cities across the US have shown that a green roof can have comparable summertime air conditioning benefits to those achieved by white or “cool” roofs. In contrast to a cool roof, however, the added thermal insulation of a green roof can result in a wintertime heating energy savings whereas the cool roof generally has a wintertime heating penalty. In general, our model shows that the annual air conditioning energy savings associated with replacing a typical roof with a green roof are on the order of 100 to 500 kWh for each 1,000 sq. ft of green roof. What is important to note, however, is that the energy savings are just one component to be considered in determining the economic and environmental value of green roofs. It is likely that the stormwater, urban heat island, and extended roof life aspects of green roofs are equally important.

GrID: The energy savings for green roofs are more modest than I would have expected. I remember some of your findings displayed at the 2007 Green Roof for Healthy Cities conference in Minneapolis had energy saving ranges between 4-12% depending on the location.

Dr. Sailor: Yes, the air conditioning energy savings by themselves are modest. The numbers I gave above are just for the Air Conditioning savings. Heating savings can be comparable or more important depending upon the location and roof design.

The data that you recall from the Minneapolis meeting were specified in terms of HVAC savings. The numbers from that poster were 3-6% annual cooling electricity savings in Minneapolis, 2-5% for Phoenix, and 3% in Orlando. For heating energy savings we had found up to 10-14% for Orlando and Phoenix, and about 4% for Minneapolis. While the model has changed some, these values are generally consistent with what we are still finding.

The nominal ranges that I described from our current model simulations are for a green roof in comparison to a roof that has an albedo of 30%. Both roofs are assumed to be insulated to modern energy code standards. The actual savings depend very much on the baseline used for comparison with the current tool providing a conservative estimate that might significantly underestimate savings for some applications. Also, it should be noted, that depending upon soil depth, vegetative lushness, local climate, and building type, a green roof can actually INCREASE the energy cost for heating or cooling in a building. The tool can provide the necessary feedback to avoid such a situation, and then help you move toward an optimum design with respect to total energy performance.

GrID: Oftentimes, engineers modeling the energy use of a building find it difficult to accurately simulate the effects of green roofs on energy use. Can you tell our readers about the plug-in your have created for the Department of Energy’s (DOE) EnergyPlus modeling software? How has this enabled mechanical engineers to more accurately model the effects of green roofs? How widely used is it?

Dr. Sailor: We developed a physically-based model of the energy balance of a vegetated roof and integrated this model into EnergyPlus. This module is now a part of the standard release of EnergyPlus and allows the energy modeler to explore how variations in green roof design can impact whole building energy performance. It is hard for me to assess how widely used it is among practitioners in the field, but I have been contacted by multiple groups around the US who are now gearing up to use the model in their research and design work.

GrID: If the impact green roofs have on energy use in a building is as modest as you describe, why would you need to model the green roof? Are there cases where it can/has make/made a significant difference?

Dr. Sailor: As I mentioned previously, if one does not pay attention to the green roof design from an energy standpoint, the roof may perform WORSE than a conventional roof. The tool can help the user avoid such potentially undesirable outcomes and then be used to optimize the design for improving energy performance beyond a conventional design. In the case of a retrofit the existing roof insulation may be significantly lower than current code or the current membrane may be much darker than the 30% reflective baseline that I use in the modeling. In such cases, the actual energy savings of the retrofit may be much larger than that reported directly in the current version of the calculator. Nevertheless, the calculator can be used to optimize the energy performance of the new roof.

GrID: What do you see as the future of green infrastructure?

Dr. Sailor: I think that historically there has been a bit of inertia within the building industry that tends to limit the pace of innovation and application of new concepts. From the perspective of an academic researcher I see great opportunities for applied research to develop new technologies and the data and modeling tools necessary to understand the building performance implications of these technologies. Green roofs and walls are technologies that are both promising, and receiving increased interest in recent years. In order for green infrastructure to reach its full potential, however, it is important to develop the tools and data needed to fully evaluate their many co-benefits.

