Goals and Philosophy

Every roof is a resource. Whether in a dense, urban area, or a suburban zone, rooftops are valuable real estate that can serve vital purposes, such as catching water, supporting edible gardens and vegetated surfaces and generating solar power. Although there are myriad other rooftop systems, Bay Localize initiated the Rooftop Resources Project to promote living roofs, rainwater catchment and solar power because of their instrumental contribution to a balanced urban ecology and resource security.

California and the nation have limited energy and water supplies. The centralized, industrial agribusiness model that currently distributes these resources is unsustainable due to its own resource demands. To ensure our resource security, we must diversity our supplies and generate our own food, energy and water resources wherever possible.

We believe that if every roof was harnessed with either solar power, rainwater catchment or a vegetated or food producing surface, our cities would be more sustainable and more prepared for resource scarcity. To meet these goals, we need greater public awareness, education, support from local government and financial incentives. Bay Localize actively supports these measures. For answers to common questions about rooftop systems and clarification of our views, we encourage you to read this document.

* Please note that Bay Localize is comprised of advocates, community organizers, policy analysts, and volunteers — as such, we are drawing on research and expertise within the field, and are not consulting for this technology. We are aiming to provide more information and resources to promote sustainable, decentralized resources.

Frequently Asked Questions

Living Roofs

Why is a living roof worthwhile? What benefits merit the implementation and investment of a green roof?

* Increase energy efficiency for the building through insulation
Plants shade the roof and cool it through their natural cycles of transpiration and evaporation. The layer of plants and growing medium insulates the building, reducing temperature fluctuations between day and night. One Canadian study found that green roofs limited the daily temperature range to 25-40 degrees Celsius compared with a range of 20-70 degrees Celsius on a traditional roof. (Source: Performance evaluation of an extensive green roof) Another study in Pennsylvania found temperature differences between traditional and green roofs of 12 degrees Celsius during the winter, and 40 degrees Celsius during the summer. (Source: Green Roof Mitigation of Stormwater and Energy Usage)

* Living roofs mitigate the urban heat island effect
Urban Heat Island Effect is the temperature difference that exists between urban areas and their less developed surroundings. Urban areas tend to be between 7-10 degrees hotter due to the concentration of heat-absorbing buildings and pavement, lack of vegetation, and hot air from cooling systems. Conventional rooftops are typically covered with sunlight-absorbing materials, so that on sunny days with an air temperature of 95 degrees F, city roofs can easily reach temperatures of 175 F or more. The bubble of hot air that encloses cities contributes greatly to smog, ozone and dust. The urban heat island effect can also alter cities' normal weather patterns, making drought, high winds, and electrical storms more common than outside cities. Living roofs act like a protective umbrella above the roof to shade and insulate buildings, limiting the roof's contribution to the heat island effect. (Source: Live Roof, Prevegetated Modular Green Roof System) Click here for a case study from New York.

* Improve air quality
Plants reduce airborne particulate matter by preventing particles from becoming airborne and filtering airborne particles out of the air. By cooling surfaces, plants can reduce convection currents that lift dust into the air. Airborne particles settle onto leaves and vegetation, and are then rinsed off by rainwater. Foliage can also trap heavy metals and absorb gaseous pollutants, although these benefits are mostly from larger plants like trees. Plants naturally consume carbon dioxide and release oxygen during the day, and though they use some oxygen at night, there is a net increase in oxygen locally.

