what is a green roof?

A green roof is an assembly of materials, including plants, that is added to a traditional roof system. Also known as “vegetated roofs”, green roofs are an energy-efficient, ecologically-sensitive upgrade to the typical roof finishes of tar, gravel, asphalt and plastic membranes, usually seen on urban rooftops. Most commonly applied to flat or shallow-sloped roofs, green roofs work with many building types, including commercial, multifamily and industrial buildings; as well as single-family dwellings, rowhouses and garages.

Green roofs are not new. Roofs with vegetated surfaces have been in use for centuries as an effective thermal insulator and watertight building technology. In the 20th Century, modern green roofs were made possible by advances in membrane and waterproofing materials, and in horticulture.  Are you Green Roof Ready ?


why choose a green roof?

Green roofs deliver multiple benefits in all three spheres of sustainability – environmental, social and economic.

environmental benefits

  • reduces heating in buildings by adding thermal mass and resistance to the roof membrane; and reduces heat loss and energy use in winter conditions
  • reduces cooling loads on a building by 50 – 90%; again reducing cooling loss and energy use in summer months
  • dramatically reduces the temperature fluctuations on the rooftop daily, seasonally and yearly:
    • in the summertime, lower ambient air temperature means HVAC equipment is operating less and with greater efficiency
    • in urban areas,a concentration of green roofs can reduce the city’s average temperatures during the summer; known as, the Heat Island Effect, cities are hotter than surrounding areas due to impervious surfaces, such as rooftops, streets, paved parking lots and playgrounds
  • reduces stormwater (rainwater)runoff:
    • rainwater is stored by the media, absorbed by the plants, then returned to the atmosphere; in summer 70-90%, is retained; in winter between 25-40%
    • green roofs also moderate the temperature of the water that happens to run off, improving water quality for marine life
    • green roofs delay the time that runoff occurs, reducing the number and volume of overflows in combined sewer systems (CSOs)
    • green roofs slow the velocity of direct runoff, reducing erosion of stream and river banks, and the sedimentation and loss of habitat that result
  • green roofs reduce noxious and volatile organic compounds (VOCs) from off gassing into the atmosphere by covering asphalt, plastic and other membrane roof surfaces
  • creates natural habitat and promotes biodiversity, including increasing the population of pollinators in cities
  • filters pollutants and carbon dioxide (CO2)out of the air; improving air quality which helps lower rates of diseases, such as asthma; sequestering CO2, reduces the pace of global warming and climate change
  • filters pollutants and heavy metals out of rainwater, keeping waterways and marine life healthy and abundant
  • serves as a sound dampener, insulating a building for noise reduction; the soil blocks lower frequencies and theplants block higher frequencies
  • reduce the space used for landfills; green roofs protect the roof membrane from damage and wear, extending the life of the membrane 2 to 3x, lowering the volume of materials deposited in landfills
  • contribute to achieving several points in multiple LEED credit categories; including Sustainable Sites, Water Efficiency, Material Resources, Environmental Quality and Innovation in Design
  • offer agricultural space, especially in urban areas; contributing to accessible and locally grown produce

social benefits

  • aesthetic value:
    • improves morale and productivity; one study for schools described results of higher grades, as well as improved attendance
    • more visually appealing than tar, gravel or plastic membrane roofs, either black or white, especially viewed from above
  • amenity value:
    • desire for more green space in cities as urbanization increases
    • accessible green roofs provide green space above the dim of city streets
    • increase marketability of properties; buildings with green roofs command higher lease rates and selling prices
  • therapeutic value:
    • horticultural therapy has shown improvement of health among aging populations and those with health issues
    • faster recovery rates for hospital patients with more natural surroundings
    • overall wellbeing is markedly improved with greenery and natural vista
  • urban agriculture
    • increases food security and reduces food miles
    • fruit and produce availability in urban ‘food deserts’
    • opportunity for community building and empowerment
    • locally-sourced food for restaurant and hospitality industry
  • recreation value:
    • rooftop terraces for residents and workers
    • space for playgrounds for urban schools and childcare facilities
  • job creation:
    • growth of the green roof market gives new job opportunities in manufacturing, plant growth, design, installation, and maintenance; according to American Rivers, a$10B investment could create 190,000 jobs by building 48.5 billion-square-feet of green roof area, or just one percent of the United States’ roof space in every community over 50,000 in populationjobs and career opportunities would encompass a wide range of disciplines at all level of skill
  • educational value:
    • provide an easily accessible sight to teach the sciences, from biology and chemistry, to engineering and technology

how to save with a green roof

A recent study commissioned by the General Services Administration, determined that the Return on Investment (ROI) for a green roof on commercial office buildings is 6.5 years.  Green roofs pay for themselves through energy cost savings and, in the District of Columbia, by offsetting stormwater management fees and charges.

local incentive programs

In the District of Columbia, the District Department of the Environment (DDOE) green roof subsidy is $7 per square foot- and up to $10 per square foot in targeted subwatersheds .  There is also a structural engineering rebate available for roofs smaller than 2,500 square ft.   The program is administered through Anacostia Watershed Society, a local nonprofit organization. Owners of residential or commercial buildings may register and apply for a green roof project of any size. DDOE will fund qualifying projects on a first-come, first-served basis.