-Brian Phelps


-Green Roof Energy Calculator

-Mitigation Impact Screening Tool


18 12 2009

Green Roof Dashboard
from Davis Center at University of Vermont

With a son who is a sophomore in college and a daughter as a high school senior, I have managed to spend a lot of time visiting college campuses over the past few years. One of the things that I have paid particular attention to (and seen an huge increase in during the past two years) is the focus on sustainability. My strong hunch is that schools are incorporating sustainable technologies because this generation of smart, college age youth demand it.

Many college campuses now sport LEED certification on at least one building – my son’s dorm at the University of Richmond (Lakeview Hall) is LEED registered and undergoing certification. It is one of nine buildings at the University which is either certified, or in process of being certified as LEED with the USGBC. Locally, Vanderbilt University completed the LEED certified The Commons at Vanderbilt residential housing complex in 2008. As I have traversed the country and seen what must be dozens of (mostly) smaller liberal arts colleges, I have seen organic gardens and solar panels at Whitman College, windmills and biomass generators at Middlebury, local and organic foods at Skidmore, a unique “homestead” intentional environmental community at Denison, beautiful rain gardens at Emory and the list goes on.

I also found a interesting resource online called the College Sustainability Report Card for 2010 (, This report card basically looks at environmental sustainability at over 325 colleges and universities in the United States and Canada based on 48 indicators used to evaluate performance within four categories.

One of those categories is “green building”. It was heartening to see that 44% of the schools have had at least one LEED-certified green building or are in process of constructing one and a whopping three-quarters of all of the schools have adopted green building policies that specify minimum performance levels such as LEED certification for new construction.

I was particularly interested in taking a closer look at some of the successes that I have witnessed at several of the schools that I have visited especially as they relate to green infrastructure. I found some additional information on Emory, Allegheny, Middlebury, University of Vermont and Macalester.


As a part of Emory University in Atlanta’s overall commitment to sustainability (with over 1 million square feet in LEED certified buildings), Emory has incorporated many innovative water-conservation technologies.. Particularly impressive to me was their implementation of rainwater harvesting and condensate recovery, especially in light of the fact that Atlanta suffered an historic drought event in the summer of 2007. On Emory’s whole campus they have to date included 6 cisterns with a collective size of over 350,000 gallons for both toilet flushing and for irrigation as well as a condensate recovery technology for over 4 million gallons of water per year.

In their new freshman residence complex including Ignatius Few Hall and Lettie Pate Whitehead Evans Hall, rainwater and condensate collection is diverted to an 89,000 gallon reservoir underground which can provide adequate volume to provide 2170 gallons per day needed to flush all toilets int eh buildings. The rainwater is collected form the roof, then filtered and slowed through a bioswale system outsde of the building and then into the below grade cistern. The condensate harvest provides a reliable source of water to supplement rainfall during those months from May through September. It is estimated that the condensate harvests is adding 300,000 gallons per year to the system.

At the nearby Whitehead Biomedical Research Facility Building, completed in 2001, the engineers devised a system of piping condensate back into nearby cooling towers to use as make-up water. This system conserves water AND diverts 2.5 million (that’s 2,500,000) gallons a year from the sanitary sewer system.

Video About Emory University’s Sustainability Efforts


It seems to me that many, many schools are incorporating green roofs as that technology provides one of the most visible elements to show-off sustainable design. In every school we visited, if there WAS a green roof, it was highlighted on the student led campus tours. The green roof were touted for their well-documented benefits such as longer roof life, reduced cost of heating and cooling, stormwater runoff reduction and habitat.

Allegheny College in Meadville, Pennsylvania impressed me with the well designed green roof on the Vukovich Center for Communication Arts. It is located within the topography of the campus site allowing for a fully accessible roof (entering the building at the green roof on the high side and entering on a lower level to the main campus commons or quad –type area. The roof includes extensive and semi-intensive depths and features lawn space as well as sedums and native grasses with an interesting incorporation of stones and cedar decking through the rooftop.

University of Vermont, just on the edge of downtown Burlington, Vermont, recently completed the 186,000 s.f. Dudley H. Davis Center. The Center features a 19,000 s.f. green roof.

Middlebury College, also in Vermont, provided a sloped green roof above the Atwater Dining Hall. I was interested in seeing their notation that in addition to the traditional green roof benefits that I have seen listed in may locations, Middlebury includes improved acoustical insulation, noting that green roof systems can reduce airborne sound levels by 40 to 50 decibels.

