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1. What is a vegetated green roof?
A green roof is a layer of vegetation planted over a drainage and waterproofing system installed on a flat or slope roof. The primary purposes for installing green roofs are energy efficiency, stormwater control, and heat island reduction.
There are three main types of vegetated roofs: extensive, intensive, and semi-intensive. The main differences between these types are based on their growing medium. An extensive green roof has a shallow growing medium—usually less than six inches. Intensive green roofs have more soil and a deeper growing medium—sometimes several feet.
The availability of activities on the vegetated roof indicates whether it is extensive or intensive. If you find a paving area, seating, or tree canopies, you can tell you are on an intensive vegetated roof. (see the illustration below for the comparison of various green roofs).
Image by Hazel
2. What are the benefits of having a vegetated green roof?
Economic benefit:
Energy-saving: Installing vegetated roofs usually involves higher upfront costs than a traditional roof. The energy saved after installing the vegetated roof could offset those costs.
Increased market value: The lush and aesthetic view provided by the green roof also improves property values and marketability.
Tax benefit: Several federal policies provide tax incentives for installing vegetated roofs in the project. Such as:
Clean Energy Stimulus and Investment Assurance Act: owners could get a Tax credit for 30% of the green roof cost.
Energy Policy Act of 2005: owners could get a Tax credit of up to $18 per square foot for SHRAE 90.1 projects.
US General Service Agency: Requires LEED Gold for new buildings
US Environmental Protection Agency: Stormwater regulation.
A study done by the University of Michigan evaluated the cost benefits of vegetated roofs (20,000 square feet). They found out that over its estimated 40 years, a green roof would save about $200,000 (solely from increased longevity, health savings, and stormwater tax credits gained). (1)
Energy benefit:
Providing proper insulation: The growing medium adds an extra layer on the top of traditional roof insulation. Therefore, it increases the R-value.
Providing evaporation cooling for the buildings: The increased R-value on the roofing system will reduce the roof temperature and lessen the HVAC loads, reducing energy consumption. This strategy will contribute to the overall heat island effect.
Help retain water and manage stormwater runoff: A green roof keeps moisture in the plants and growing medium, thus slowing and reducing the amount of stormwater entering the storm drain, reducing flooding and erosion.
3. What are the primary concerns of a vegetated green roof?
The cost or the extra embodied energy: Vegetated green roofs take twice as much time to build as conventional roofs. Therefore, the longer the tops are used, the less average annual impact their embodied energy will have on our environment. As you know, the total embodied energy is considered from extraction-transportation-manufacture-construction.
Requiring more maintenance: Unlike conventional roofs, most vegetated roofs require minimum maintenance and irrigation. Even if the building owner chooses sedums or xeriscape for their vegetation material, monitoring or replacing the trays is still needed periodically.
Potential of leaking: Though there are drainage, insulation, and waterproofing layers underneath the vegetation, plants’ roots are likely burrowing into the structure during the life span.
Prone to wind-lifting: If too much air pressure is caught underneath the green roof when the wind blows overhead, it may cause a pressure shift, pulling the greenery off the top.
Prone to fire hazard: While green roofs provide fire retardation for your building, there is still a risk of the greenery catching fire due to natural events in your area.
4. What are the industry design standards for a vegetated green roof?
There are guidelines approved by the American National Standards Institute (ANSI) that has worked in close partnership with SPRI and have been developed for Fire, Wind Uplift, and Root Repellency.
Wind Uplift: ANSI RP-14 Wind Design Standard for Vegetable Roofing Systems was created to guide and protect buyers from uplifting wind issues. This ANSI RP-14 Wind Design Standard provides design and installation references for professionals.
Fire Hazard: This potential disaster caused by fire can be minimized by implementing ANSI VF-1 Fire Design Standard for Vegetative Roofs.
Root Repellency: Regarding the roots delving into the roof of the building, tearing up the material, and compromising the overall structure. ANSI VR-1 is the standard for testing the resistance of vegetative roof coverings to normal root and rhizome penetration.
That is not a typical green roof in the industry. But it is probably one of the oldest “green roofs “ in our habitation history.
Notes and References:
See Page 13, Figure B on “Green Roofing at the University of Michigan Final Report“. Environ 391: Sustainability and the Campus, Dr. Mike Shriberg December 12,2012
Need a broader view on how green roofs impact the natural environment around us? Please check Technical Preservation Service of Sustainability by National Park Service.
Building Industry’s technical standards update from time to time. Check out this page ensure the current ANSI/SPRI standard for yout projects.
Fist published: 7.8.2021, updates: 1.30.2023