Tapping the Potential of Urban Roof Tops

Ta p p i n g T h e poT e n T i a l o f U r b a n ro o f To p sro o f To p r e s o U rc e s n e i g h b o r h o o d a s s e s s m e n T

Final Repor t

October 31, 2007

D E S I G N , C O M M U N I T Y & E N V I R O N M E N TD E S I G N , C O M M U N I T Y & E N V I R O N M E N T

Ta p p i n g T h e poT e n T i a l o f U r b a n ro o f To p sro o f To p r e s o U rc e s n e i g h b o r h o o d a s s e s s m e n T

Final Repor t

Bay Localize is an Oakland-based organization that catalyzes a shift from a globalized, fossil fuel-based economy to a localized green economy that strengthens all Bay Area communities. Bay Localize is a nonprofit project of the Earth Island Institute.

This report is generously supported by the Community Foundation Silicon Valley, Laurence Levine Charitable Fund, San Francisco Foundation, Ollie Fund, and Bay Localize supporters. It was prepared by Brian Holland and Sarah Sutton of Design, Community and Environment, Kate Stillwell of Holmes Culley, and Ingrid Severson and Kirsten Schwind of Bay Localize.

For more information, contact:

Bay Localize 436 14th Street, Ste 1127 Oakland, CA 94612 510-834-0420 www.baylocalize.org

October 31, 2007

D E S I G N , C O M M U N I T Y & E N V I R O N M E N TD E S I G N , C O M M U N I T Y & E N V I R O N M E N T

TABLE OF CONTENTS

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1. EXECUTIVE SUMMARY/INTRODUCTION ........................................................ 1-1

2. EXISTING CONDITIONS .............................................................................. 2-1

3. ROOFTOP RESOURCE PROTOTYPES ............................................................. 3-1

4. FINDINGS .................................................................................................. 4-1

Appendices Appendix A: Assumptions and Methodology

B A Y L O C A L I Z E T A P P I N G T H E P O T E N T I A L O F U R B A N R O O F T O P S R O O F T O P R E S O U R C E S N E I G H B O R H O O D A S S E S S M E N T T A B L E O F C O N T E N T S

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List of Figures Figure 2-1. Aerial view of buildings in study area with existing

rooftop resources....................................................................... 2-4 Figure 2-2. Aerial view of study area indicating distribution of

building types. ......................................................................... 2-14 Figure 3-1. Cross-section of Extensive Green Roof Prototype. .................. 3-3 Figure 3-2. Cross-section of Intensive Green Roof—Vegetables

prototype................................................................................... 3-9 Figure 3-3. Cross-section of intensive Green Roof—Herbs

prototype................................................................................. 3-14 Figure 3-4. Cross-section of Rooftop Hydroponic Garden

prototype................................................................................. 3-18 Figure 3-5. Diagram of assembly of rainwater catchment

system using 50-gallon drum. .................................................. 3-24 Figure 3-6. Diagram of integrated Rainwater Harvesting and

Solar Photovoltaics prototypes. .............................................. 3-25 Figure 4-1. Aerial view of study area with buildings assigned

rooftop resources prototypes. ................................................... 4-3

List of Tables Table 2-1 Building Typology-Typical Characteristics ............................ 2-12 Table 2-2 Building Type Distribution..................................................... 2-13 Table 3-1 Prototype Characteristics.......................................................... 3-2 Table 4-1 Prototype Assignment and Productivity ................................ 4-10

1 EXECUTIVE SUMMARY/INTRODUCTION

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“Built-out” is a phrase often used in planning and development fields to de-scribe dense, urban communities that have few remaining vacant buildable parcels. As the Bay Area adopts smart growth and transit-oriented develop-ment policies emphasizing high-density housing, neighborhoods throughout San Francisco, the East Bay, Peninsula, and South Bay are becoming increas-ingly built-out. This density presents a challenge in identifying available land for important uses such as open space, community gardens, and stormwater and energy infrastructure. In cities across the country, however, a new land-scape is being discovered where building rooftops meet the sky. Previously regarded as unusable space, the landscape of rooftops is being re-claimed for productive and sustainable purposes. Whereas in the past, roofs have been a liability—emitting heat into the urban atmosphere, shedding pol-lutants into the watershed, requiring costly repair and replacement—some cities are transforming roofs into assets. They are being used as catchment areas for irrigation water, renewable energy platforms, recreational open space, food and educational gardens, reduction of stormwater surges, and aes-thetic improvement. In short, rooftops are being harnessed to improve cities and enhance the quality of life of inhabitants. A rooftop resource development philosophy is emerging and taking root in the Bay Area. Building owners and developers are looking at the options of solar power, rainwater catchment and living roofs to maximize their build-ings’ efficiency and function. Designers and planners are coming together to map out strategies for green roof implementation. Public works departments and utilities are stimulating adoption of solar photovoltaic systems. And citi-zens are seeking ways to better utilize rooftops for energy, food and commu-nity empowerment. I. PROJECT OBJECTIVES

Information on green roofs, solar technologies, and rainwater harvesting is available in abundance. This study seeks to fill gaps in that knowledge, par-

Rooftop garden atop St. Simon Stock Catholic School Bronx, New York. Source: St. Simon Stock Catholic School.

B A Y L O C A L I Z E

T A P P I N G T H E P O T E N T I A L O F U R B A N R O O F T O P S

R O O F T O P R E S O U R C E S N E I G H B O R H O O D A S S E S S M E N T E X E C U T I V E S U M M A R Y / I N T R O D U C T I O N

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ticularly with regard to low-cost strategies on existing buildings and the po-tential productivity for future developments. The study analyzes rooftop resource implementation and benefits for the Eastlake district in Oakland. Objectives include:

♦ Analysis of the suitability of rooftop resource strategies in different built contexts, highlighting retrofits to existing buildings without structural improvement;

♦ Design of conceptual rooftop resource prototypes that are feasible for ex-isting buildings;

♦ Analysis of productivity for edible garden designs on future development in the area; and

♦ Quantification of the productivity benefits of rooftop gardens, renewable energy, and rainwater catchment technologies.

Several unique contributions are addressed in this study, including:

♦ Focus on Existing Buildings. Most informational resources for green roof development focus on new construction; therefore, less information is available for building owners and policymakers to use when consider-ing the potential for green roof retrofits on existing buildings.

♦ Regional Context. Much of the available information on green roofs was developed in different social, political, economic, environmental and meteorological contexts, from Chicago to Germany to Portland to Mont-real. Also, while rooftop resource development in cities across the US and the globe is supported with public financial incentives, the Bay Area and the state of California fall short in implementing many of these poli-cies.

♦ Urban Agriculture. This study also differs from many existing docu-ments in that an emphasis is placed on rooftop vegetable gardening as a strategy for intensifying urban agriculture activities, which can improve nutrition and food security in urban neighborhoods while reducing de-pendence on an energy-intensive global food economy.




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♦ Neighborhood Scale. This study looks beyond the analysis of green roof benefits at the building scale to focus primarily on projecting out-comes at the neighborhood scale.

II. PROJECT APPROACH

As detailed in later chapters, this assessment analyzes the potential for green roofs, rooftop gardens, solar photovoltaics, and rainwater harvesting on exist-ing buildings and future developments, and identifies possible benefits to the Eastlake neighborhood in Oakland. A model was developed for this study to produce quantitative estimates of rooftop productivity. Buildings in the Study Area were categorized into types to generalize their characteristics, including the weight-bearing capacity of the roof structure. Rooftop resource prototypes were then designed to serve as test retrofits, providing data on loading characteristics. The prototypes were tailored to meet the special needs of existing buildings and were correlated with produc-tivity estimates per square foot. Prototypes were then assigned to each build-ing based on their suitability. Vacant lots were categorized as “opportunity sites” that could hold intensive, edible roof gardens. Finally, the total area and productivity estimates of each prototype were used to determine aggre-gate benefits to the Eastlake Study Area. III. PROJECT FINDINGS

The findings of the assessment demonstrate a great deal of potential for har-vesting food, energy, and water on Bay Area roofs. Rooftop gardens, solar photovoltaic systems, and rainwater harvesting technologies can all be fitted on existing buildings. There are clear opportunities and constraints to each strategy as well as some surprising benefits. In addition to well-documented benefits such as water quality and energy efficiency improvements, provision




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of open space amenities, and aesthetic appeal, rooftop resources in the Study Area can provide:

♦ Clean, renewable electricity satisfying approximately 25 percent of de-mand;

♦ Fresh, leafy-green vegetables for all area residents; and

♦ Supplemental rainwater for irrigation for approximately 83 percent of the area’s buildings

These benefits are attainable, but not without significant effort invested by State and local government, the private sector, communities and individual households. IV. REPORT STRUCTURE

The report is organized into four chapters: Introduction, Documentation of Existing Conditions, Description of the Rooftop Resource Prototypes, and Study Findings. Methodological approaches and assumptions are described in the text or footnoted, and also described in greater depth in Appendix A. Figures are distributed throughout the text to provide accessible graphic illus-tration of concepts. V. ACKNOWLEDGEMENTS

Preparation of this study was aided by several professional advisors and community volunteers. Deserving of special acknowledgement are: ♦ American Soil and Stone ♦ Andrea Solk, Sustaining Ourselves Locally ♦ Association of Bay Area Governments ♦ Babak Tondre ♦ Center For Sustainable Economy




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♦ City Slicker Farms ♦ Community Foundation Silicon Valley ♦ Institute for Simplified Hydroponics ♦ Intertribal Friendship House ♦ Laurence Levine Charitable Trust ♦ Mark Richmond, Practica Consulting ♦ Natylie Baldwin ♦ Rana Creek ♦ REC Solar ♦ San Francisco Planning and Urban Research Association (SPUR) ♦ Stewart Winchester, Merritt College ♦ Tufani Mayfield ♦ United Nations Food and Agriculture Organization

Building Survey Volunteers Aaron Lehmer Andrea Mann Bob Strayer Carolyn Bush Charles Hardy David Jaber Debbie Collins Dominic Porrino Ellen Doudna Inga Sheffield Kelley Lake Kirsten Schwind Lisa Katz Maija Dzenis Mark McBeth Nelson Chick Oliver Lear Paula White




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Sarah Kennedy Sharon Kutz VI. CONCLUSION

This Neighborhood Assessment conclusively demonstrates that rooftop re-sources can be developed on existing buildings in the Bay Area without struc-tural improvements. Furthermore, future developments would gain consid-erable benefits by planning for intensive, edible roof gardens. Hydroponic rooftop gardens and solar photovoltaics show the most promise for existing buildings, while intensive and extensive green roofs and rainwater harvesting present additional challenges, some of which may be overcome in time if greater investment is warranted. Today, it is possible for building owners to install rooftop technologies and improve water quality, save energy, grow fresh produce, generate clean electricity, and contribute to greater community resilience and livability. The promise of a healthier environment and greater resource security makes it imperative that we begin planning and implement-ing for these sustainable rooftop systems now. Education and leadership can bring about the kinds of benefits that so many cities have successfully demonstrated. Policy and government support are essential keys to fostering the implementation of these systems. Rooftops are currently untapped resources and a package of appropriate design, develop-ment incentives, and public support is crucial to fulfilling their great poten-tial.

2 EXISTING CONDITIONS

2-1

This chapter describes the current state of rooftop resource implementation in the Bay Area and specifically in the Study Area. It also documents the ar-chitectural history and existing demographic and regulatory setting of the Eastlake neighborhood in order to identify dynamics that may affect rooftop resource development. The latter half of the chapter presents the Building Typologies that were developed for the purposes of the assessment and de-scribes their distribution in the Study Area. I. ROOFTOP UTILIZATION

The role of rooftops has historically been a peripheral consideration in the development of urban infrastructure and largely remains an afterthought in water, food and energy systems planning. Roofs have been used for collecting water or insulating homes for millennia, but widely-held perceptions dismiss these traditionally “low-tech” strategies as being old-fashioned or only appli-cable in rural contexts. While solar thermal and photovoltaic technologies have been applied on roofs for decades, these practices have yet to gain wide-spread adoption. However, new interest in green building is once again fo-cusing attention on rooftops. Green roofs, rainwater harvesting systems, and rooftop photovoltaics are being installed at an increasing rate while California remains a national leader in solar electricity generation. A. Rooftop Utilization in the Bay Area The Bay Area is well-known for its focus on environmental sustainability and for good reason. With regards to rooftop resource strategies, the region is ahead of the curve but far from taking full advantage of its resources.

Buildings within study area. Source: Ingrid Severson.