Macalester College in St. Paul, Minnesota impressed me, not in size but in determination. The two green roofs on campus were the result of student designs and even some student labor! The first green roof at Macalester was a 300 s.f. tray system installed above a walkway between tow residence halls and the newer 1350 s.f. green roof on Kagin Commons. I happened to be on campus the day the Kagin Commons green roof was unveiled.

I believe the influence of these campuses and so many others will influence the bright minds of our next generation of decision makers and leaders.

Kansas City Stormwater Overflow Control Plan

4 12 2009

Source: Kansas City, Missouri Overflow Control Plan Overview Document

This year Kansas City embarked on a massive $2.3 billion stormwater overflow control plan to address sewer overflows throughout the city. Its inclusion of a major $28 million green infrastructure pilot project has gained a lot of attention. The project has been recognized as the largest green infrastructure project in the United States. The Marlborough Neighborhood Pilot Project, as it is called, is located in the Middle Blue River Basin, one of the four major watersheds addressed by the plan. The entire pilot project encompasses nearly 100 acres of primarily residential neighborhoods. This program is anticipated to be expanded over a larger 744 acre area that will eventually include over 25 acres of mixed green infrastructure strategies (i.e. rain gardens, bioswales, permeable pavement, and green roofs) that have the capacity to sequester 3.5 million gallons of water. The green infrastructure strategies employed are designed to replace two underground tanks of similar capacity. In total the pilot project and its expansion are budgeted to cost $68 million.

Video of compiled images from Mark O’Hara’s Greenbuild Presentation about the Kansas City Plan. The video shows various Green Infrastructure Strategies recommended in the plan. Video compiled by Hawkins Partners Images provided by BNIM (Click here to see it if  video is not present)

In addition to the Marlborough Neighborhood Pilot Project, the plan also recommends the enhancement of the area’s highly acclaimed 10,000 Rain Garden Program. Over the past two years, the initiative is reported to have installed several hundred rain gardens, bioswales, and rain barrels. The purpose of the expansion it to develop an incentive program to accelerate the program’s progress and complement the public investments being made.

Wet retention basin projects have been identified as an appropriate strategy for treating stormwater downstream from six separated sewer system (SSS). The plan acknowledges that green infrastructure is beneficial and should be included where it is practical. The plan states:

“Every decision should be viewed as an opportunity to incorporate a green-solutions approach. The City has adopted an “every drop counts” philosophy, meaning it is important to reduce stormwater entering the system wherever practicable. This will be accomplished through changing the way the community develops and redevelops its sewer and stormwater infrastructure, educating citizens regarding steps they can take to reduce the amount of stormwater entering the sewer system, enabling citizens to take those steps, incorporating green infrastructure in the design of public infrastructure, and making targeted public investments in green infrastructure projects early in the Plan implementation.”

Areas identified that should be considered for green infrastructure projects include those meeting the following criteria:

  • Areas for which no or minimal conventional structural controls are proposed.
  • Areas in which widespread implementation of green solutions by the community at large offer the greatest opportunities for reducing the size and cost of conventional structural controls included in the Plan.
  • Areas for which it would be particularly desirable to further reduce the projected overflow
    activation frequency following completion of recommended controls.
  • Areas in which sewer separation is proposed but where no Water Services Department (WSD) investment in treating the separate stormwater runoff has been included in the Plan.
  • The plan’s ambitious Marlborough Neighborhood Pilot Project is very encouraging, particularly as a stand alone project. It is very significant and the City should be commended for their efforts. However based on the $2.3 billion budget established by the plan, it is evident that green infrastructure will play a supporting role. The plan was developed during the recent significant shift in the way we address stormwater management across the country over the last few years. It is not surprise to see this. What is encouraging is the magnitude of the pilot project and the extensive monitoring that will be conducted.

    The monitoring component will provide valuable data for the City and others across the country. In addition to understanding green infrastructure’s effectiveness to control Combined Sewer Overflows (CSOs) and improving water quality, monitoring it will provide insight into conflicts with local codes and ordinance, social-economic benefits, construction techniques, associated cost, and maintenance practices.