* Decrease outside noise through acoustic insulation
Plants, soil and trapped air and moisture in the soil are great acoustic insulators. Studies have shown that green roofs can reduce indoor sound by as much as 40 decibels, a great benefit for buildings close to overhead highways and airports. (Source: Live Roof, Prevegetated Modular Green Roof System)

* Stormwater retention
In urban areas, impermeable surfaces (streets, sidewalks, rooftops, etc) dominate the landscape and prevent rainwater from being absorbed by the covered soil. Instead, as much as 75% of rainwater runs into sewer systems (versus about 5% runoff in forested areas), carrying contaminates from the air and roadways (salt, mercury, oil, etc). (Source: Live Roof, Prevegetated Modular Green Roof System) This water flows untreated into lakes and streams, which in some cases may cause overflows of sanitary sewer systems, causing the dumping of untreated sewage. Living roofs can absorb up to 70% of a one inch rainfall, reduce runoff, lessen the risk of sewer overflows and flooding, and filter the water as it percolates through the soil. (Source: ASLA et al., "Landscape Architects Release Green Roof Performance Report: Roof Retained 27,000 Gallons of Stormwater in First Year") Even when the living roof doesn't absorb all the water from a rainfall, the live roof media will filter and delay the water till after sewer system peak flows, which helps prevent sanitary sewer system overflows.

* Provide a potential space to grow food
While extensive green roofs are typically planted with ground cover, edible plants can be grown in a vegetable or herb garden or using hydroponics. A year-round garden can produce 1.5 pounds of food per square foot from an intensive vegetable garden or 2.5-4 pounds of food per square foot from a hydroponic garden. (Source: Tapping the Potential of Urban Rooftops)

* Provide habitat for wildlife
Roofs planted with native species can provide habitat patches for wildlife and add to networks of endangered habitat and vegetation types. For example, plants on the California Academy of Science's new living roof create a new patch of habitat for the endangered mission blue butterfly in a location that fits into a network of potential habitat patches, allowing the butterflies to migrate throughout the Bay Area. (Source: John King, San Francisco Chronicle, Urban Planning, Up on the Roof).
Contribute points to LEED certification
Living roofs qualify as a LEED point that can help bring a project to LEED certification. LEED or Leaders in Energy and Environmental Design is a building environmental certification program developed and operated by the U.S. Green Building Council.

What rebates and financial incentives/funding sources are available?
Funding in the form of rebates are not available that we are aware of, although there are multiple government funding sources as grants and loans that have been compiled and listed by the company Elevated Landscape Technology. These may be viewed at the company's website. These are the only financial incentives of which we're aware. If you know of any additional sources, please inform us!

How do I know if a green roof is right for my building?
The load capacity of your existing building is one of the most important considerations, as living roofs weigh more than conventional roofs. Extensive green roofs and hydroponic rooftop gardens are lighter options than intensive vegetable and herb gardens. Flat and low-pitch roofs are most appropriate for living roofs; slippage may occur if the slope is too steep, though stabilization techniques like horizontal strapping, battens, and grids can allow steeper slope roofs to support living roofs. Your building use (e.g. public or private) and the designed roof occupancy can influence permitting requirements. You should also consider the amount of maintenance you are willing to accommodate; gardens require year-round tending and maintenance, whereas extensive green roofs are low-maintenance and can actually reduce total roof maintenance required relative to conventional roofs by protecting the roof membrane from damage caused by UV radiation and temperature fluctuations. For further information regarding green roof applicability, see the report, Tapping the Potential of Urban Rooftops.

Aren't living roofs too heavy for most roofs to handle? How heavy are green roofs?
As per California code, all buildings have a minimum load bearing of 20 lbs per square foot, and some buildings exceed that minimum. Depending on their design, living roofs can weigh as little as 10 lbs per square foot. The soil type and depth, variety of plants, and type of living roof installed determine its weight. Intensive green roofs require thicker substrates than extensive green roofs because they support trees, crops and greater soil depth, while hydroponic systems nearly eliminate the growing substrate and its concomitant heavier weight. Existing roof ballast, like gravel, can be removed to allow the installation of heavier living roof options. The earthquake safety of a roof is not affected by load additions less than five percent of the total building mass. Please refer to the Neighborhood Assessment Report, Tapping the Potential of Urban Rooftops, for a more in-depth analysis of the weights of living roofs and their applicability to new and existing buildings.