R O O F T O P R E S O U R C E S N E I G H B O R H O O D A S S E S S M E N T E X I S T I N G C O N D I T I O N S

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1. Solar Photovoltaic Installations In the nine-county Bay Area, over 5,000 photovoltaic systems have been in-stalled.1 The largest include the 675 kilowatt (kW) installation on the City of San Francisco’s Moscone Center, a 766 kW system at the Rodney Strong Winery in Healdsburg, and Alameda County’s 1,180 kW installation on the Santa Rita Jail in Dublin. In addition, much of the region’s photovoltaic ca-pacity exists in smaller systems under 15 kW, many of which serve residential buildings. In seven counties of the Bay Area (excluding Napa and Solano Counties), these systems comprise approximately 18,000 kW, or 18 mega-watts (mW) of electricity generating capacity.2 2. Green Roofs and Rooftop Gardens Despite a number of high-profile green roof projects in the Bay Area, the green roof trend has been somewhat slow to take hold in the region. An out-standing exception is the Gap Headquarters in San Bruno, which was con-structed with a 69,000 square-foot extensive green roof in 1997. The Califor-nia Academy of Sciences building under construction in San Francisco’s Golden Gate Park will also have a large, extensive green roof. Intensive green roofs and rooftop gardens and parks have also been built, including park en-vironments atop parking garages at Civic Center, Yerba Buena Gardens and the North Beach Place mixed-use project in San Francisco, and at the Kaiser Center office complex in downtown Oakland. Nevertheless, a number of cities have consistently outperformed Bay Area locations in terms of green roof implementation, including Chicago, Wash-ington D.C., New York City and Portland, Oregon.3 As far as could be de-

1 Liz Merry, “Status of Photovoltaic Installations in California,” Solar Energy Resource Guide, NorCal Solar, 2007.

2 Ibid.

3 Green Roofs for Healthy Cities, “Green Roof Industry Survey Final Report,” http://www.greenroofs.org/storage/2006grhcsurveyresults.pdf (accessed April 14, 2007).




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termined, no municipalities in the Bay Area region have green roof incentive programs as do Chicago and Washington, D.C. 3. Rainwater Harvesting Regional attention to harvesting rainwater has fluctuated with concerns over environmental conditions or scarcity. Many residents discovered rainwater harvesting, for example, in the drought of 1976-77, when reservoirs across the region were dangerously drawn down and mandatory restrictions were im-posed on water use. While data on regional rainwater catchment implemen-tation is unavailable, it is likely that a limited number of residential buildings are fitted with rainwater harvesting systems, and that this number is increas-ing, albeit very slowly, with elevated awareness of California’s water resource and sustainability challenges. B. Rooftop Utilization in the Study Area A limited level of rooftop utilization is already occurring in the Study Area. There are at least six rooftop solar water heating installations, all on apart-ment buildings. There are no green roofs on occupied buildings in the area, but a vegetated plaza sits atop an underground parking structure. The plaza is planted with a variety of trees, grasses, and shrubs, providing an attractive semi-public space with stormwater retention benefits. It is possible that some rainwater harvesting systems are in use but none were identified through ae-rial photograph analysis or the field survey. Figure 2-1 illustrates existing rooftop resources in the area. II. REGULATORY AND POLICY SETTING

The Study Area is within the jurisdictional boundary of the City of Oakland and is subject to a number of State and City regulations pertaining to rooftop uses. This section introduces these regulations and their applicability to the study.

Volunteers identifying building types. Source: DC&E.

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Figure 2-1. Aerial view of buildings in study area with existing rootop resources.

0 250 500 Feet

Intensive Green Roof

Solar Water Heating

No Resource

Study Area




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A. Zoning Code The Zoning Code is contained in Title 17 of the City of Oakland’s Municipal Code. The Code classifies, regulates, restricts and segregates land uses, build-ing characteristics, and population densities according to the land use goals established by the community in the General Plan. Minimum requirements for usable open space are established for residential uses. In Chapter 17.126, the Code sets minimum standards for usable open space on residential parcels, including rooftop uses. Residential parts of the Study Area are mostly zoned R-50 (Medium Density Residential) or R-60 (Medium-High Density Residential), with the remainder zoned at higher densities. Usable open space requirements for these classifica-tions range from 150 to 200 square feet per dwelling unit. Rooftop areas can satisfy a maximum of 20 percent of this required open space, or 30 to 40 square feet per dwelling unit. B. California Building Code The State Building Code is contained in Title 24, Part 2 of the California Code of Regulations. The Code regulates the construction and function of buildings to ensure fire and life safety and adequate structural design. Perti-nent sections of the code include Chapter 5, Section 509 (Guardrails), Chapter 10 (Means of Egress), Chapter 13, Section 1301 (Solar Energy Collectors), Chapter 15 (Roofing and Roof Structures), and Chapter 16 (Structural Design Requirements). The following considerations will affect the extent to which usable rooftop spaces can be created. 1. Occupancy Load and Means of Egress Since construction of an accessible space on a rooftop alters the use of the roof, the municipal Building Department will ensure that Building Code re-quirements are met when reviewing plans for the improvement. Code re-quirements will vary depending on how the occupancy of the roof space is

Mixed use building. Source: DC&E.




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defined and on the maximum number of occupants expected and allowed to use the space, which is the “occupant load.” The most relevant example is with regard to means of egress.4 Accessible roof spaces that accommodate many occupants will be required to provide more than one exit, while spaces intended for ten or fewer occupants are adequately served by one exit. This is a critical variable for rooftop gardening since very few buildings have two exits from the roof. Therefore, for rooftop gardening to be possible, the Building Department must ensure safety by either calling for two exits or determining that the rooftop garden’s occupancy load will be ten or less, rendering one exit sufficient. The Building Department is responsible for assigning an occupancy load to the rooftop space, in accordance with the following direction from Chapter 10, Section 1003 of the California Building Code:

♦ Areas with fixed seats. Occupant load for areas with fixed seats is de-termined by assigning one occupant per seat provided in the area. For example, an area with 12 seats has an occupant load of 12.

♦ Areas without fixed seats. Here the occupant load is determined by di-viding the occupied square footage by an “occupant load factor” in Table 10-A of the California Building Code. For uses not included in the table, such as gardening, a factor for a similar type of use will be used. Specula-tively, a case could be made that gardening is similar in intensity of use to such uses as manufacturing or a commercial kitchen, where a limited number of people are involved in a productive activity over a large area. If these factors are used, as much as 2,000 square feet can be occupied for gardening without exceeding the maximum desirable occupant load of 10.

Because rooftop gardening is a relatively rare phenomenon in the region, no interpretation of the Code with regards to occupancy has been established. It

4 Means of egress are Code-compliant exits. Any occupiable space, such as a rooftop garden, must have at least one Code-compliant means of egress.




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is possible that Code officials will be wary of rooftop uses more intensive than gardening and will consider the space a gathering place, thereby requir-ing an additional exit. The outcome will depend on the municipality and on assurances that can be made to limit the number of occupants. 2. Guardrails Chapter 5 of the Building Code requires a guardrail around habitable space in order to protect life safety. Some buildings in the Study Area have fixed para-pets lining the perimeter of the roof area and extending as high as a few feet. Others have no perimeter barrier at all. In any case, a code-compliance bar-rier that extends 42 inches in height is required. C. Accessibility Local, State, and federal governments address accessibility for the mobility-impaired through several codes and laws. At the federal level, the Americans with Disabilities Act (ADA) requires that equal access be provided for the mobility-impaired when alterations to public spaces are made. Chapter 11 of the California Building Code also sets forth stipulations for accessibility, which are enforced by municipal Building Departments. Both the ADA and Chapter 11 must be satisfied. 1. Americans with Disabilities Act Compliance ADA requirements for building alterations do not apply to buildings that are used for strictly residential purposes; only buildings considered “public ac-commodations”—such as restaurants, hotels, theaters, doctors’ offices, phar-macies, retail stores, museums, libraries, parks, private schools, and day care centers—are subject to ADA rules. Some rooftop garden retrofits that are accessible to the public would fall under ADA and would need to include accessibility features to the roof, in the form of either elevators or ramps. It is likely that these features would prove prohibitively expensive to install and would create a major disincentive for creating accessible rooftop spaces on existing buildings.




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ADA requirements apply only to public accommodations and do not address residential environments. Even for public accommodations, elevators would not be required in many cases. According to the Department of Justice, ele-vators are generally not required in facilities under three stories or with fewer than 3,000 square feet per floor, unless the building is a shopping center or mall; the professional office of a health care provider; a terminal, depot, or other public transit station; or an airport passenger terminal.5 In addition, accessibility requirements may be waived as an undue hardship if accessibility features cost more than 20 percent of the total alteration cost, which would apply in the case of rooftop gardens. 2. California Building Code, Chapter 11 Accessibility requirements in the State Code are similar to ADA require-ments, but also include residential uses in their scope. Like the ADA, the Code allows for exemptions based on “unreasonable hardship,” which waives accessibility requirements when the cost of accessibility features exceeds 20 percent of the total alteration cost, and the total alteration cost is less than $120,000 (both of which are true for rooftop gardens). Installation of a new elevator in an existing building in order to access a new garden on the roof may be acknowledged as unreasonable hardship, particularly if the structure is not a major commercial or institutional building. Every effort should be made to provide universal accessibility to rooftop gar-dens when feasible. The Americans with Disabilities Act and California Building Code require that these improvements be made whenever feasible, but may provide flexibility when the costs of accessibility improvements are unreasonably high, as with elevator installation in existing residential struc-tures and other small buildings.

5 US Department of Justice, “Americans with Disabilities Act Questions and An-swers,” http://www.usdoj.gov/crt/ada/qandaeng.htm.




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III. STUDY AREA CHARACTERISTICS

Comprising roughly one-quarter of a square mile, the Study Area consists of the Eastlake commercial district and surrounding residential neighborhoods southeast of Lake Merritt in Oakland. The area provides a fertile testing ground for rooftop resource feasibility due to its great diversity, both in its socioeconomic conditions and its built environment. A. Demographics Eastlake is a unique neighborhood demographically, presenting a complex mix of economic and ethnic attributes in a compact area. Extrapolation of census data suggests that the Study Area is home to approximately 7,000 resi-dents in approximately 3,500 dwelling units. The median income of around $31,000 is low relative to the City of Oakland as a whole, but the number of people below the poverty line is lower than the city average.6 A wide range of income levels exists in the area. The neighborhood is widely perceived as one of the most ethnically diverse in the region. Dozens of languages are spoken by immigrants from around the world. Asians and African-Americans are the largest ethnic groups, compris-ing roughly one-quarter of the population each, with Hispanics accounting for another 14 percent and the remainder White, or other races. Of residents with Asian ancestry, many trace their roots back to Vietnam and other Southeast Asian countries.

6 Estimates based on U.S. Census Bureau, U.S. Census 2000.

Mixed-use building on 2nd Avenue.

Pitched-roof houses.

Parking garage in study area. Source: Ingrid Severson.




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B. Architectural History Known variously over the years as Rancho San Antonio, Clinton, Brooklyn, and New Chinatown, Eastlake is a neighborhood with a complex and fasci-nating history that is reflected in the building stock found there today. While the built remnants of the Ohlone communities that originally occupied this area have disappeared, historic features dating back to Spanish settlement can still be found. San Antonio Park, lying just southeast of the study area, once served as the main plaza of Rancho San Antonio, the cattle ranch started by the East Bay’s original Spanish settlers in the 1820s. The area was incorporated into Oakland in 1872. At least one 19th century home still stands in the neighborhood, along with several early 20th century Victorian homes. Many multi-family residential and commercial buildings found in the area today were constructed in the early 20th century, leading up to World War II. After the War, Eastlake experienced the same pattern of disinvestment that impacted many urban neighborhoods across the country. Deteriorating buildings continue to impact quality of life in Eastlake today and many structures in the study area are in fair or poor condition. Mid-century urban renewal projects also had an effect with hundreds of structures demolished in these decades and replaced with apartment buildings—1,108 apartments in 57 buildings total.7 Many of these renewal-era buildings are still found in the study area and factor significantly into the rooftop resource assessment. Since the era of urban renewal, the built conditions of the study area have not changed as rapidly. Investment in the 1980s and 1990s was primarily directed to areas near Lake Merritt where apartment towers, strip retail and big-box retail were developed. However, it appears that Eastlake may be in the early stages of economic transition. The neighborhood is located in the City of Oakland’s Central City East Redevelopment Area, in which infrastructure for development projects may be financed through tax-increment financing,

7 Urban Ecology, Clinton Park Plan, August 1999.

Apartment tower.

“Shops” building type.

Repair shop. Source: Ingrid Severson.