    The plan stresses that it is an evolutionary document, referring to it as an “adaptive management” approach. The approach involves evaluation of the strategy throughout the life of the project based on their experiences and data gathered through the monitoring efforts. While green infrastructure may not be the predominant tool of choice at this point, the longer-term nature of the plan provides the opportunity to adjust its course as confidence increases in green infrastructure. The City’s plan can become more green overtime as it builds upon its successes.

    Fairly or unfairly, like many pilot projects much rests on the success of the Marlborough Neighborhood Pilot Project. Many, both locally and nationally will be watching it with great interest. Failure of such a high profile project could significantly set back the growth of green infrastructure as the stormwater management tool of choice. Therefore, it is critical it is done to the highest standards possible. The project will serve as an example for those involved in stormwater planning and design to have full confidence and understanding of the complexities of utilizing natural systems. Natural processes are complex making them more difficult to quantify. A paper prepared in 2007 by the Center for Neighborhood Technology titled “Managing Urban Stormwater with Green Infrastructure: Case Studies of Five U.S. Local Governments”, identified the lack of performance data as a barrier to green infrastructure implementation. The more research we do and data we collect the better off we will all be.

    I anticipate this will be a successful demonstration of green infrastructure. It is exciting to see another city embrace green infrastructure on such a large scale. We will all eagerly await the results and follow its realization. Construction is expected to start soon.

    -Brian Phelps

    New Downtown High-rise Includes Green Roof

    30 11 2009

    The Pinnacle at Symphony Place, a 29-story office building in downtown Nashville, opened this month. The building includes 520,000sf of Class A office space. It is home to law offices of Bass Berry and Simms and the headquarters of Pinnacle Financial Partners. The building designed by the award winning architectural firm Pickard Chilton with Nashville architects EOA Architects is anticipated to receive LEED-Silver certification from the United States Green Building Council (USGBC). With the inclusion of a 28,000sf rooftop garden, the building contributes significantly to Nashville’s ever growing green infrastructure

    The green roof, designed by our office, is located on the 7th floor above the parking garage and includes a series of spaces that can be enjoyed by the building’s tenants. The area is comprised of 9,400sf of pedestal pavers and 19,000sf of vegetated areas. One hundred percent (100%) of the pavers were selected to exceed the minimum solar reflectance standards established by the LEED rating system. The striping pattern continues the prominent vertical fins on the facade of the building into the rooftop garden area. The planting areas are a combination of extensive green roof (planting media depths ranging between 5-9”) and semi-intensive areas (planting media depths ranging between 18”-30”). The semi-intensive areas were planted to reflect a more traditional landscape around each of the gathering areas and provide areas for trees to shade and scale the spaces. In an effort to establish a more pedestrian scaled environment and additional interest a series of 12ft pyramidal trellis structures were incorporated in the extensive green roof areas.

    It is estimated that the green roof can retain nearly 67% of the annual rainfall falling directly on it. In addition, it reduces the peak flows, is significantly cooler than neighboring conventional roofs, reduces thermal heat gains in the water that enters the stormwater system, and provides a beautiful space to look upon and enjoy.

    We are honored to have been a part of such an exciting project and look forward to watching it grow. We have been pleasantly surprised by the significant growth the plant material has shown in a short time period. As it matures, we will keep you up to date on its progress.

    -Brian Phelps

    Triple Bottom Line of Green Infrastructure

    18 11 2009

    Before and After of Green Infrastructure Improvements
    (Source:“Green Cities Clean Waters” Plan)

    In an earlier post titled “Making Green Infrastructure Common Place” we discussed the recent release of Philadelphia’s $1.6 billion dollar “Green Cities Clean Waters” Plan. Its thrust is to transform over 4,000 acres of impervious areas within the City’s Combined Sewer System to green space over the next 20 years through the use of green infrastructure strategies. This would involve converting over 34% of all existing impervious areas. Of this, the conversion will primarily be made on public property and right-of-ways. Green streets, the most widely used management tool, will comprise nearly 38% of these improvements (see graphic). The report claims this is “the largest green stormwater infrastructure program ever envisioned in this country”. While green infrastructure has been utilized and proven in many parts of the country, the sheer magnitude and commitment of the city is a radical departure from the conventional approach to stormwater management practices.