Would I need to replace my roof to implement a living roof?
This depends on multiple factors, and is best determined on a case-by-case basis. Generally, if your roof is in good condition, a green roof is applied to the original roof with additional layers (such as a root barrier and water proofing membrane) that serve as protective layering. Please refer to our Systems Resource List for further information.

Is a permit required?
If you must alter your roof to construct an accessible space, the plans must be approved by the municipal Building Department.

I've heard that living roofs commonly leak (how do I prevent that)?
Green roofs require a waterproof membrane; fluid-applied membranes block water best, and single-ply membranes — overlapped sheets or tiles of impermeable synthetic material — also work, though the joints may be more susceptible to leaking. Protective layers can be added to preserve the waterproofing of the membrane.

Don't living roofs create a fire hazard?
Living roofs may actually reduce the spread of fires, particularly if the growing medium is irrigated. Dead, dry vegetation on the roof can constitute a fire hazard, so maintaining your living roof, especially during its initial year, is important (although the medium below the vegetation will not burn). Incorporating fire breaks like strips of gravel or pavement can mitigate fire concerns. (Source: Nigel Dunnett and Noell Kingsbury, Planting Green Roofs and Living Walls)

Return to Top

Solar Power

Is a solar roof right for me?
The ideal conditions for solar power are a south-facing pitched roof in an unshaded location, but other roofs can support solar panels. If you're unsure, solar professionals can assess your home's solar potential with a Solar Site Assessment. Please refer to the Systems Resources List for further information.
How expensive is a solar power system?
Although the initial cost of installing a solar system can be high, over time solar power is cheaper than buying solely from the grid. In the Bay Area in 2007, installing a medium (15-50kW) solar power system costs $8-10 per watt, while a smaller solar system will cost upwards of $9.50 per watt. The average residential project was 3.6kW, or about $34K before rebates and tax credits. (Source: Tapping the Potential of Urban Rooftops)

What are the available rebates?
The California Solar Initiative currently offers a rebate of $2.50 per watt of solar power installed. Financial incentives available for solar power can be found on specialized websites like the California Solar Center, Go Solar California, or the Northern California Solar Energy Association.

How long does a solar PV system last?
Solar panels typically have a power warranty of 25 years, meaning that in their 25th year, they are guaranteed to produce 80% or more of their original output. Inverters usually have a 10 year warranty but don't require replacement until their 15th year or later.

When does a solar power system pay itself off?
Solar panels pay off fastest for higher-tier energy customers because the solar-generated power can reduce their energy rates to a lower tier. To estimate your annual savings, multiply one year's expected solar-generated energy in kW by the electricity rate. The rate of inflation increases the speed of payoff because inflation will increase the rate that the power company pays you for your energy, but your cost of energy production will not increase. Solar power systems also increase property resale value. For more information, please read this document from OnGrid Solar.

What is net metering?
Net metering allows electricity users who have their own generators (solar panels in this case) to balance their energy use and bills with the energy they sell back to the grid. Utilities allow net metering for systems generating up to 1 MW. California limits net metering to 2.5% of a utility's service territory, and producers' net excess generation is carried forward to future bills for up to 12 months.

Why didn't the Rooftop Resource Project include solar thermal?
Bay Localize values the development of solar thermal. We recognize the efficiency benefits of solar thermal and support the implementation of this technology. At the time the project was initiated, we had a limited scope of what we could analyze, thus focused on the solar electric technology. We are currently promoting solar thermal through our Local Clean Energy Campaign.

How would I get started on a solar project for my roof?
To receive the California Solar Initiative rebate, your first step is to have an energy audit of your house to determine your current energy usage and potential efficiency improvements you could make. Then decide what size PV system your house needs and consider whether solar is right for you, which you can do by having a solar installer conduct a solar site assessment. A rule of thumb for determining the kW of PV your building would need for a net bill of $0 is to divide your average daily energy usage (in kW) by five.