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providing a public incentive for private development. A multi-million dollar streetscape improvement project was also initiated in 2002 for the business district on 12th Street and International Boulevard between 5th Avenue and 8th Avenue. Rehabilitation and new construction projects in Eastlake may pro-vide an opportunity to incorporate rooftop features into the neighborhood. IV. BUILDING TYPOLOGY

Buildings in the Study Area are classified by type for the purposes of this as-sessment. The typology categorizes flat-roofed buildings into eleven types to allow for generalized estimates of structural properties and roof loading ca-pacities. Buildings with pitched roofs are not included in the typology since they were assigned the rainwater harvesting and solar photovoltaic proto-types. Both systems are sufficiently lightweight to be installed on virtually all pitched roof buildings, regardless of the building type. To account for the potential of future development in the area, vacant lots were identified as op-portunity sites in which new construction could plan for the inclusion of rooftop systems. Table 2-1 describes the typical characteristics of each flat-roofed building type and associated roof loading capacities. In addition to assigning a building type to each flat-roofed structure, a field survey conducted by the consultants and Bay Localize volunteers recorded discrete characteristics such as occupancy type, height, construction type and era, and presence of a “soft story.” Estimates of loading capacity are refined to account for differences in these factors when they do not match the typical assumed characteristics of the building type. A. Building Type Distribution A wide variety of building types are found in the Study Area. Table 2-1 de-scribes the building type split over the area. Table 2-2 illustrates the distribu-tion of building types in the area.

“Big Box building. Source: Ingrid Severson.

Typical office building. Source: DC&E.

One of nine vacant lots identified as opportu-nity sites. Source: Ingrid Severson.

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2-13

TABLE 2-2 BUILDING TYPE DISTRIBUTION

Building Type Number

of Buildings Percentage

of Total

House 20 2.7

Apartment Building 160 21.3

Apartment Tower 8 1.1

Mixed Use 36 4.8

Shops 40 5.3

Warehouse 14 1.9

Big Box 7 0.9

Repair Shop 6 0.9

Office Building 7 0.9

Community Building 30 4.0

Parking Garage 1 0.1

Pitched Roof Building 419 56.01

Total 748 99.91

Community building. Source: Ingrid Severson.

Apartment building. Source: DC&E.

Warehouse building. Source: Ingrid Severson.

E. 19th St.

E. 18th St.

E. 17th St.

Foothill Blvd.

E. 15th St.

International Blvd.

E. 12th St.

E. 11th St.

E. 10th St.

1st A

v e.

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Ave.

3 rd

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5 th

A ve .

6 th

A ve .

7 th

Ave .

8 th

A ve .

Clinton Square Park

Lake MerrittPa

rk B

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Lake

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F i g u r e 2 - 2 . A e r i a l v i e w o f S t u d y a r e a i n d i c a t i n g d i s t r i b u t i o n o f

b u i l d i n g t y p e s .

0 250 500 Feet

House

Apartment Building

Apartment Tower

Mixed Use

Community Center

Big Box

Shops

Office Building

Parking Garage

Warehouse

Repair Shop

Opportunity Site

Unknown

Pitched Roof

Study Area

3 ROOFTOP RESOURCE PROTOTYPES

3-1

For the purposes of this assessment, five rooftop resource prototypes were developed. These prototypes provide assumed characteristics that can be ap-plied to each of the rooftop resource models, including weight and productiv-ity. This chapter describes the design concepts, architectural and maintenance requirements, and potential benefits of the five prototypes. It also analyzes the synergies and conflicts that could arise if the prototypes are implemented in concert. The rooftop resource prototypes are a set of design concepts that represent various strategies for rooftop utilization. The design process was informed by the objective of utilizing rooftops in a manner that is productive, sustainable, and feasible. In addition, the goal of assessing rooftop capacity on existing buildings in their current condition—rather than on new construction or on structurally reinforced buildings—made loading a central consideration in the prototype design. Table 3-1, found at the end of the chapter, describes the components, cost ranges, and yields of the prototypes. The prototypes are necessarily generalized to allow for variations in roof size and type, roof slope, building type, wind variables, client budget, and other conditions. They should be taken as examples of possible configurations and should not be relied upon for specifications for any site. Before any specific rooftop resource is developed, professional consultation should be obtained to determine precise design loads and roof loading capacity. I. EXTENSIVE GREEN ROOF

A. Design Concept Extensive green roofs are the most common type of green roof found today, valued for their many environmental benefits. The prototype follows the typical configuration of extensive green roofs in arid climates, in which low-growing, drought-tolerant ground cover is planted in 4 to 6 inches of growing substrate and placed on an assembly of filter fabric, a drainage layer, root

Raised vegetable garden on rooftop. Source: Resource Centres on Urban Agriculture and Food Security.



R O O F T O P R E S O U R C E S N E I G H B O R H O O D A S S E S S M E N T R O O F T O P R E S O U R C E P R O T O T Y P E S

3-2

TABLE 3-1 PROTOTYPE CHARACTERISTICS

Prototype Major

Components Maximum

Weight

Annual Productive

Yield

Extensive Green Roof

½" Drainage Mat 4" Mineral Substrate Sedums

22 psf drainage and

energy benefits

Intensive Green Roof—Vegetables

2¼" Drainage Board 18" Organic/Mineral Substrate Variety of Vegetable Crops

108 psf 1.86 psf

vegetables

Intensive Green Roof—Herbs

1 ¼" Drainage Board 8" Organic/Mineral Substrate Herbaceous Plants

51 psf perennial yield

Hydroponic Rooftop Garden

Growing Container Reservoir Container 4" Inert Substrate Variety of Vegetable Crops

16 psf 4 psf

vegetables

Solar Photovoltaics

Multicrystalline PV Panels Mounting Hardware

5 psf 1 kilowatt per 100 square feet

Rainwater Harvesting

Conveyance-Gutters/Leaders Debris Screen First-flush Diverter Roof Washer Storage Tank/Cistern

N/A

average 3,000 ga. per

structure (1" rain on 100 sf

= 60 gal.) barrier, and a waterproof roof membrane. This type of assembly can be in-stalled directly on the roof or placed in trays that are installed as a modular system. Extensive green roofs differ from intensive green roofs in the mini-mal depth of the substrate and more limited planting possibilities. Figure 3-1 illustrates the prototype assembly and vegetation. 1. Green Roof Assembly The Extensive Green Roof prototype is intended to extend across the maxi-mum feasible area of the building to confer the greatest storm water and en-ergy benefits. Plants are established in four inches of substrate, considered a minimum depth for plants to endure dry Bay Area summers. In this case a

Extensive green roof assembly. Source: American Hydrotech.

Extensive green roof. Source: American Hydrotech.

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3-4

blend with high mineral content and low organic content should be devel-oped to provide moisture and air retention while minimizing the load im-posed upon the roof. While the exact blend depends highly on the site, one very appropriate medium is pumice, which is one of the lightest mineral ma-terials mined within 500 miles of the Bay Area. Expanded shale is another very lightweight mineral that exists in California; however, it is currently shipped in from Colorado and other Western states, resulting in higher trans-portation costs and environmental impacts. Lava and scoria are available from the Clearlake area in Northern California, but they are heavier than pumice and expanded shale. Whichever mineral medium is selected should be combined with a minimal amount of organic material, such as locally-available compost. Beneath the substrate, a ½-inch thick recycled polyethylene drainage mat aer-ates and drains the media, and attached filter fabric prevents it from clogging the drainage layer. Finally, a root barrier and waterproof membrane protect the roof deck from the living layer above. A popular alternative to the type of assembly described above (which is built-up directly on the roof) is the modular approach, in which the above compo-nents are assembled in container trays and installed on the roof. Modular systems present a number of benefits. Perhaps their most notable benefit is the flexibility they allow in installation and removal. They can often be in-stalled without re-roofing, while the soil membrane system may require re-placement or major repair to the roof membrane. In addition, building own-ers may be more open to experimenting with a green roof installation know-ing that the trays can be easily removed if desired. Whether the soil membrane approach or the modular system is chosen for a particular roof, the described components of the extensive green roof assem-bly are almost identical in other respects.

Extensive green roof modular tray system. Source: Green Roof Blocks.

“Green Paks” modular tray system. Source: Green Roof Blocks.




3-5

2. Plants The Extensive Green Roof prototype is planted with of a mix of regionally-appropriate sedums, such as Sedum album, Sedum spathuifolium, Sedum spu-rium, and Sedum sexangulare. The specific species chosen for a particular rooftop context depends on many factors, including: ♦ Initial budget and maintenance budget. ♦ Physical conditions, such as shading and wind. ♦ Roof slope. ♦ Retrofit schedule and seasonal variables.

Planting methods can vary, from direct seeding in the growing medium dur-ing installation to application of pre-planted container trays or vegetated mats. Generally, materials for vegetated mats and modular tray systems are more expensive but labor costs are reduced, while seeding or transplanting plugs directly is more labor-intensive but reduces materials costs. B. Architectural Requirements Because extensive green roofs are a low-impact, low-maintenance rooftop re-source, they can be useful on buildings with a wide range of characteristics. Their implementation on existing buildings in the Bay Area is constrained, however, by their weight. The lightweight Extensive Green Roof prototype weighs approximately 22 pounds/square foot (psf),1 precluding installation on many building types. This weight value is at the low end of the spectrum for extensive green roofs. Because the prototype is not intended to be used as occupiable space, egress requirements are more lenient than for rooftop gardens. Older stairways

1 All green roof loading estimates are based on design loads from the German green roof standard, “Guidelines for the Planning, Execution, and Upkeep of Green-roof Sites,” published by FLL (Forschungsgesellschaft Landschaftsentwicklung Land-schaftsbau e.V.), 2002 Edition.

Sedums on extensive green roofs using modular systems. Source: Green Roof Blocks.




3-6

with steeper inclines to the roof, or common ladder and hatch access, would both be sufficient to provide necessary maintenance. C. Maintenance Requirements Extensive green roofs are intended to be a low-maintenance technology. Dur-ing the first year after installation, plants need to be irrigated as they establish themselves. Planting hardy, drought-resistant, regionally-appropriate varie-ties such as those specified in this prototype will limit irrigation needs over the long-term since these plants are accustomed to the arid conditions of Bay Area summers. However, minimizing substrate depths to the level entailed in this prototype would likely require that some irrigation occur on a regular basis, depending on the conditions of the site. The extensive green roof needs to be inspected only a few times a year to ensure that all components, includ-ing the membrane, are functioning as intended. Extensive green roofs can reduce roof maintenance demands and as much as double the life of the roof membrane by protecting it from extreme tempera-ture changes, ultraviolet radiation, and accidental damage.2 D. Cost Range The extensive green roof prototype is a relatively low-cost rooftop resource strategy, though initial costs are higher than that of a conventional roof. De-pending on the type of labor that is used, accessibility and size of the roof, and the planting method, initial materials and labor costs are estimated at $15

2 City of Chicago, Design Guidelines for Green Roofs, http://egov.cityofchicago.org/webportal/COCWebPortal/COC_ATTACH/design_guidelines_for_green_roofs.pdf (accessed April 8, 2007).




3-7

to $22 per square foot. This estimate assumes that the prototype is applied when a new roof is needed and therefore excludes the cost of a new roof membrane, which generally adds another $8 to $12 per square foot. Pre-planted trays with irrigation in place cost approximately $25 to $30.3 In considering the financial viability of extensive green roofs, it is important to note that they are more cost-competitive with conventional roofs when using a life-cycle costing approach, which incorporates savings from reduced maintenance and longer roof life, as well as ongoing energy efficiency savings. Life-cycle costing is a valuable method for understanding the benefits of many “green” technologies, which may cost more initially but which may result in cost savings over the operating life of the product. E. Benefits While the extensive green roof prototype does not yield a harvest of food, energy, or water, it does confer a number of environmental and aesthetic benefits, including:

♦ Storm Water Retention. Extensive green roofs are valued for their storm water retention capacity and a good deal of research is being con-ducted to quantify these benefits. By capturing and holding water in the vegetation and substrate layers, the extensive prototype can mitigate flooding and combined sewer system backup (where applicable) in heavy rain events. This prototype can also improve the quality of runoff water by capturing and holding pollutants in the substrate. Studies indicate that an extensive green roof can absorb as much as 70 percent of rainfall

3 All costs are derived from Steven Peck and Monica Kuhn, “Design Guidelines for Green Roofs,” Ontario Association of Architects and Gabrielle Fladd, “Green Roof Matrix” (cost analysis paper presented in San Francisco Planning & Urban Research Association, Green Roof Task Force meeting, San Francisco, CA, June 2007).




3-8

from a storm event, releasing the remainder over an extended period of time.4

♦ Energy Efficiency. The extensive prototype can improve building per-formance, effectively acting as a heat trap, while also shading the roof and cooling the roof surface through evaporation and vegetative transpira-tion. A study in Canada found that when implemented broadly on a city or regional scale, extensive green roofs can significantly reduce ambient urban temperatures, thereby lowering energy demand for air condition-ing. The living roof on Chicago’s city hall building stands as a popular case study modeling the cooling effect of green roofs. It has been re-corded as 7 degrees cooler than surrounding roofs on an annual average, and up to 30 degrees cooler during the summer time.5

II. INTENSIVE GREEN ROOF-VEGETABLE GARDEN

A. Design Concept The Intensive Green Roof—Vegetable Garden prototype is designed to sup-plement the environmental and aesthetic benefits of the extensive prototype with food production and an open space amenity. Vegetables would be grown in 18 inches of growing medium, the minimum depth to support a large variety of vegetables. The assembly of the prototype is similar to that of the extensive prototype, consisting of a water proof membrane, root barrier, drainage layer, filter fabric, substrate layer, and plants. Figure 3-2 provides a section of the green roof assembly.