    Map of Green Street Locations
    (Source:“Green Cities Clean Waters” Plan)

    So why did Philadelphia decide to rely so heavily on green infrastructure as a means of reducing overflows in their CSO system? Quite simply it was cheaper, significantly cheaper. The plan estimates over the next 20 years the plan is to be implemented, the “triple bottom line” benefits (social, environments, economic) of the plan alone will add up to a present value of $2.2 billion dollars. The following is a breakdown of the benefits that comprise this figure.

    • Heat Stress Mortality Reduction (35%)
    • Recreation (22%)
    • Property Value Added (18%)
    • Water Quality and Habitat (14.5%)
    • Air Quality (4.6%)
    • Avoided Social Costs from Green Jobs (3.7%)
    • Energy Savings (1.0%)
    • Carbon Footprint Reduction (0.6%)
    • Reduction in Construction- Related Disruptions (0.2%)

    So instead of employing conventional underground infrastructure that is one-dimensional, and is estimated to cost $16 billion, the city has decided that implementing a multi-dimensional strategy with multiple benefits made more sense. But not only is it more desirable, it is politically easier to implement because it makes the city a more beautiful and healthy place. So if you are going to have to spend the money anyway, why not make it count.

    The shortcomings of the conventional “tanks and tunnels” approach were not only that it exceeded the EPA’s affordability standard for stormwater management (2% of median household income), but it also did not address water quality issues and could require green infrastructure tools anyway to meet these requirements. In addition, the report points out that the conventional solution isn’t aligned with the EPA’s broader goals of sustainability, reduces streams baseflow thereby damaging the resources that is designed to protect, and doesn’t offer any secondary triple bottom line benefits. Furthermore, since the conventional solution is not delivered incrementally it is not flexible and does not offer any benefits immediately.

    Green infrastructure on the other hand offered the city the opportunity to revitalize and restore the city’s streams and rivers, enhance the quality of the built environment throughout the city, improve air quality, reduce the heat island effect, and sequester carbon. While accumulating these benefits, the approach was more flexible, offered immediate benefits, and, most importantly, the cost of implementation was offset by the dollar value of the benefits. (see Volume 2: Triple Bottom Line Analysis of the plan for specifics)

    While conventional infrastructure has its place, the combination of the two can play a significant role in addressing many of the issues facing our cities. It is critical that we continue to move toward making these strategies common place. By doing so we can make our cities healthier and more beautiful for all of us to enjoy, while at the same time responsibly managing our stormwater.

    -Brian Phelps

    A Different Kind of Green Roof

    4 11 2009

    Research has begun on a light weight alternative to extensive green roofs (the least intense form of a green roof) for when structural loads or costs might otherwise deter a client from choosing to pursue a green roof. It is being referred to as a ‘green cloak’ and uses fast growing vine species that attach to a trellis suspended above the roof. Laura Schumann, a graduate student at the University of Maryland completed her thesis on the cost benefits for temperature and stormwater using green cloaks. More complete information on temperature and stormwater reduction can be found on the University’s website.

    While green cloaks will likely never provide near the benefits of an actual green roof system, a major potential is that they are probably a less expensive option when installing a green roof is just too cost prohibitive and a client is still looking for a way to save on energy costs. In addition to reducing cooling costs and slowing the runoff of stormwater from roofs, one of the most intriguing facets may be the potential for using vine and trellis systems on sloped roofs where it is currently challenging to implement traditional green roof systems. Another aspect is that vines have the potential to provide cover for vertical surfaces and may help provide even greater temperature benefits when combined to cloak walls as well.

    The vine species researched in the study included 5 different species: cross vine (Bignonia capreolata), kudzu (Pueraria lobata), Japanese Honeysuckle (Lonicera japonica), porcelain berry (Ampelopsis brevipedunculata), and Virginia creeper (Parthenocissus quinquefolia).

    Virginia Creeper

    Testing Virgina Creeper's effects on Building Temperature (Photo from Univerisity of Maryland's website)

    One drawback may be that green cloaks might not be as aesthetically pleasing to the masses as green roofs and could be a hard sell for more refined urban or retail areas. And it may also be difficult to provide full coverage for large roofs, however even partial coverage could provide huge cost savings in cooling costs for big box retailers or manufacturer’s with large warehouses where load bearing capacities for roofs are low and aesthetics are not as much of a concern. Either way, this is another potential option available for designers to help reduce energy costs, the urban heat island, and reduce stormwater runoff.