In Montgomery County, Maryland,the RainScapes Rewards Rebate Program offers a rebate to homeowners of up to $1,200.00; 300 square feet or ¼ of roof- retrofit. Commercial property owners can receive up to $5,000; 300 square feet or ¼ of roof- retrofit.

Contact Andrew Benenati, GRP, Green Roof Project Manager, if you have questions and would like to take advantage of these incentives

energy cost savings

Green roofs save energy, and that means they save energy costs. There are real savings (10-30%) for homeowners, commercial building owners, the agents who manage properties; and in urban communities, there are savings for local governments that maintain the built infrastructure.

  • reduces heating demand in colder seasons, and cooling demand in warmer weather
  • dramatically reduces the temperature fluctuations on the rooftop daily, seasonally and yearly:
    • lower air temperatures near the roof surface mean HVAC equipment is operating less and with greater efficiency, saving on maintenance and replacement costs
    • in fact, green roofs on neighboring buildings in densely-built communities contribute in a small way to reducing one’s own energy bills, especially in warm summer weather; this is known asthe Heat Island Effect
  • cites are hotter than surrounding areas due to impervious surfaces, such as rooftops, streets, paved parking lots and playgrounds

More and more cities are enacting legislation and setting policies for reducing stormwater runoff. The volume, as well as the trash and pollutants carried in runoff are enormous and damaging to our waterways and ecosystems. Federal and state mandates, court orders, and the soaring costs of remediation are forcing municipalities and utilities to charge fees to property owners and rate payers for the costs of restoring the damage and of implementing new gray and green infrastructure technologies.

stormwater management costs

Green roofs capture rainwater by storing it in the growing media; over time it is absorbed by the plants, and a portion is returned to the atmosphere by evaporation and transpiration; in summer 70-90%, is retained; in winter between 25-40% is retained

In the District, DC Water, the local utility, is charging every rate payer a monthly fee known as Impervious Surface Area Charge (IAC) to offset the costs of managing stormwater in the city where a single set of pipes conducts both sewage and stormwater together to be treated at the Blue Plains facility. This Combined Sewer System or CSS was built in the 19th and early 20th centuries. Under a Consent Order from the courts, DC Water is required to substantially reduce the volume of stormwater runoff in the CSS area of the city. The remediation solution chosen is to construct miles of concrete tunnels below grade and a large concrete catch basin. The current cost estimate is over $2 billion.

For FY 2012, the monthly fee for the IAC to every homeowner with a property between 700 & 2000 square feet is $6.64. This area range is counted as one (1) Equivalent Residential Unit (ERU). For larger properties, the monthly fee is considerably higher. The rate nearly doubled from the previous year, and is expected to be nearly $30/month/ERU in 2019.

The Consent Order requires that DC Water create a discount program for ratepayers with green infrastructure technologies operating on their properties. The program is in development, and the expectation is that up to a 70% discount will be available for ratepayers.

Also on the DC Water bill is a Stormwater Fee charged by The District Department of the Environment (DDOE). This fee is to partially pay for costs of reducing runoff of stormwater in the other 2/3s of the city where a newer system is installed. The Municipal Separate Storm Sewer System (MS4) is a system of two sets of pipes; one for sewage, the other for stormwater, Sewage is carried to and treated at the Blue Plains facility. Stormwater is carried to and released directly into our waterways; the Anacostia and Potomac Rivers, Rock Creek, and their many tributaries.

DDOE is required by an EPA Permit to reduce the volume of stormwater entering the waterways. The fee for one (1) ERU is currently $2.67. Similarly, DDOE is required to create a discount program for properties with green roofs, rain gardens or rain barrels to offset the fee.

Low-income residents may qualify for assistance with their water bill through DC Waters S.P.L.A.S.H. program administered by the Greater Washington Urban League and the Customer Assistance Program administered by the DDOE Energy Office. For more information about S.P.L.A.S.H., contact the Greater Washington Urban League at (202) 265-8200. For more information about the Customer Assistance Program, contact the DDOE Energy Office at (202) 673-6700.

additional cost savings

Green roofs extend the life of the roof membrane 2 to 3x, or 30 to 60 years; this is a huge savings on roof replacement costs that typically run $10-30 per square foot, depending on size, for each replacement