4 ASLA et al., “Landscape Architects Release Green Roof Performance Report: Roof Retained 27,000 Gallons of Stormwater in First Year,” http://asla.org/press/2007/ release091907.html. 5 Karen Liu, “A National Research Council Canada Study Evaluates Green Roof Sys-tems’ Thermal Performances,” http://www.professionalroofing.net/article.aspx? A_ID=130 (accessed April 8, 2007).

Intensive roof garden. Source: American Hydrotech.

Intensive green roof—vegetable garden. Source: The Rooftop Garden Project, Alternatives, 2004.

F i g u r e 3 - 2 . C r o s s - s e c t i o n o f I n t e n s i v e G r e e n R o o f — V e g e t a b l e s p r o t o t y p e .

B A Y L O C A L I Z ER o o f t o p R e s o u R c e s N e i g h b o R h o o d A s s e s s m e N t




3-10

In this case, however, the vegetated roof is not intended to cover the entire roof area. Instead, paths are created by closing in growing areas with retain-ing walls, constructed of lightweight materials such as wood or recycled plas-tic. A protective surface would be installed to protect the roof from foot traf-fic damage. This arrangement provides accessible spaces similar to ground-level container gardens. While a multitude of different arrangements are pos-sible, depending on the existence of fixed obstructions on the roof, it is esti-mated that this prototype would provide an average growing area of 60 per-cent of total roof area. This estimate takes into account space unavailable due to fixed obstructions as well as space needed for paths and equipment storage. 1. Green Roof Assembly The prototype includes 18 inches of substrate, in this case a blend of about equal parts mineral and organic content. Conventional topsoil and potting soil are inappropriate media for rooftop environments, particularly on exist-ing buildings, due to their weight. Instead, the medium used in this proto-type would include approximately equal parts organic material, such as com-post or bark humus, and mineral material such as pumice or scoria. The drainage layer in this prototype is a 2¼ inch thick recycled polystyrene drain-age board. The drainage course is filled with lava or similar mineral material for structure. Other components are similar to the extensive green roof pro-totype. 2. Plants The prototype is designed to provide year-round vegetables that could thrive in the Bay Area. A selection of vegetables was developed based on several criteria, including: ♦ Regional suitability ♦ Growing medium depth requirements ♦ Plant weight at maturity (structural consideration) ♦ Height at maturity (wind loading considerations) ♦ Full sun tolerance ♦ Normal to low water needs ♦ Growing season

Crop Variations A wide variety of vegetables may

be suitable for rooftop environ-

ments in the East Bay, despite

such challenges as wind, heat, and

evaporation. Experimentation is

needed to better establish planting

possibilities. While the prototype

suggests several popular garden

vegetables, other appropriate

edible crops may also include

broccoli, celery, chard, collards,

eggplant, kale, mustard, green

onions, and peppers. In addition,

some of these crops may be

planted in less than 18 inches of

growing media, which was used

in this prototype as a conservative

value for growing a large variety

of edible plants in potentially

harsh conditions.




3-11

Based on these criteria, plants were selected that could be rotated to grow on a year-round basis. Cool-season crops in the prototype are spinach, mustard, carrots, and beets. Tomatoes, cucumbers, and winter squash are included as warm-season crops, along with leaf lettuce, which in many cases can be grown year round. Variations on this arrangement should provide an opportunity to grow cool-season crops twice yearly—planted in the early Spring and Fall—while planting warm-season crops in late Spring. Additional crops that could be grown on this prototype are listed in the side box. B. Architectural Requirements The vegetable garden prototype would introduce a new roof load of ap-proximately 108 psf to the structure.6 Providing this type of structural sup-port would require a uniquely-tailored structural design incorporated into new construction. As an occupiable space, the vegetable garden prototype would require a code-compliant stairway or elevator, as well as guardrails or fencing around the roof edge. C. Maintenance Requirements The prototype would require substantial maintenance, much of which is as-sociated with normal vegetable gardening activities. Maintenance demands would include regular irrigation, pruning, weeding, fertilizing, and pest con-trol. Water needs could be increased relative to ground-level gardening due to higher rates of heat- and wind-induced evaporation. Depending on budget, irrigation could take place through hand-watering or sub-surface drip irriga-tion, the latter of which would reduce water use and labor requirements. In 6 All green roof loading estimates are based on material design loads in the German green roof standard, “Guidelines for the Planning, Execution, and Upkeep of Green-roof Sites,” published by FLL (Forschungsgesellschaft Landschaftsentwicklung Land-schaftsbau e.V.), 2002 edition.

Intensive green roof assembly. Source: American Hydrotech.




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addition to regular maintenance of the growing area, inspection and repair of the roof membrane would be required on an occasional basis. D. Cost Range Initial costs depend greatly on the type of labor that is used, accessibility of the roof, and planting method, as well as the size of the roof. Given these variables, it is estimated that materials and labor costs to install this prototype would range from $30 to $45 per square foot of growing area, plus an addi-tional $20 to $40 per linear foot for guardrails and an optional $2 to $4 per square foot if irrigation is installed.7 This estimate assumes that the prototype is applied when re-roofing is needed and therefore excludes the cost of a new roof membrane. It also assumes that structural and architectural require-ments of the Building Code are already satisfied. E. Benefits In addition to providing storm water retention and treatment, energy effi-ciency and aesthetic benefits, this prototype provides an open-space amenity to building residents. Food production on the roof also results in a number of valuable outcomes, such as new wildlife habitat, recreational and educa-tional opportunities, and readily accessible fresh, healthy produce. Ade-quately maintained year-round, this prototype would yield approximately 1.86 pounds of vegetables per square foot annually.8

7 Stephen Peck and Monica Kuhn, “Design Guidelines for Green Roofs,” http://egov.cityofchicago.org/webportal/COCWebPortal/COC_ATTACH/design_guidelines_for_green_roofs.pdf (accessed March 24, 2007). 8 Nancy Garrison, “Home Vegetable Gardening,” University of California Coopera-tive Extension, http://vric.ucdavis.edu/veginfo/veginfor.htm. This figure is an aver-age of vegetables’ approximate yields, multiplied by three seasons.




3-13

Implemented on a broader scale, this type of green roof can improve local food security and reduce “food miles traveled,” thereby reducing the energy and climate impacts of food transportation. A study conducted in 2001 at Iowa State found that in the conventional American agriculture system, food travels an average of 1,500 miles from its origin to its point of consumption.9

III. INTENSIVE GREEN ROOF—HERB GARDEN

A. Design Concept

The concept of this prototype is largely similar to the Vegetable Garden pro-totype, except that herbs are grown instead of the wider variety of vegetables. Plants included are rosemary, thyme, and cilantro. The growing medium remains the same—a lightweight, soil-free mix of approximately equal parts organic and mineral material—but substrate depth is reduced to 8 inches, a minimum for many herbs that would be suitable for a rooftop environment. The drainage layer is also similar to the Vegetable Garden prototype, but its thickness is reduced to 1¼ inches. Figure 3-3 illustrates the green roof assem-bly and vegetation. B. Architectural Requirements An approximate load of 51 psf would result from installation of this proto-type.10 As an accessible garden, this prototype would be considered an occu-piable space, subject to code requirements for stairways and guardrails.

9 Leopold Center for Sustainable Agriculture, Food, Fuel and Freeways, An Iowa Per-spective on How Far Food Travels, Fuel Usage, and Greenhouse Gas Emissions, June 2001. 10 All green roof loading estimates are based on the German green roof standard, “Guidelines for the Planning, Execution, and Upkeep of Green-roof Sites,” published by FLL (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V.), 2002 edition.

Intensive roof garden atop the Fairmont Royal York Hotel in Toronto grows vegetables, herbs, and edible flowers. Source: Lorraine Flanigan.

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3-15

C. Maintenance Requirements Maintenance needs would be significant and would include regular irrigation, pruning, weeding, fertilizing, and pest control. Irrigation needs would be reduced compared to the Vegetable Garden prototype, but a sub-surface drip irrigation system would still be desirable. As with all green roofs, regular inspection of the roof membrane would be required, and occasional repair of the membrane could prove necessary. D. Cost Range Given the cost variability factors that apply to all green roofs, it is estimated that materials and labor costs for this prototype would range between $28 to $40 per square foot, plus an additional $20 to $40 per linear foot for guardrails and an optional $2 to $4 per square foot if irrigation is installed.11 E. Benefits In addition to the environmental and social benefits previously noted, this prototype would provide an ongoing supply of culinary herbs.

11 Stephen Peck and Monica Kuhn, “Design Guidelines for Green Roofs,” http://egov.cityofchicago.org/webportal/COCWebPortal/COC_ATTACH/ design_guidelines_for_green_roofs.pdf (accessed March 24, 2007). This estimate as-sumes that the prototype is applied when re-roofing is needed and therefore excludes the cost of a new roof membrane. It also assumes that structural and architectural requirements of the Building Code are already satisfied.




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IV. HYDROPONIC VEGETABLE GARDEN

Hydroponics, also known as ‘organoponics’ in Latin America, is a horticul-tural method that supplies plant roots with liquid nutrients, eliminating the need for organic material that provides nutrients under conventional meth-ods. Plants are provided with nutrient solution, and are either grown in an inert mineral substrate or are suspended above the solution without substrate. The hydroponic model substantially reduces the weight of vegetable cropping systems by eliminating the growing medium. No fewer than six different techniques can be used to operate the system, some of which involve such equipment as water pumps, air pumps, comput-erized monitors, timers, and lighting, when used indoors. This model of hy-droponics does not require artificial lighting as there is adequate sunlight for growing plants on the roof. The water supply is plumbed from the building to feed into the hydroponic system. A unique design component of the roof-top hydroponic prototype is a shade-cloth or light screen meshing as a ‘lid’ to protect the growing medium from being blown away in the wind. This re-taining cloth may be stapled along the edges of the container trays. A. Design Concept The Hydroponic Rooftop Vegetable Garden prototype utilizes a low-tech, low-cost method that minimizes weight while maximizing vegetable produc-tivity, variously called Simplified Hydroponics or Popular Hydroponic Gar-dens (PHG). The design is based on concepts developed and implemented around the world by the UN Food and Agriculture Organization (FAO), the UN Development Program (UNDP), and the Institute for Simplified Hydro-ponics. In this prototype, containers are filled with lightweight mineral substrate to a depth of 4 inches. Perlite is used as a base, and combined with inert organic material such as rice hulls, peanut hulls, grain chaff or coconut coir. The ad-

Commercial hydroponics. Source: Aaron Lehmer.

Rooftop hydroponics. Source: The Rooftop Garden Project, Alternatives, 2004.




3-17

dition of these organic materials provides a more balanced substrate, thus fos-tering a healthier medium for the plants. This growing medium is ideal for its extremely low weight, but it should be noted that other lightweight bases could be used, such as pumice. Appropriate plants include cooking and salad greens, most summer and winter vegetables and herbs. Root vegetables can be grown hydroponically but require greater substrate depths than specified in the prototype. Planting methods may vary among vegetable types, but generally seedlings are transplanted into the substrate, where they have regu-lar access to the nutrient solution. The prototype utilizes the “flood and drain” method of hydroponics, in which the nutrient solution is circulated back and forth between the growing container and a reservoir container. The growing container is constructed with a drain approximately one inch above the base, allowing for some accu-mulation of solution in the bottom of the container but draining the remain-der. If the growing container is elevated and placed at a minimal slope, drain-age can be gravity-fed. Alternatively, a pump can be used to continually flood and drain the growing container, recirculating the nutrient solution. Accounting for roof obstructions, pathways and storage areas, it is assumed that growing area would constitute 60 percent of the total roof area. Figure 3-4 illustrates the components of the prototype. B. Architectural Requirements Under normal conditions, the prototype would add approximately 9 psf to the roof load. However, in a heavy rain event, the substrate may be saturated and an inch of water could accumulate in the bottom of the growing con-tainer, resulting in a maximum load of approximately 16 psf. This prototype would be considered an occupiable space, subject to code re-quirements for access and guardrails.