    – Sara Putney


    Inspiration (photo from 'Green Cloak' Presentation, David R. Tilley, University of Maryland)

    Green Roofs Address D.C.’s Environmental Problems

    30 10 2009

    asla green roofPhoto Source: ASLA

    It has been three years since the American Society of Landscape Architects (ASLA) finished the 3,000sf green roof on top of the their headquarters building in Washington D.C. The green roof is unusual in that it is sloped to cover the mechanical units on the roof. An informative video (link to video) was posted on Youtube this month highlighting the stormwater benefits of the ASLA roof. Nancy Somerville, ASLA’s CEO was interviewed during the video and she stressed the important role green roofs could play in helping address Washington D.C.’s and the nation’s difficult stormwater issues (i.e. water pollution, Combined Sewer Overflows). An EPA report estimated 850 billion gallons of untreated sewage and stormwater are discharged nationally each year as combined sewer overflows. (EPA Fact Sheet [pdf]) As Ms. Somerville points out, green roofs can filter the stormwater falling on the roof as well as act as a sponge and significantly reduce the amount of stormwater coming off of the roof. A green roof with 4″ deep planting media has been shown to retain 63% of the rain fall hitting the roof.

    During the first year, ASLA conducted a study (link to ASLA green roof website) to quantify the specific benefits of the their green roof. The data showed that 74% of the water was retained on the roof. Interestingly, the water quality of the stormwater discharge leaving the roof included an increase in pH and temperature as compared to the rain fall. In addition, the test results showed a significant increase over the concentration originally present in rain water for Chemical Oxygen Demand (COD), phosphate, total phosphorus, total suspended solids, and total dissolved solids. According to the report most of these contaminants were within the allowed freshwater chronic concentration values established by the E.P.A. and none of the concentrations were above the acute level. Unfortunately, the study did not compare the green roof with a conventional roof. The report concluded that “Green roofs have significant potential for reducing stormwater carried pollutants in major metropolitan areas such as Washington DC. However, more comprehensive and extensive monitoring studies are needed to evaluate specific performance measures of specific designs and develop accurate predictive tools.” The following are a few specific findings highlighted in their press release (.doc):

    • The roof typically retained 100 percent of a one-inch rainfall.
    • The heaviest rainfall during the monitored period was March 16, 2007. A total of 2.48 inches of rain fell during the 24-hour period with the roof retaining 51 percent, the equivalent of 1.3 inches of rain.
    • The green roof did not add any nitrogen to the runoff. Because of the amount of water retained, the roof provided a significant reduction in the amount of nitrogen introduced back into the watershed.
    • Typical of “young” green roofs, the analysis showed higher amounts of some other nutrients such as phosphorus, as well as heavy metals in the runoff—all below EPA standards and below levels expected from street runoff. Based on other green roof research, nutrient levels are expected to decrease in a few years. The heavy metals may be coming from the roof materials or from settled particulate matter/pollutants.
    • It is important to note that this study did not look at runoff from a conventional roof compared to the green roof runoff—and the results would be expected to look different. Water quality testing will be repeated in two years to see how the results change over time with a goal of comparing the green roof runoff to conventional roof runoff.
    • The green roof has been as much as 32 degrees cooler than conventional black roofs on neighboring buildings.
    • Engineering analysis showed that the green roof created a 10 percent reduction in building energy use during winter months and negligible difference in the summer.

    On a city wide level, the Casey Tree, a non-profit dedicated to restoring, enhancing and protecting the tree canopy of the Nation’s Capital, conducted a study (link to study) of the Washington D.C. area that examined the impact of green roofs and tree plantings. They concluded that if 55 million square feet of green roofs were installed throughout the Washington D.C. area, they would reduce the reduce CSO discharges by 435 million gallons or 19% each year.

    These studies illustrate the effectiveness of including green infrastructure within the overall strategy for cleaning up our nation’s stormwater.

    -Brian Phelps