Green Roof

Green (or vegetated) roofs are covered with various native or adapted plants. The benefits of a green roof include increased stormwater protection, extended roof lifespan, lowered building heating and/or cooling costs, increased biodiversity, noise attenuation and heat island reduction. Green roofs last 2.5 to 3.5 times longer than traditional roofs.
Photo credit: Greensulate

Reduced Heat Island Effect

Urban areas are often warmer than surrounding rural areas because of the prevalence of dark-colored hardscape, such as concrete or asphalt sidewalks, roofs, and other man-made surfaces.

Such surfaces absorb heat and radiate warmth, which increases the temperature of the surrounding area. In fact, on a sunny day, black surfaces may become 70 degrees F warmer than white or reflective (high albedo) surfaces. The "heat island effect" is the absorption of heat by these surfaces and the subsequent heat radiation to surrounding areas, which results in a higher than normal average temperature.

Just as rural areas benefit from a greater percentage of grass and other vegetation, which absorbs sunlight and does not radiate the heat, green roofs in urban areas can have the same positive impact, reducing the heat island effect due to their vegetation which absorbs sunlight for photosynthesis.

Because vegetation does not radiate much heat, green roofs can also reduce temperature fluctuations in buildings. Traditional roofs, such as those with asphalt tiles or shingles, can absorb up to 83% of the sun’s heat and UV rays. When a traditional building roof absorbs heat, the heat radiates inside the building causing temperature fluctuations and forcing the building’s HVAC systems to work harder to heat and/or cool the building.

Stormwater Protection and Native Vegetation

In urban areas, heavy rains may cause sewer overflow due to increased stormwater runoff. Green roofs can help prevent rainwater from collecting on asphalt or concrete where it typically picks up and transports pollutants and suspended solids to surface waters through municipal sewer systems.

Studies show that green roofs retain 50-60% of stormwater runoff, on average. In addition to reducing stormwater quantities, vegetative roofs can improve stormwater quality by naturally absorbing and filtering rainwater.

A green roof may have additional environmental benefits if it contains native or adapted vegetation. Native or adapted plants, according to the United States Green Building Council (USGBC), the developer of the LEED rating systems, consist of “plants indigenous to a locality or cultivars of native plants that are adapted to the local climate and are not considered invasive species or noxious weeds”. Such plants promote biodiversity and provide habitat, while also reducing the amount of water and fertilizer required for healthy growth.

Green Roofs Improve Solar Panel Performance

Another benefit of green roofs is that they have been shown to increase solar panel productivity. Two recent studies have proven this hypothesis, indicating that vegetation lowers a roof’s ambient temperature, which boosts solar photovoltaic panel output.

A study in Berlin found that solar PV panels installed on green roofs produced 6% more energy, on average, than those on (conventional) bitumen roofs. A similar study from the Bronx Design and Construction Academy also supported these findings. However, the Bronx study found that solar PV panels mounted on vegetative roofs improved solar photovoltaic performance by only 3%. Regardless of the amount of the increase, the incremental performance of PV systems on green roofs reduces the payback period for solar photovoltaic installations. Also, since green roofs reduce heating and cooling loads, the solar panels will need to produce less energy to meet the building’s needs.

Free LEED GA Practice ExamDisadvantages of Green Roofs

While green roofs have many environmental benefits, they also have disadvantages. Typically, green roofs are more expensive to install than conventional roofs and could have higher maintenance costs. Also, a vegetative roof is typically not a do it yourself project. Green roof professionals, or an experienced roof vendor (and/or installer) with a track record and knowledge of green roofs should be consulted.

Leaks are atypical with green roofs, however troubleshooting and fixing a leak on a green roof may be more difficult and labor intensive than with a conventional roof.

Green Roofs and LEED

Using a green roof can help to earn points toward various LEED credits, including:

Sustainable Sites (SS) Credit 5.1, Protect or Restore Habitat
SS Credit 5.2, Maximize Open Space (if the project achieved SS Credit 2, Development Density & Community Connectivity)
SS Credit 6.1, Stormwater Design - Quantity Control
SS Credit 6.2, Stormwater Design - Quality Control
SS Credit 7.2, Heat Island Effect –  Roof

Water Efficiency (WE) Credit 1, Water Efficient Landscaping

Energy and Atmosphere (EA) Prerequisite 2, Minimum Energy Performance)
EA Credit 1, Optimize Energy Performance)

Green Roof Cost

Green roofs can range from $15 to $35 per square foot, including the cost of materials and labor. Green roofs may also cost more to maintain over time, depending on the size and accessibility of the roof, in terms of landscaping maintenance.

While more expensive than their conventional counterparts, green roofs can pay for themselves over time because they reduce energy costs. Indeed, in some buildings, green roofs have been shown to reduce annual energy demand by over 50%. Studies by both the Heat Island Group and the Florida Solar Energy Center showed that green roofs used up to 40% less energy for cooling than buildings with conventional dark roofs. Also, because green roofs reduce heating and cooling loads, HVAC equipment in a building may be smaller, which can save money for a developer when planning ahead.

Comparatively, cool roofs, another green alternative to conventional roofs, are less expensive (though completely different aesthetically) and vary from $0.75 per square foot for reflective, acrylic paints to over $3 per square foot for PVC single-ply membranes. Because of their reduced up-front cost, a cool roof may pay for itself in 2-3 years, unlike a green roof.

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AvatarJohn Tomlinson's picture
Environmental Consultant
Westminster, CO
AvatarVance Barnes's picture
LEED Professional
Germantown, WI
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Engineer
Richmond Hill

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