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C. Benefits The hydroponic prototype combines many of the benefits of the Intensive Green Roof-Vegetable Garden prototype with those of the Rainwater Har-vesting prototype. The prototype would appeal to residents as an attractive open space amenity. In terms of energy efficiency, the hydroponic containers would shade the roof and vegetation would provide ambient cooling through evapotranspiration, but the thermal mass benefits would be less than a con-ventional green roof. Generally, the prototype would not deliver energy effi-ciency gains comparable to green roofs. Instead of the storm water retention associated with green roofs or the ground-level water storage of a rainwater harvesting prototype, the hydro-ponic system would capture rainwater in the growing containers and drain it to the reservoir containers for reuse. While centralized water storage would be prohibitive due to excessive loading, this decentralized water storage across the roof would distribute and minimize the load. Depending on the design, the Hydroponic prototype could capture and reuse all of the 14.3 gallons per square foot of rainwater that falls in an average year. Additional irrigation water would be conserved in dry months by recycling the water back and forth between the growing container and the reservoir container. In general, hydroponic systems represent a more productive growing method than conventional gardening. Because sufficient nutrients are supplied close to the base of the plant, roots do not spread horizontally to satisfy their nu-tritional requirements. This growth pattern allows for closer spacing of plants. In addition, hydroponically grown plants have an advantage over soil based plants in that energy that would be expended in root growth is utilized instead for leaf, flower, and fruit growth. Based on estimates of the FAO, the Hydroponic Vegetable Garden prototype would yield approximately




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4 pounds of vegetables per square foot.12 However, as this volume level is based on figures from professional hydroponic technicians, the actual volume may vary as much as two pounds less than the figures in this report. Typical field conditions can average a yield of 2.5 pounds per square foot.13 V. SOLAR PHOTOVOLTAIC ELECTRICITY

A. Technology Concept Solar photovoltaic (PV) technology offers an opportunity to produce clean, renewable electricity on a wide range of residential and non-residential roof-tops. While the technology has existed for many years, recent advances are improving the efficiency of PV cells, which are the individual units that pro-duce electricity. These cells are combined into modules that are available in single crystal, multi-crystalline, and amorphous silicon (thin-film) varieties, differing in their efficiency and cost. Today, single crystal and multicrystalline systems are the most common and cost-effective types of installations. Single crystal and multicrystalline cells are comparable in most ways, though the former is generally the more effi-cient and the latter generally the more affordable. Both are commonly in-stalled in the Bay Area and both are considered “high-efficiency” systems. For the purposes of the PV prototype, the characteristics of the more com-

12 Charles Schultz, “Soilless in Singapore,” Growing Edge Magazine, http://www.growingedge.com/magazine/back_issues/view_article.php3?AID=170324 (accessed April 1, 2007).

Juan Izquierdo, FAO. Personal communication with Brian Holland, DC&E, May 2007. 13 Willow Rosenthal, City Slicker Farms. Personal correspondence with Ingrid Severson, October 2007.

Ufafabrik factory in Berlin—Templehof green roof research. Source: www.ufafabrik.de.




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mon multicrystalline type will be assumed. These panels have a typical gen-erating capacity of 1 kilowatt (kW) per 100 square feet.14 Over time, other types of photovoltaic technology may become more attrac-tive options. Thin-film photovoltaics may become more cost-effective as manufacturing methods improve, and advances in building-integrated photo-voltaics may one day make it possible to generate electricity affordably through building materials themselves. PV dealers and contractors should be consulted to determine the most appropriate type of installation for a specific application and point in time. In addition to consideration of solar cell type, any PV system must be in-stalled with an acceptable tilt and orientation to be effective. Ideally the array should face south, but southeastern and southwestern orientations are also acceptable. In applying the prototype on pitched roofs, assumed orientation is either southeast (135 degrees) or southwest (225 degrees). For flat roofs, the prototype has a southern orientation. Currently, public financial incentives are directed toward electricity production during times of peak demand, which results in greater impetus for southwestern-facing installations. Once again, this condition may change over time as the structure of financial incen-tives will be a key determinant in the arrangement of any installation. With regard to tilt, an ideal tilt angle is that which sets the panel perpendicu-lar to the sun, which is the latitude of the location. The latitude of the Study Area is 37.7 degrees north. While installing a PV system at an ideal tilt some-times involves additional cost, this prototype assumes that all arrays are set at the area’s ideal tilt.

14 Liz Merry, “Solar Electric System Basics,” Solar Energy Resource Guide, NorCal Solar, 2007.




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B. Architectural Requirements Roofs fitted with photovoltaics must be able to support the weight of a typi-cal multicrystalline PV panel and mounting hardware, which is approxi-mately 5 psf. The location should be mostly unshaded by trees or adjacent buildings and panels should be set back from roof obstructions that cast shad-ows, such as enclosed stairway landings or mechanical equipment. C. Cost Estimate Installation costs for a residential PV system in Oakland averaged $8.68/watt between 2006 and 2007 with rebates of about $2.50 per watt.15 While most panels have a power warranty of 25 years (meaning at year 25 they are guar-anteed to produce 80 percent of their original output), initial costs are often paid back in only ten years, providing at least 15 years of no-cost electricity.16 Inverters typically have a warranty of 10 years, and need to be replaced be-tween year 15 and 20. The cost of inverters has been decreasing significantly quicker than the cost of the PV panels. D. Benefits Solar photovoltaics can play an important role in a clean energy future, reduc-ing the region’s dependence on fossil fuel imports and mitigating greenhouse gas emissions associated with natural gas power plants. In the near term, rooftop photovoltaics can also protect residents from rate spikes and reduce peak loads on the electrical grid, diminishing the need for costly and underuti-lized infrastructure to accommodate peak demand. Depending on the orien-

15 NorCal Solar. “September 2007 Update and City Solar data spreadsheet.” http://www.norcalsolar.org/local-activism/bay-area-solar-installs-2007-6.html (ac-cessed October 10, 2007). 16 Andy Black, “Payback and other Financial Tests for Solar Electric Systems,” http://www.ongrid.net/papers/PaybackOnSolarSERG.pdf (accessed April 1, 2007).




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tation, a 1kW module of the prototype is expected to produce between 1,278 and 1,415 kilowatt hours (kWh) annually.17 VI. RAINWATER HARVESTING AND REUSE

A. Technology Concept Rainwater collection has been practiced for centuries as a way of stretching a scarce resource. Harvesting water from the roof is a simple and elegant solu-tion, powered not by electricity or sunlight, but by gravity. Since drainage is a standard component for most roofs, much of the infrastructure needed to harvest rainwater is already in place on existing buildings. In the U.S., rain-water is most often used for irrigation, less often for fire protection or toilet flushing, and rarely for potable water supply. This prototype will be de-signed for irrigation purposes as the most feasible use for the Study Area. Figure 3-5 provides a diagram of one potential arrangement of rainwater har-vesting components. The actual design of the system will depend on site-specific conditions. 1. Catchment and Conveyance The first step in rainwater harvesting and reuse is to capture the precipitation. Typically, pitched roofs are fitted with external gutters and downspouts to carry water off the roof and away from the exterior walls of the house. In this case, the prototype rainwater harvesting system entails installation of a cistern connected to the downspout, intercepting water that would otherwise

17 All calculations of PV capacity and production were generated in the “PV Watts” online modeling tool, developed by the National Renewable Energy Laboratory. It is available at http://rredc.nrel.gov/solar/codes_algs/PVWATTS/.

Cistern at residence of Robert van de Walle. Source: Robert van de Walle.

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reach the ground and run away from the foundation of the building.18 Leaf screens are installed at the gutter/downspout connection and at the down-spout/cistern connection to keep debris from clogging the system. A roof-wash system should be fitted to the gutter to divert the first flush of storm water, which may be laden with pollutants from the roof surface. While some flat or low-slope roofs have external drainage systems like the one described, others do not have drainage or use internal downspouts that drain directly to the storm sewer or to a ground-level discharge spout. In these cases, capturing rainwater is a more expensive proposition as the drain-age system needs to be modified with external conveyance equipment. 2. Filtration After collection and conveyance, several features should be incorporated to filter out roof debris and pollution before storage. A simple debris screen should be fitted to the gutter or downspout to catch leaves and other large particles. In addition, a first-flush diverter is commonly used to capture the first few gallons of rainwater during a storm, which is usually more laden with pollutants that have accumulated on the roof between rain events, such as dust and bird droppings. The first flush of storm water is then drained separately from the rainwater harvesting system. The recommended capacity of the diverter varies by roof type, regular presence of pollutants, and regular duration between rain events, but a general rule of thumb suggests one gallon of diversion capacity is a minimum for each 1,000 square feet of catchment area.19

18 Not all rain that falls on the roof will be drained, due to such factors absorption and evaporation. A runoff coefficient of 0.85 is assumed for pitched roofs in this proto-type, meaning that 85 percent of fallen precipitation will be conveyed into the rain-water harvesting system. The assumed runoff coefficient for flat roofs is 0.50, account-ing for pooling conditions or gravel ballasted roofs. 19 Texas Water Development Board, Texas Manual on Rainwater Harvesting, Third Edition, 2005.

Rainwater captured for irrigation at Stopwaste.org headquarters in Oakland. Source: Sarah Sutton.




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Another potentially necessary filtration component is the roof washer. This feature is usually installed to filter smaller debris and organic materials. Many roof washers take the form of a container with one or two canister filters in-side, installed directly before the cistern. The necessity of this component will depend on the type of irrigation system used; drip irrigation systems in particular may become clogged if this type of micro-filtration is not em-ployed. 3. Storage After catching and filtering the rainwater, it must be stored for later use. Storage is the limiting factor in this prototype, since space is often limited for installation of cisterns. Below-ground cisterns have been used for individual buildings or community systems, particularly when they can be planned into new construction, but the expense of excavation and pumping water back up into the distribution system will prove cost-prohibitive for many residents and building owners. Instead, this prototype assumes that above-ground cisterns are the most feasi-ble option in the Study Area. Cisterns vary in type and size. Plastic or steel cisterns are commonly available and are durable and movable. Reclaimed containers such as trash cans or steel/plastic drums could also be used. Cis-tern size would be based on the size of the roof, rainfall patterns, the amount of space available for siting and the proportion of rainwater versus municipal water use that is desired. While some lots could accommodate a 1,500-gallon cistern, others would be confined to a 500-gallon or even 110-gallon tank. Based on observations of density and open space in the Study Area, an aver-age storage capacity of 1,000 gallons is incorporated into the prototype. 4. Distribution Water can be distributed to meet landscaping needs either through drip irriga-tion or watering by hand. Ideally, the cistern is sited at the highest elevation on the lot, enabling gravity-fed irrigation. This is often not the case in the Study Area. If drip irrigation is used, a pump would be needed to pressurize the system in some circumstances.

Residential underground cistern in Sausalito by 450 Architects. Source: Richard Parker.

Residential cistern. Source: Marc Richmond.




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B. Architectural and Site Requirements As described previously, buildings should have an external drainage convey-ance system, most commonly found on pitched roof residential buildings. The site requirements for storage will depend on the size of the cistern. Di-mensions of storage tanks vary, but a circular 1,100-gallon cistern can measure as little as 6 feet in diameter by 6 feet in height, while a 55-gallon barrel can fit in spaces of just a few feet wide. C. Cost Estimate Costs vary widely depending on the need for new conveyance gutters and pipes, the size and type of the storage unit, the type of distribution, and the type of labor used for installation. If reclaimed 55-gallon barrels are installed at existing downspouts and residents water by hand, costs can be as little as $100. On the other hand, storage tanks alone can cost between $0.45 and $1.00 per gallon, or $225 to $500 for a 500-gallon cistern.20 California-based vendors sell 1,000-gallon, polyethylene tanks for an average of $550 with costs for shipping averaging around $200. Plastic tank prices will fluctuate based on the current petroleum market.21 D. Benefits This prototype assumes a storage capacity of 1,000 gallons per structure, which would be stored for summer irrigation. Rainwater harvesting can cre-ate numerous benefits to water quality and water supply. These include:

20 Ag Extension Communications, Montana State University, "http://www.montana.edu/wwwpb/pubs/mt9707.html (accessed April 1, 2007). 21 Ingrid Severson. Phone calls to vendors, October 2007.




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♦ Conservation of potable water.

♦ Conservation of energy required to deliver and treat potable water.

♦ Minimizing need for upgrades to water treatment facilities.

♦ Minimizing the stress on water delivery systems.

♦ Reduction of storm water runoff.

♦ Reduction of pollutants entering the watershed through runoff.

♦ Reduction of the thermal impact of runoff to wildlife and vegetation due to heat gain from the roof.

Rainwater harvesting and associated water conservation are increasingly viewed as critical responses to potential climate change impacts. Models sug-gest that the Sierra snowpack could decrease by as much as 70 to 90 percent and that early Spring flow from this source could decrease by 30 percent un-der a medium-warming scenario.22 Combined with growing demand, particu-larly in Southern California regions impacted by supply constraints of the Colorado River Basin, these trends could stretch the region’s water supply in dramatic ways. In response, rainwater harvesting offers a feasible and effec-tive means for conserving water and protecting water quality. VII. MULTIPLE PROTOTYPE INTERACTIONS

This section identifies opportunities and constraints for implementing multi-ple rooftop resource strategies in the same roof space. While there are many unknown variables that influence these possibilities, research is beginning to look at what type of synergies may come about through interaction of these systems. This section considers a few of the most likely interactions.

