Planted roofs reduce building energy use by cooling roofs and providing shading, thermal mass and insulation.

Building-Level Energy Reduction

The thickness, or thermal mass, of a planted roof provides building insulation, retains heat, and reduces energy loads, while also acting as a barrier protecting the roof membrane from exposure. When internal building temperatures rise (server rooms, lighting, people, etc.), a planted roof transfers excess heat through its membrane, media, and plants where it is cooled through modes of evaporation, conduction, and convection. See Living Architecture: Green Roofs for Public Buildings for more information.

Peak Energy Demand Reduction

Planted roofs take longer to heat up and cool than conventional roofs because of their higher heat capacity (thermal mass). The membrane of a planted roof reaches its peak temperature several hours after that of a conventional roof. This “thermal lag” leads to a reduced mid-day peak heat gain as well as to warmer nighttime temperatures for planted roofs, which can benefit buildings occupied 24-hours a day.

At a municipal level, there is potential to avoid significant energy production at power plants due to this thermal lag. This in turn conserves water (used to create power), minimizes the creation of ground-level ozone, and ultimately reduces city-wide energy dependence.

An energy modeling study on peak summertime cooling demands and CO2 reductions at the city level was performed by Ryerson University. They calculated the impacts of urban heat islands and smog reductions using the city of Los Angeles, California, and determined that the city of L.A. could realize a savings of $100 million a year by turning 75% of its eligible rooftops into planted roofs.

Source: Living Architecture: Green Roofs for Public Buildings

The vegetation on a planted roof removes fine particulates (PM), black carbon, and pollen out of the air, which lends well to incorporating solar panels within a planted roof’s design.  By removing particulates out of the air, planted roofs can reduce maintenance cleaning requirements of PV equipment and enable the solar cells to absorb more light and operate more effectively.

Design Considerations:

• Plan to incorporate solar panels in the early in the design.
• Only install PV over extensive, shade-friendly vegetation.
• Shade provided by panels may promote biodiversity for shade-loving species.

Source: Living Architecture: Green Roofs for Public Buildings

Reduced temperature fluctuations reduce thermal stress.  Benefits of a planted roof are realized through the extended life of the roof membrane, nearby equipment, and its related systems.  

A conventional roof lasts 15-20 years, while a planted roof typically lasts 40+. The extended roof life means that a significantly higher return on investment can be expected. As more planted roofs are installed, reroofing activities will become less frequent. This results in saved money, time, and resources (through funding requests, contracting, design, oversight, etc.). Re-roofing half as often also saves “embodied energy.” This includes diversion of waste and reduction in all carbon forming activities involved in the re-roofing process such as manufacture, transportation, installation and disposal of materials.

Source: Living Architecture: Green Roofs for Public Buildings

Rooftop gardens and farms have been recognized as a promising form of urban agriculture, and a way to take advantage of a significant amount of flat space that receives steady sunlight throughout the day.

Using rooftop space for food production can:

• Help reduce the distance food travels to reach consumers.
• Reduce carbon emissions associated with food distribution.
• Provide fresh and local food options to building occupants and the local population.
• Provide an outlet to educate the local community about food production and seasonal variety.
• Boost property values through the addition of a new building service.
• Help create jobs.

Urban agriculture can appear in a variety of forms, such as container gardens, hydroponics, aquaponics, vertical farming, multi-tiered farming, technologies, apiculture, and rooftop gardens. This last form of urban agriculture, rooftop gardens, is one that can utilize available space over a somewhat limited environment.

Be sure to consider:

• Local zoning regulations and policies.
• The load-bearing capacity of existing roof infrastructure.
• Farming labor requirements, security and medium depths when selecting crops.
• Roof access concerns, such as safety.

Source: The Benefits and Challenges of Green Roofs on Public and Commercial Buildings