4 Things You Need to Know About Green Roofs
What is a vegetated green roof?
A green roof is a layer of vegetation planted over a drainage and waterproofing system installed on top of 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 is an indicator of 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).
What are the benefits of having a vegetated green roof?
Economic benefit:
- Energy-saving: though the installation of 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 (of 20,000 square feet). They found out over its estimated lifespan of 40 years, a green roof would save about $200,000 (solely from increased longevity, health savings, and stormwater tax credits gained).
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.
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. (As you know, the total embodied energy is considered from extraction-transportation-manufacture-construction.) Therefore, the longer the tops are used, the less average annual impact their embodied energy will have on our environment.
Requiring more maintenance:
Unlike conventional roofs, most vegetated roofs require minimum maintenance and irrigation. Even the building owner chooses sedums or xeriscape for their vegetation material, it is still needed to monitor or replace the trays periodically.
Potential of leaking:
Though there are drainage, insulation, and waterproofing layers underneath the vegetation, it is likely plants’ roots still burrowing into the structure during the life span.
Prone to wind-lifting:
If there is too much air pressure 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.
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 and 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.
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