22 California Energy Commission, “Our Changing Climate: Assessing the Risks to California,” July 2006.




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A. Extensive Green Roof and Photovoltaics This combination presents both positive and negative interactions, and exist-ing research has not adequately tested its feasibility. Installation of solar pan-els above the green roof vegetation would create a good deal of shade and would keep precipitation from falling evenly over the vegetation. Neverthe-less, some Sedum varieties have demonstrated shade tolerance, including Se-dum ternatum and Sedum telephium. Shading would reduce evaporation as well, potentially allowing for reduction of substrate depths beyond what is otherwise feasible in the seasonally arid Bay Area climate or the elimination of installed irrigation. A research plot maintained by University of Applied Sciences Neubranden-burg and the Technical University of Berlin has had success not only in green roof plant growth under a photovoltaic installation, but also in demonstrating increased PV output in this scenario. Their research indicates that green roofs can improve the efficiency of photovoltaics mounted above them. Ambient temperatures on the test plot were reduced 16 degrees Celsius compared to an adjacent conventional roof, which improved the efficiency of the PV and re-sulted in an average 6 percent increase in energy yields.23 The combined load of the Extensive Green Roof prototype and the Solar Photovoltaic prototype would be approximately 27 psf, precluding the possi-bility of combining these prototypes on existing buildings without structural retrofit. B. Green Roofs and Rainwater Harvesting Installation of these technologies in concert is technically feasible. Precipita-tion that is not taken up by the green roof vegetation is sometimes drained to 23 Manfred Kohler, et. al., “Positive Interaction Between PV Systems and Extensive Green Roofs,” Green Roof Infrastructure Monitor, Green Roofs for Healthy Cities. April 2007.

Ufafabrik factory in Berlin—Templehof green roof research. Source: Ufafabrik.de.




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downspouts as in the conventional scenario. However, the cost-effectiveness of this strategy is questionable due to the retention and absorption capabilities of the green roof. Runoff coefficients of green roofs can range from 0.50 to 0.80, allowing as little as 20 percent of the precipitation to drain into the wa-ter storage system. C. Photovoltaics and Rainwater Harvesting Photovoltaic systems do not intercept or impede the flow of water from the roof, so these prototypes can usually be implemented together. Because photovoltaic panels do not absorb any water, the runoff coefficient of a roof fitted with an installation may be improved relative to that of normal asphalt roofing material, resulting in a marginally higher catchment capacity. In ad-dition, photovoltaic systems require periodic washing to remove dust and dirt buildup and the wash water could be harvested under this scenario.




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4 FINDINGS

4-1

This chapter presents the conclusions of the analysis, including discussion of how prototypes were assigned to each building and what considerations were not taken into account in the assignment process. The chapter also describes the benefits of rooftop resource development in terms of increased productiv-ity of energy, food, and water. Finally, the chapter concludes with a sum-mary of incentives for and barriers to future rooftop utilization. I. PROTOTYPE ASSIGNMENT

Each building in the Study Area is assigned one or more rooftop resource prototypes. The following criteria are incorporated into the assignment proc-ess:

♦ Prototype Load and Roof Loading Capacity. Loading is a primary consideration in matching prototypes with suitable buildings. The analy-sis shows that the type of rooftop resource development that can take place on existing buildings is heavily dependent on the building type and associated loading capacity.

♦ Roof Access and Building Code Access Requirements. Access is an-other primary consideration in the assignment. The Intensive Green Roof and Hydroponic prototypes would be highly difficult to install and maintain without stair or elevator access. As occupiable spaces, these prototypes are also required by the Building Code to have stair or eleva-tor access. Therefore, the assignment of these prototypes depends on the existence or potential construction of a code-compliant stairway or eleva-tor.

♦ Occupancy Type. In some cases, more than one prototype would meet the above primary criteria for a building. Occupancy type is a secondary criterion that allows for consideration of what prototype the building oc-cupant would more likely choose based on its costs and benefits.



R O O F T O P R E S O U R C E S N E I G H B O R H O O D A S S E S S M E N T F I N D I N G S

4-2

The following sections describe how buildings were designated with each pro-totype. Figure 4-1 illustrates the pattern of potential rooftop resource devel-opment in the Study Area. A. Intensive Green Roof Because of the loads associated with the Intensive Green Roof prototype, it could not be developed on any occupied buildings in the Study Area. How-ever, the prototype was assigned to the one parking garage in the area. In addition, nine vacant lots in the study area were identified that will likely be developed over the next several years. As reflected on Figure 2-2, these lots were classified as “Opportunity Sites” and were fitted with the Intensive Green Roof prototype. Because new construction developments can plan to accommodate the load of a living roof, these buildings will carry this feature more readily than existing buildings with a retrofit of a green roof. Advances in growing media may soon lead to the production of extremely light-weight materials that can further reduce the weight of intensive green roofs and allow limited vegetable production on existing buildings. At this time, however, these products are not readily available in the San Francisco Bay Area. B. Hydroponic Vegetable Garden Post-war residential and institutional buildings are estimated to have the high-est loading capacities in the Study Area, ranging between 15 and 20 pounds per square foot (psf). The prototype adds a maximum of approximately 16 psf to the roof load when applied across the entire roof area. However, the load is less than 15 psf when applied to 60 percent of the area, as specified in the prototype.

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0 250 500 Feet

Intensive Green Roof-Vegetable Garden

Hydroponic Rooftop Garden

Solar Photovoltaic with Rainwater Harvesting

Solar Photovoltaic

Rainwater Harvesting

No Resource

Study Area




4-4

The limiting factor in this case is roof access. Buildings under four stories are not required by Building Code to provide stairway access to the roof, and most do not. At the same time, buildings with occupiable space on the roof-top are required to provide stairway access to the roof. The implication is that many one-, two-, and three-story buildings are not currently equipped to accommodate this prototype. For the purposes of the assessment, the following assumptions were made for flat-roofed buildings with adequate loading capacities:

♦ Buildings four stories and higher were assumed to have code-compliant stairway access since it is currently required, and are assigned the Hydro-ponic prototype.

♦ Two- and three-story buildings were assumed to have only ladder and hatch access and are not assigned this prototype.

♦ One-story buildings with adjacent open space were assigned the proto-type, with the expectation that a code-compliant external stairway could be installed at relatively little cost. The existence of adjacent open space was recorded during the field survey.

These assumptions are necessary because roof access could not be determined on a building-specific basis. However, a limited number of two- and three-story buildings may have stairway access to the roof and should not be ex-cluded from consideration in the future on that basis only. C. Extensive Green Roof Loading is the main constraint in retrofitting buildings with extensive green roofs. The prototype, which was designed to minimize the green roof load, weighs approximately 22 psf. The strongest roofs in the Study Area have loading capacities of between approximately 17 psf and 20 psf, found in post-War residential and institutional buildings. Therefore, it is likely that the only cases in which extensive green roofs can be applied are those where addi-




4-5

tional dead load capacity can be obtained by removing pea gravel or rock bal-last from the roofs of these building types. Gravel has sometimes been used to surface conventional built-up roofs, where layers of felt and asphalt are built up from the roof deck. Just a few inches of pea gravel are laid on top of the roof, weighing as little as 4 or 5 psf. How-ever, by reconfiguring this assembly with a green roof, that additional capac-ity may be obtained, potentially allowing for retrofit with the 22 psf exten-sive green roof. In other cases, heavier rock ballast has been used to secure loose laid single-ply membrane roofs, a roof type that has gained in promi-nence since the early 1980s. The commonly referenced standard for ballasted single-ply roofs calls for a minimum of 10 psf of rock ballast.1 If the ballast is removed, additional dead load capacity is again generated, and the extensive green roof may become a feasible roofing option. The applicability of this approach will vary depending on location. It is pos-sible that very few gravel surfaced or rock ballasted roofs exist in the Study Area, partly because of the age of the building stock (rock ballast was not commonly used before the 1980s) and partly because of windy conditions that may have restricted their use in the past. More investigation and collabora-tion will be needed between engineers, roofing contracts, and code officials to determine the feasibility of gravel or ballast replacement for green roof retro-fits. For the purposes of this study, the Extensive Green Roof prototype was not assigned to any buildings in the Study Area. In addition to the uncertainty surrounding the cost and technical feasibility of gravel and ballast replace-ment and the prevalence of these roof types in the Study Area, it was deter-mined that the Hydroponics prototype was a more productive and low-cost strategy for rooftop greening on the buildings in question. Though extensive green roofs can also be installed on pitched roof structures of a limited slope,

1 American National Standards Institute/Single Ply Roofing Industry, Wind Design Standard for Ballasted Single Ply Roofs, November 19, 2002.




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these buildings were not designated with green roofs because they are also more suitable for other prototypes, such as solar photovoltaics and rainwater harvesting. D. Solar Photovoltaics The Photovoltaic prototype requires buildings with 5 psf of roof loading ca-pacity. The southern-oriented prototype was assigned to all flat-roofed build-ings with between 5 psf and 15 psf of loading capacity. All pitched roof buildings were designated with the Photovoltaic prototype.2 With regard to orientation, the street grid and buildings in the study area are oriented 45 degrees off of north, so the prototype is for southeastern and southwestern-facing installations. The tilt was assumed to be ideal for the area, at approximately 37 degrees, though installation costs can be reduced without substantially sacrificing performance by installing panels flush with the pitched roof. The Study Area has high solar insulation with minimal shading, due to sunny conditions, generally uniform building heights and an absence of mature trees. E. Rainwater Harvesting and Reuse The Rainwater Harvesting prototype was assigned to all pitched roof build-ings, including those with the Photovoltaic prototype, and to selected flat roof buildings. Pitched roofs shed water more efficiently than most flat roofs.

2 It is assumed that all pitched roof structures in the study area will have adequate load-ing capacities for photovoltaics, and that usable roof space will average 40 percent for pitched roofs buildings and 40 percent for flat roofs, accounting for shading and roof obstructions.




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In addition, most pitched roof buildings in the study area are detached resi-dential units, which are assumed to have some open space to accommodate water storage. Parcels with flat roof buildings were assessed for the existence of adequate open space and only these buildings were designated with the Rainwater Harvesting prototype. No buildings with green roofs or hydro-ponic gardens received the prototype.3 II. OTHER CONSIDERATIONS

A. Cost-Benefit Analysis The assignment methodology took into account the technical and regulatory constraints and opportunities for rooftop resource development and only used cost-benefit considerations as a secondary criteria. Clearly, however, a number of financial factors will play a major role in determining how roof-tops are utilized. Initial costs bar many residents from taking advantage of rooftop resources, despite cost savings that accrue on an ongoing basis for each technology. Also, higher initial costs often correspond with higher im-pact technologies, such as solar photovoltaics or intensive green roofs. The cost-benefit calculus will inevitably change over time to reflect evolving economic conditions and social values. For example, as government moves to reduce greenhouse gas emissions, it is possible that electricity will become more expensive due to market-based mitigation mechanisms like carbon taxes or cap-and-trade systems. Similarly, government may institute new incentives for rooftop resource development in response to citizen concerns about community livability and sustainability. These incentives will change the direction of rooftop utilization depending on their objectives.

3 For pitched roof buildings, a runoff coefficient of 0.85 is assumed. Flat roof build-ings may have inadequate slope to drain most of their water, or could have gravel bal-last that would absorb water, so the runoff coefficient is reduced to 0.50 to reflect these possibilities.




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While it is outside the scope of this study, analyzing the costs and benefits of each rooftop resource prototype is an important task that will assist building owners in making tough decisions about how to use their building to its full-est potential. B. Seismicity The most prominent earthquake-related factor relating to additional roof loading is the existence of a soft story, as described in Chapter 2. This factor was incorporated into the loading capacity assumptions used in the assign-ment process. However, for some individual applications of rooftop re-sources, additional seismic effects should also be explicitly considered in greater detail. For instance, ground accelerations experienced during earthquakes cause building damage in proportion to the building’s mass. In general, additional load that is less than 5 percent of the total building mass does not significantly affect the earthquake safety of that building. Many buildings weigh 50 psf for each level above grade. In this case, if 10 psf is added to the roof, the total weight is increased by 20 percent for a one-story building and 5 percent for a four-story building. Thus, the seismic resistance of one- and two-story spe-cialty buildings (those which are not conventional wood construction) should be explicitly considered by a qualified professional during an overall assess-ment of the roof loading capacity. III. PRODUCTIVITY OF ROOFTOP RESOURCES

Based on the prototype characteristics and the assignment schema above, the total output of rooftop resources can be estimated for the Study Area. The outcomes of the exercise tell a story about what is feasibly achievable, in just one neighborhood of many, if a commitment is made to utilizing rooftops.




