Green Roofs

A green roof is a vegetative layer grown on a rooftop of a building. Also known as roof gardens, these green roofs serve as a large contributor to energy efficiency of buildings.

green roof

Green roof (flickr: Arlington County)

Green roofs reduce the temperature of the building and the surrounding air in multiple ways:

    1. Shading: They provide shade to the top of a building
    2. Evapotranspiration: the plants in a green roof absorb water through their roots, and then use surrounding heat from the air to evaporate the water

While some roofs can reach temperatures of up to 90°F in the summertime, these two features can allow green roofs to actually be cooler than the surrounding air temperature! This in turn mitigates the urban heat island effect.

Green Roofs Today

Green roof production has grown significantly over the past decade; with a 115% increase in green roof production recorded from 2010 to 2011 alone. In 2012, the North American Green Roof Industry recorded 982 green roof projects, which was a 24% increase from 2011. Washington D.C. installed the most green roofs in North America this past year with 1,326,872 square feet. Succeeding Washington D.C. were Chicago, New York City, Toronto, Philadelphia and Cincinnati. Be sure to check out GRHC’s (Green Roofs for Healthier Cities) Annual Green Roof Industry Survey for 2012 for more current green roof statistics!

The Green Features of a Green Roof

Multiple layers work together to produce a green roof’s high efficiency and quality:

 

greenroof

  • Vegetation: The top layer of a green roof consists of vegetation. Chosen plants should be based on the type of roof, surrounding climate, and condition of the building. For flatter roofs and stronger buildings, taller and heavier vegetation can be used. Lighter and shorter vegetation is recommended for slanted roofs.
  • Growing Medium: The growing medium is the foundation for all the vegetation on the top later. The medium type, mixture and depth area all depend on the plants chosen for the green roof. A wind erosion layer, commonly made of burlap jute blankets, can also be installed to prevent wind from blowing top soil off the roof.
  • Root Barrier: Root barriers are associated with deep-rooted plants such as trees and shrubs. A root barrier prevents roots from reaching the membrane on the bottom layer, which when punctured, could potentially cause roof leaks and decay. Common materials used for this layer are foil or plastic.
  • Drainage: A drainage layer is installed to adequately remove excess water from a green roof. Green Roof Solutions states that drainage systems are designed to ensure storm water can be used by the top plants for extended periods of time without oversaturation of the entire green roof system. Note that this layer can also be found above the root barrier layer as well.
  • Insulation: The insulation layer is the protector of the membrane layer below. The insulation prevents the weight of the green roof from crushing the insulation and impairing the membrane. A light weight extensive green roof can include insulation below the membrane.
  • Membrane Protection: The roof’s membrane needs protection to prevent deterioration.  The protective layer can be a slab of lightweight concrete, insulation, thick plastic, copper foil or any combination of these.
  • Membrane:  A membrane is the bottommost layer of a green roof, and is the main separation between the green roof above and the structural supports below. Due to the excessive water amounts associated with green roofs, the membrane must be strong enough to support this water weight and to not let it seep through.

Types of Green Roofs

There are generally two types of green roofs that differ by cost, depth of growing medium, plant types and the potential for accessibility or use. Extensive green roofs are simpler, lighter weight systems commonly seen on residential buildings. The concept of extensive green roofs is to design a rugged, durable roof that requires little maintenance and no human interaction. Used vegetation consists of hardy, weather-resistant plants that can adapt to extreme climates. Intensive green roofs, on the other hand, more so resemble a traditional roof. There is almost no limit to the plants that can be used on these roofs, or the weight. Intensive roofs are commonly used for public areas, where people can walk and enjoy the area. Common differences between the two roof types are listed below:

 

Extensive Green Roofs

Intensive Green Roods

Common Roof Type

Residential, Slanted Roof

Commercial, Flat Roof

Planting Medium

1.6-6 in.

6+ in.

Manageable Weight

10-35 lb / ft2

150 lb / ft2

Used Vegetation

Drought-tolerant succulents, Grasses

Plants, Shrubs,
Gardens, Trees

Base Cost

$10 / ft2

$25 / ft2

Amount of Required Maintenance

Low

High

A green roof may include components of these two types of green roof systems and be referred to as either semi-intensive or semi-extensive green roof.

Benefits of Green Roofs

A few of the many benefits to green roofs are listed below. For more benefits, be sure to check out greenroofs.org!
  • Aesthetic Improvement: Aside from their sustainable aspects, green roofs can be a beautiful addition to any building, and can be designed to become a public area for building occupants and guests. Greenroofs.com states that these public areas can increase a sense of community and overall public safety!
  • Energy Reduction: The needs for heating and cooling a structure reduce drastically due to the thermal efficiency of green roofs.  Acting as natural shade and a heat-absorber for a building, a green roof can significantly reduce cooling costs by as much as 25%.
  • Increased Market ValueInstallation of a green roof has been shown to increase the marketability of a building and add a value of 6%-15%. Significant aesthetic effects accrue to the owner, occupants and the surrounding community.
  • LEED Certification Points: A green roof has the potential of earning up to 34 points for a building. Refer to this link for more information.
  • Moderate the Urban Heat Island Effect: The urban heat island effect is caused by specific areas (usually urban areas or cities) being hotter than nearby rural areas. This heat primarily comes from rooftops in urban areas since they are commonly painted black or other dark colors. Therefore, the installation of green roofs can lower the surrounding air temperature and help moderate the urban heat island effect.
  • Reduced Air Pollution: The reduced energy demand from green roofs reduces air pollution and greenhouse gas emissions related to energy production. Plants also remove air pollutants and greenhouse gas emissions through two processes: dry deposition and carbon sequestration and storage. Lastly, green roofs help slow the rate of ozone formation by reducing overall air temperatures.
  • Saved costs: Although installation costs are higher than traditional roofs, the lifespan and low-maintenance of a green roof can result in vast cost savings. Research from the University of Michigan showed that with initial roof costs of a green roof and a conventional roof relatively being $464,000 and $335,000, the green roof would actually save around $200,000 over its lifetime! People can also install green roofs on their own, cutting down the costs significantly!
  • Stormwater Management: Green roofs- depending on size, density, and type- can capture stormwater and return it to the atmosphere through transpiration and evaporation. According to greenroofs.org, 70-90% of precipitation can be retained in the summer time, and 25-40% in the winter time. Green roofs can also reduce stormwater runoff by holding water within the substrate and therefore delaying the runoff time. By having a green roof, buildings could also eliminate roof drains and other stormwater management infrastructure.
  • Financial Incentives: Be sure to check out GBA’s Green Building Incentives [CM4] packet for more information!
  • Urban Food Production: Green roofs provide the possibility of growing edible gardens on top of buildings!  Local and convenient, green roofs can source food just above occupants’ heads. Urban food production reduces the woes of long-distance food supply.  They also contribute to community nutrition and social interaction.

Related Links

Local Products

Green Roof Incentives

Featured Green Roof

Phipps Conservatory’s Center for Sustainable Landscapes, said to be one of the greenest buildings in the world, incorporated a green roof that not only insulates the building but offers a beautiful landscape for any event.  Read more here!