(Source: How to Build Your Own Rainwater Catchment System - A Simple Cost Efficient Approach)

Isn't rainwater catchment expensive?
If your roof already has gutters and downspouts, you can set up a rainwater catchment system using reclaimed barrels or drums for free or as little as $100. (Using trash cans is not advised as they are not designed to withstand water pressure for extended periods of time.) The cost of storage containers increases along with the size — a 500 gallon cistern can cost between $225-$500, 1000 gallon polyethylene tanks retail for about $750 including shipping, while plastic tank prices will fluctuate along with the petroleum market. (Source: Tapping the Potential of Urban Rooftops) Tank costs can also be quantified by the gallon — polypropylene tanks range between $0.35-$1.00/gal. while metal tanks can cost between 0.40-$0.60/ gal. (Source: How to Build Your Own Rainwater Catchment System - A Simple Cost Efficient Approach) Currently there are no governmental incentives for rainwater catchment, but some water districts offer rebates.

Aren't some roofs toxic and detrimental as a catchment surface for rainwater?
A metal roof is the number one choice for collecting rainwater, but almost any material is fine. Roofs made from tar and gravel or asbestos are not ideal, as these materials can leave unwanted chemicals in collected water. There are no known toxins on rooftops that cause damage to landscapes. Acid rain doesn't occur in the Bay Area so is not an issue. Between rains, pollutants like dust and bird droppings can accumulate on rooftops, but this pollution can easily be removed from your water storage using a first flush diverter that drains away the first few gallons of rainwater from each storm event. Additional filtration systems, like a debris filter and a roof washer, can be added to remove organic materials.

How long can water sit before it goes bad, if it would be stored for a long period of time?
A proper cistern design can prevent water quality problems arising from aging. These problems can range from temperature increase, taste, odor or color changes, decay of disinfectants, formation of disinfectant byproducts, and bacteria regrowth. A cistern can be designed so that there is no stagnant backwaters where the water never turns over. In a tank, the inlet should be opposite the outlet. Abandoned runs of pipe should be capped at the beginning, not the end. Generally, water age is not a problem in well-designed, small systems if the input water is of good quality. The most common exception is rainwater harvesting or other systems that collect water with a high load of organic matter, traces of bird feces, etc. Don't collect this untreated water in a black tank in full summer and leave it to fester all summer. Prevent any potential algae/bacteria by storing the water in a (preferably shaded) cool, dark-colored, non-transparent cistern. Techniques to improve water quality in storage: "settling", deflocculation, ozonation, ultraviolet light, chlorination, and aeration. (Source: Art Ludwig, Water Storage)

Is rainwater catchment appropriate for our limited season of precipitation?
The common rule of thumb for rainwater catchment efficiency is: one thousand square feet of rooftop space can gather 600 gallons of water with only inch of precipitation. Our Neighborhood Assessment report projected that a thousand gallon cistern is an appropriate sized storage container and can store and refill water three times in one year, providing three thousand gallons of water throughout the year. Considering that Bay Area individual residential, outdoor water use averages around eight thousand gallons annually, rainwater catchment can be valuable as a supplemental water resource. (Source: Tapping the Potential of Urban Rooftops)

What kinds of tanks are best?
Space limitations are the primary constraint for water storage. Aside from this factor, all tanks sold in the market are appropriate. Efficiency-wise, the bigger the tank, the better. Tank options include reclaimed 55-gallon barrels, 500- or 1000-gallon polyethylene tanks, and plastic cisterns. Cisterns can also be constructed by hand, as a metal system, or ferrocement. These systems will be a cheaper alternative to commercial systems. A recommended book on this subject is Water Storage by Art Ludwig.

Aren't mosquitoes a problem?
Cisterns must be completely covered and sealed to avoid mosquito infestation and breeding. As long as cisterns are properly designed, placed and maintained, mosquitoes should not be an issue.

Is permitting required?
Permitting of rainwater catchment for landscape irrigation use is not required anywhere within the Bay Area.

Return to Top