4-9

Table 4-1 on the following page illustrates the application of prototypes and their benefits. A. Electricity from Photovoltaics In the ¼-square mile Eastlake neighborhood, 8.5 megawatts (MW) of renew-able electricity capacity could be installed without sacrificing other rooftop uses that may be more appropriate. This total includes 4.6 MW on pitched roofs and 3.9 MW on flat roofs. These photovoltaic installations would gen-erate over 11 million kilowatt hours (kWh) of electricity per year. With an-nual per capita electricity consumption of around 6,700 kWh,4 photovoltaics would satisfy approximately 25 percent of the Study Area’s electricity de-mand under this scenario. Please refer to Appendix A, Assumptions and Methodology, for an explanation of the solar calculations. B. Vegetables from Intensive Green Roofs and Hydroponic Gardens Under this study’s scenario of rooftop utilization, 18 structures would be de-veloped with the Hydroponic Rooftop Garden prototype, and ten structures with Intensive Green Roof-Vegetable Garden prototype, providing approxi-mately two acres of growing area. These gardens would yield approximately 273,373 pounds, or 124 metric tons, of vegetables annually. This higher-than-average yield is the effect of year-round growing methods as well as hydro-ponic productivity greater than that of conventional methods.

4 California Energy Commission, “Per Capita Energy Use by State in 2003,” http://www.energy.ca.gov/electricity/us_percapita_electricity_2003.html (accessed April 18, 2007).




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TABLE 4-1 PROTOTYPE ASSIGNMENT AND PRODUCTIVITY

Prototype

Number of

Structures

Total Annual Yield

Extensive Green Roof 0 None

Intensive Green Roof –Vegetables 10 34.1 metric tons*

of vegetables

Intensive Green Roof – Herbs 0 0

Hydroponic Rooftop Garden 18 90 metric tons of vegetables

Solar Photovoltaics 668 11,609,024 kWh/year of electricity; 8.5 megawatts

of capacity

Rainwater Harvesting 623 1,869,000 gallons of

irrigation water * Productivity for the Intensive Green Roof. Vegetables prototype was derived from one exist-ing parking garage and nine existing, vacant lots (“Opportunity Sites”) that were projected as being built up with new structures integrating this prototype.

Current annual consumption of the nutritional “dark-green leafy” and “deep yellow” vegetables included in the prototype is about seven pounds per cap-ita.5 Based on current consumption, the buildings with this prototype could meet this type of vegetable demand for approximately 38,127 Oakland resi-dents. However, current consumption of these vegetables is significantly lower than that recommended by the USDA, partly because of limited access to affordable fresh produce. USDA recommendations for leafy greens and deep yellows translate into about 31.6 pounds per capita. Using this figure, the garden prototypes could produce enough of these vegetables to satisfy the recommended consumption for approximately 8,500 residents, which is more than the population of the Study Area itself.

5 USDA Economic Research Service, Moving Toward the Food Guide Pyramid: Implica-tions for U.S. Agriculture, 1999, http://www.ers.usda.gov/publications/aer779/ (accessed on April 1, 2007).




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C. Irrigation Water from Rainwater Harvesting According to a report by the Public Policy Institute of California, outdoor water use in the Bay Area averages approximately 0.19 acre-feet, or 8,276 gal-lons, per household annually.6 Approximately 26.9 million gallons, or 82.5 acre-feet of rainwater falls on the roofs of the buildings assigned to the rain-water catchment prototype in the study area each year. If all of this rainwater could be captured, stored and reused, outdoor water use needs would be met for over 3,000 households, assuming a consumption rate at the state average. However, storing this volume of water would present an enormous technical challenge within an urban setting in that most of the storage would need to be installed underground, much of it in the public right-of-way. Large volumes of water storage are particularly necessary because 83 percent of Oakland’s average precipitation falls between the months of November and March, while almost all of the irrigation demand occurs between the months of April and October. The quantity of water that can be stored and used as irrigation water using this prototype is more than just the 1,000 gallons of storage capacity, since the cistern will empty during irrigation and re-fill with the next rain. A common method in calculating rainwater harvesting and reuse is the “water balance method,” in which monthly storage and demand are compared to determine the balance of water that remains in the cistern at the end of the month. For example, a system may start with 1,000 gallons of water in the cistern, use 1,400 gallons through the month, capture another 1,000 gallons during the month, and end the month with 600 gallons in storage. However, use of this method requires an understanding of irrigation demand for the specific site, while the purpose of this study is to estimate the potential

6 Ellen Hanak and Matthew Davis, “Lawns and Water Demand in California,” PPIC, http://www.ppic.org/content/pubs/cep/EP_706EHEP.pdf (accessed April 15, 2007).




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for harvested water at the neighborhood scale. Without an understanding of water demand (that is, how large an area is being irrigated, what types of plants and soils exist on the site, etc.), it is exceedingly difficult to quantify the amount of rainwater that can be stored and used. The best estimate, there-fore, relies on an assumption: that the harvested water is being used for a landscape that will require intermittent irrigation during the year, and that each structures’ cisterns are filled and emptied three times in the course of a year. This is a conservative estimate that assumes limited irrigation from October through April, allowing the cisterns to fill up a total of three times per struc-ture, thus processing a total of 3,000 gallons of water in a year. If applied to all buildings designated with the rainwater harvesting prototype, the Study Area could capture and use 1,869,000 gallons, or 250,446 cubic feet, of rain-water annually.7 This amount of water will be captured from 83 percent of the buildings in the study area, providing for a portion of the irrigation needs of each building. IV. STRUCTURAL IMPROVEMENT OPPORTUNITIES

In individual applications of rooftop resources, there will be some buildings that can support rooftop resources with additional productive capacity if structural improvements are conducted. This section describes a few of the most feasible retrofit options for various prototypes.

7 Build it Green, Advanced Training Manual for Certified Green Building Professionals, February 24, 2007 e-mail from Marc Richmond, Practica Consulting, October 25, 2007.




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A. Extensive Green Roof Prototype Buildings were identified that could potentially support an extensive roof after conducting relatively minor improvements to or verifications of the roof structure. Improvements were considered minor if they did not significantly disrupt building operations and could be conducted for an overall cost on the order of $10 per square foot of the total building area (2007 dollars). For wood buildings, such improvements could consist of sistering additional rafters to the existing rafters. Buildings constructed of concrete masonry units may only require installation or verification of steel beams. However, any proposed improvements to a particular building to increase its roof load-ing capacity should be designed by a qualified professional. B. Intensive Green Roof-Herb Garden Prototype For some buildings, if major improvements are planned, a few additional structural improvements could provide sufficient loading capacity for the weight of the Intensive Green Roof-Herb Garden prototype. One opportunity is to add to roof framing during a roof replacement. Many buildings could feasibly support the weight of a shallow substrate intensive roof if the structural roof framing is significantly supplemented. In particu-lar, a simple case is a steel building with bare metal decking at the roof but concrete topping over metal deck at floor levels. Adding concrete topping to the roof deck could increase the roof loading capacity substantially. This ef-fort may also require adding supplemental roof framing beams. When planning a seismic upgrade or other major renovation for any building type, roof framing could be added as required to accommodate a heavier roof system. Many building types are natural candidates for seismic upgrades, ei-ther because of local mandates or because of documented poor response in prior earthquakes. These include:




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♦ Unreinforced masonry (“brick”) buildings, for which California Senate Bill 1633 (introduced 2/2006) mandates safety upgrades.

♦ Concrete frame buildings constructed before 1975, which was prior to more stringent, ductile detailing requirements for reinforcing bars.

♦ Tilt-up (“big box”) buildings constructed before about 1990, which was prior to more stringent requirements for anchoring heavy walls to light-weight roofs.

♦ Steel moment frame buildings constructed between 1978 and 1995, which typically used heavy framing with welds that have exhibited brittle re-sponse.

♦ Soft-story or open-front buildings. Strengthening costs increase disproportionately if strengthening is required for columns and foundations. In buildings of five stories or more, strengthen-ing existing columns and foundations may not be required if the additional weight of a heavier rooftop system may be small relative to the total building weight. Likewise, buildings of two stories or less may not require column and foundation strengthening if column and wall sizes were chosen for their conventional size and are thus stronger than necessary. These conditions must be verified by a qualified professional before any additional load is in-troduced. Buildings on hard or stiff soils, which are not prone to liquefaction in an earthquake, are less likely to require foundation strengthening. Buildings with little deterioration, including more modern buildings, will require less strengthening than a building with visible deterioration. V. POLICY OPPORTUNITIES

The results of the Eastlake neighborhood assessment demonstrate a profound opportunity to utilize Oakland rooftops for economic development, envi-




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ronmental sustainability, and food and energy security. However, a number of financial and regulatory barriers lie in the way of rooftop resources devel-opment. This section describes incentive programs and code revisions that could spur a transformation in the way Oakland’s built environment con-tributes to its quality of life. A. Incentive Programs As described previously, high initial costs limit the degree of implementation that is possible for many residents. Local governments and service providers have a role to play in offering incentives for rooftop resource development when such strategies make good sense from a policy or financial perspective. 1. Green Roofs Currently there are few incentives offered for green roof installation. Own-ers that install green roofs may qualify for a federal tax credit under the En-ergy Policy Act of 2005, which provides a credit equal to 10 percent of the cost of insulation materials. However, State government has not instituted incentives for green roofs, nor does the City of Oakland offer programs or incentives for the technology. The City of Chicago, however, is a national leader in green roof implementation as the City offers direct incentives in the form of $5,000 grants for green roof installation on residential and small commercial buildings. Green roofs can be an asset to service providers, who may benefit by encour-aging wider implementation of the technology. For example, a green roof’s storm water retention qualities reduce the volume of runoff that must be cap-tured, conveyed, treated, and discharged, thereby alleviating demand for new infrastructure. In Germany, where approximately 10 percent of all buildings are fitted with green roofs, storm water taxes are levied in many cities based on the runoff generated by an individual property. While this type of tax may not be feasible in the Bay Area, water districts like the East Bay Munici-pal Utilities District do benefit from green roofs and may find it possible to offer storm water rebates to “low-impact” customers. Similarly, PG&E cur-




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rently offers rebates for a variety of high-efficiency appliances, insulation, high-performance windows, and cool roofs, but does not have a rebate for energy-saving green roofs. Green roofs also can contribute to several points in the Leadership in Energy and Environmental Design (LEED) green building certification program. LEED is widely recognized for improving the performance of commercial buildings and providing a marketing incentive to developers and building owners through its respected “green” brand. Depending on their design, green roofs can assist in obtaining points relating to improving energy effi-ciency, reducing storm water runoff, mitigating the urban heat island effect, restoring open space, and providing water efficient landscaping. Finally, developers in Portland, Oregon are able to increase their profitability by installing green roofs in exchange for bonuses in floor-to-area ratio (FAR).8 This approach, sometimes called “amenity zoning,” utilizes zoning code pro-visions as additional incentives. 2. Photovoltaics Incentives are available for solar photovoltaic installation at the State and Federal levels of government. California has been a leader in encouraging the adoption of solar technology, and most recently stepped forward to enhance solar incentives through the $3.3 billion California Solar Initiative. The Cali-fornia Solar Initiative provides a rebate based on PV system performance. Currently, residential users receive $2.20 per watt of photovoltaic power in-stalled; in the Bay Area, this rebate is administered through Pacific Gas and Electric. Incentives for builders and developers installing photovoltaics on new residential construction are also available through the California Energy Commission. The CSI legislation also increased the number of customers that are permitted sell their renewable electricity into the power grid. These State incentives are making residential PV applications, in particular, increas-ingly affordable. At the Federal level, businesses can receive a tax credit of 30

8 FAR is a standard measure of commercial density.




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percent of the PV system cost, while homeowners can receive a credit of up to $2,000. 3. Rainwater Harvesting No incentives for rainwater harvesting are available from local, State, or Fed-eral government. However, commercial buildings may be eligible for a rebate through EBMUD’s water conservation program. EBMUD’s Water Smart Landscape Rebate Program offers $1,000 for installation of high-efficiency irrigation, but no rebate for rainwater harvesting systems on residential build-ings. Other cities in the U.S. are finding it desirable to offer incentives to encour-age water conservation and water quality improvements through rainwater harvesting. For example, the City of Austin provides rebates of between $45 and $500 for rainwater harvesting systems that provide irrigation water. The Portland Bureau of Environmental Services provides $5,000 grants for “downspout disconnection,” which includes rainwater harvesting systems. B. Regulatory Barriers There are few regulatory barriers to implementation of photovoltaic or rain-water harvesting strategies. As a newer technology, green roofs are not as well understood, and in some cases regulations have yet to adapt to the design and construction industry’s enthusiasm for the technology. One barrier to adoption of green roofs is a lack of recognition of their bene-fits in current zoning codes. All open space is not equal, but very few codes specifically reward installation of environmentally beneficial features such as green roofs. One exception is the City of Seattle’s new Green Factor pro-gram, which allows developers to meet open space requirements by choosing among a suite of landscaping components, each with a corresponding point value, instead of merely setting a minimum area requirement. Each compo-nent is assigned a value based on its benefits. Because green roofs are consid-




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ered a highly-beneficial approach, they satisfy a larger portion of the total point requirement and enable an overall reduction in open space area. This reduction can maximize the amount of area that can be built upon and im-prove the financial performance of a project. In a broader sense, green roofs are a relatively new technology in the Bay Area and code officials may need time to inform themselves of some of the technical considerations. Building codes do not necessarily discourage green roofs, but for some municipal staff, questions may remain about code impli-cations. Part of this uncertainty results from the absence of a credible green roof standard in the U.S.; the German FLL-Greenroof Guidelines, (For-schungsgesellschaftLandschaftsentwicklung Landschaftsbau e.V., Guideline for the Planning, Execution and Upkeep of Green Roof Sites, Release 2002) is widely used but lacks regional applicability to the US and the Bay Area, and in some cases may not reflect the concerns of code officials. Fortunately, an ASTM (American Society of Testing and Materials) green roof standard is in development that will soon provide additional guidance to professionals in designing, approving, and installing green roof systems. VI. CONCLUSION

This neighborhood assessment shows conclusively that rooftop resources can be developed on existing buildings in the Bay Area, without structural im-provements. In addition, new construction can be designed with increased loading capacity, allowing for living roofs that are able to provide high yields of fresh, organic produce and underground cisterns with a generous capacity for rainwater storage. Hydroponic rooftop gardens and solar photovoltaics show the most promise for existing buildings, while extensive and intensive green roofs and rainwater harvesting present additional challenges, some of which may be overcome in time as greater investment is warranted. Today, building owners can install rooftop technologies and improve water quality, save energy, grow fresh produce, generate clean electricity, and contribute to greater community resilience and livability.




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Education and leadership can bring about the kinds of benefits that so many cities have successfully demonstrated. Policy and government support are essential keys to fostering the implementation of these systems. Rooftops are currently untapped resources, and a package of appropriate design, develop-ment incentives, and public support is crucial to fulfilling their great poten-tial.

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A P P E N D I X A

A S S U M P T I O N S A N D

M E T H O D O L O G Y

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APPENDIX A ASSUMPTIONS AND METHODOLOGY

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In order to produce meaningful estimates of rooftop productive capacity, assumptions were made at each step of the study process. This appendix provides background information on what assumptions were included and describes the methodology by which information was analyzed and conclusions drawn. A. Existing Conditions Analysis Existing conditions were documented through a combination of aerial photograph analysis and a field survey of the study area. A GIS (Geographic Information Systems) base map of building “roofprints” was created based on the aerial photograph and information about roof slope and existing rooftop resources was obtained from the photograph. A field survey was conducted by volunteers to document building types and construction characteristics. The volunteers were trained in simple techniques for identifying building characteristics and ten groups were each assigned a portion of the study area to walk and analyze. Collected information was entered into the GIS database and correlated with estimates of roof loading capacity. B. Estimating Roof Loading Capacity Roof loading capacity for each building type was calculated using the following procedure:

1. Unobservable structural properties, such as rafter size and spacing, were assumed for each type based on the building’s observed features and on the professional experience of the project team’s structural engineer.

2. For each building type, a total roof loading capacity was calculated, corresponding to its assumed properties.



R O O F T O P R E S O U R C E S N E I G H B O R H O O D A S S E S S M E N T A P P E N D I X A

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3. The code-level applied roof loading of 20 pounds per square foot was subtracted to obtain the remaining roof loading capacity.

4. On a case-by-case basis, each building’s loading capacity was adjusted based on any special circumstances where an observable feature differed from the typical features of that type. Adjustments included:

a. Age Penalty Buildings constructed in an area other than that assumed for calculating typical properties were penalized to account for greater possible deterioration. For example, if an apartment building matched all typical features listed in Table 2-1, except its construction era was pre-War, we subtracted 5 pounds per square foot from the building’s loading capacity. This is because the material properties assumed for an apartment building are based on mid-century construction. b. Height Penalty Buildings with more stories than the typical range specified for that type were penalized to account for the additional loading on the vertical load-bearing members, i.e. walls and posts. c. Construction Material Adjustment For a building whose primary construction material was weaker or stronger than the typical construction material for that building type, the loading capacity was either reduced, in the case of weaker materials, or adjusted upward, in the case of stronger materials. Brick buildings that were assumed to be wood under their building type were particularly penalized. d. Soft-Story Penalty A soft-story is a story level (usually the first story above grade) which has significantly less earthquake resistance than the adjacent stories. In an earthquake, the soft story will “sway” or “lean” much more prominently than the stiffer stories, and the earthquake damage will be concentrated in that story. Buildings with open fronts were penalized in order not to overload a building with potentially low earthquake resistance.




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e. Cumulative Adjustments If a building had more than one adjustment, the adjustments were added (cumulative).

Special consideration was given to buildings whose roof loading capacity equaled or exceeded 10 pounds per square foot, but did not reach the load of the Hydroponic prototype (16 pounds per square foot). For these buildings, a general average of 60 percent of rooftop area was designated for the hydroponic systems, assuming that the remaining area would require a protective surface for human traffic. The surfacing for this prototype was assumed to weigh five pounds per square foot. The consideration for this design scheme would average less than 15 psf, given an averaging for the distribution of weight between the hydroponic system and the pathways; this enabled the Hydroponic prototype to be applied to many buildings with loading capacities of 15 psf. C. Productivity Calculations A primary goal of the study was to estimate the quantity of food, energy, and water that could be harvested from the Eastlake study area, and by extrapolation, East Bay neighborhoods in general. While some explanatory text regarding these calculations is included in Chapter 4, and the Findings section, the following provides a more thorough description of the assumptions and the calculations process. 1. Extensive Green Roof Despite presenting a variety of other benefits, this prototype was not assigned to any buildings or analyzed for productivity because food, energy and water are not produced on the extensive green roof.




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2. Intensive Green Roof-Vegetable Garden Neighborhood-wide productivity in this prototype is a function of plant selection, growing area, and a “productivity coefficient” for each crop. The criteria used for plant selection is described in Chapter 3 and Prototypes. The growing area was assumed to average 60 percent of the area of any roof, which is intended to account for mechanical equipment, vents, stairway landings and other roof obstructions, as well as space for paths to maintain the roof and storage of maintenance equipment. This proportion was applied to the total area of roofs designated with the prototype (61,232 square feet, or 1.4 acres), to determine the total growing area of 36,739 square feet, or 0.84 acres. Note that this calculation assumes development of all nine opportunity site with intensive green roof vegetable gardens. Productivity coefficients for this prototype describe the amount of food that can be grown in a space of a certain size—for the purposes of this study, pounds of vegetables per square foot. Coefficients were derived from University of California Cooperative Extension data.1 These figures clustered between 0.30 to 1.0 pounds per square foot for each harvest. To account for variation in site-specific plant selection, and in recognition of the fact that there were no outliers to skew the outcome, the coefficients were averaged to obtain a typical coefficient of 0.62 pounds per square foot per harvest. Because a year-round gardening approach is assumed for the East Bay, this coefficient was multiplied by three growing seasons, resulting in an annual coefficient of 1.86 pounds per square foot of growing area. By multiplying this figure by the total growing area above, an annual yield of 68,334 pounds, or approximately 34 tons, was determined.

1 Nancy Garrison, Urban Horticulture Advisor, UC Cooperative Extension--Santa Clara, “Home Vegetable Gardening,” http://vric.ucdavis.edu/ veginfo/commodity/garden/tables/table4.pdf (accessed April 1, 2007).




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3. Intensive Green Roof-Herb Garden This prototype was not assigned to any buildings or analyzed for productivity, but was included for the sake of testing options for lighter-weight edible garden prototypes, and could yield a perennial harvest of culinary or medicinal herbs. 4. Rooftop Hydroponic Garden Yields for the hydroponic prototype were calculated on a similar basis as the Intensive Green Roof-Vegetable Garden prototype. Growing area was again assumed to comprise an average 60 percent of any roof designated with the prototype. Total roof area assigned the hydroponic prototype was 82,731 square feet, or 1.9 acres. This resulted in growing area of 49,638 square feet, or 1.1 acres. The productivity coefficient was obtained from estimates by the UN Food and Agriculture Organization, which has stimulated the use of Popular Hydroponic Gardens (PHG) like the one described in the prototype in many nations, and was confirmed with the Institute for Simplified Hydroponics, which pioneered the use of PHGs in the developing world.2 Their initial estimate of 40 kilograms per square meter (8.2 psf) annually,3 however, was based on six annual harvests of lettuce as opposed to 3 annual harvests of leafy greens.4 The FAO figure was divided into a per harvest yield, then multiplied times three growing seasons to obtain an estimate annual yield of 4 psf. This was multiplied by total growing area to determine the total annual yield of approximately 198,554 pounds, or 90 metric tons, of vegetables.

2 Peggy Bradley, Institute for Simplified Hydroponics, personal email communication with Ingrid Severson, May 31, 2007.

3 Charles Shultz, “Soilless in Singapore,” Growing Edge Magazine, retrieved from http://www.growingedge.com/magazine/back_issues/view_article.php3?AID =170324 (accessed April 1, 2007).

4 Juan Izquierdo, FAO, personal communication with Brian Holland, DC&E, in May 2007.




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5. Solar Photovoltaic Electricity Assumptions regarding electricity output were based on regional sources and on the PVWATTS calculator produced by the National Renewable Energy Laboratory. The first step in these calculations was to determine the roof surface area that would be suitable for a photovoltaic installation. Because buildings were oriented 45 degrees off of north-south, it was assumed that half of the area of pitched roofs (southeast and southwest faces) would have suitable solar access. The total “plan-view” area of southeast or southwest-facing pitched roofs was 861,225 square feet. Total surface area of 1,148,212 square feet was determined using a typical roof pitch angle in a trigonometric function. Based on aerial analysis and field observation, it was estimated that 40 percent of this space, or 459,285 square feet, would be suitable for panel installation, due to constraints relating to shading and architectural detail. Finally, the area of flat roofs assigned the prototype was determined separately and multiplied by 40 percent to determine the suitable flat roof area of 392,463 square feet. The next step was to determine an annual “productivity coefficient” for electricity generation. The PVWATTS Version 2 calculator was used to calculate annual kilowatt hours (kWh) of electricity produced per square foot, specific to the Eastlake area weather conditions. It was determined that approximately 12.78 kWh/square foot could be produced on southeast faces and approximately 13.59 kWh/square foot on southwest faces, assuming an ideal tilt of 37.7 degrees. Southern-oriented panels could produce approximately 14.15 kWh/square foot on flat roofs with panels oriented south. Of course, these estimates will vary depending on environmental conditions at the site and on the specific equipment in use. However, the PVWATTS calculator is a widely-used tool that incorporates a variety of factors, including a DC to AC derate factor of 0.77 that considers differences between Standard Testing Conditions rating and actual field performance, as




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well as losses at the inverter, transformer, diodes, connections, AC and DC wiring, and losses due to soiling.5 Using the productivity coefficients generated by the PVWATTS calculator, total annual electricity yield of 11,609,024 kWh was calculated for the Eastlake area, produced by 8.5 MW of photovoltaic panels. 6. Rainwater Harvesting Description of the assumptions and methodology for this calculation was included in Chapter 4, Findings. To reiterate, total “plan-view” area of roofs designated with this prototype was determined using GIS. Total catchment capacity was calculated by multiplying roof area by annual precipitation and applying a runoff coefficient of 0.85 for pitched roofs and 0.50 for flat roofs to account for absorption, pooling and evaporation, and for spillage in the conveyance of water.6 Catchment capacity is somewhat irrelevant, however, if cisterns and other storage infrastructure are not available to hold the captured water. An average storage capacity of 1,000 gallons was assumed based on aerial photograph analysis and field observation of available open space. This is the capacity available at any one time, but over the course of a year more than 1,000 gallons can be stored as water is used for irrigation, freeing up additional storage capacity. As most rainfall in the Bay Area occurs from late fall through early spring and the highest demand for irrigation water occurs in the summer, it is assumed that the harvested water will account for only a portion of the irrigation needs for individual properties. The water demands will vary upon a number of factors including plant selection, solar exposure and soil type. Seasonal variations in rainfall and drought will also impact water need

5 For more information on the PVWATTS Version 2 parameters, see http://rredc.nrel.gov/solar/calculators/PVWATTS/system.html.

6 Email from Marc Richmond, June 24, 2007.




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throughout the rainy months. Therefore, a conservative estimate assumes that the cisterns will be filled and emptied an average of three times per year, resulting in 3,000 gallons harvested per structure or 1,869,000 gallons harvested annually and used for irrigation in the study area. To account for these flows, a basic monthly water balance calculation was performed that assumed all water captured could be usefully applied to the landscape between rains in the fall, winter, and spring, and the remainder of the captured water would be used in the dry season during the summer.




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Tapping the Potential of Urban Roof Tops

Education

Transcript of Tapping the Potential of Urban Roof Tops