Most Green Roofs are designed not as outdoor spaces for people but as green spaces that provide multiple benefits of stormwater control, natural cooling, and wildlife habitat.
Green roofs today are commonly described as either low-profile (extensive) or high-profile (intensive). Low-profile green roofs include only 2” to 6” (50–150 mm) of planting media and support short ground-cover vegetation, such as sedums, grasses, mosses, and prairie mixes. High-profile green roofs have much more soil—sometimes up to several feet—and can support a wider variety of tall grasses, meadow plants, shrubs, and even small trees. Structural requirements for high-profile green roofs are much more significant than those for low-profile systems.
Benefits of “greening” the VBA roof…
Stormwater control.
Stormwater runoff is a significant problem in urban and suburban areas. Where stormwater runoff from hard surfaces, such as city streets, parking lots, and roofs, flows directly into nearby rivers and lakes, it can cause downstream flooding and introduce pollutants into those bodies of water
Part of the key to reducing stormwater flows is to detain it (hold it back). Detention basins are the usual means of doing this, but green roofs also detain stormwater. A green roof will absorb rainfall for quite a while (depending on moisture level in the soil and the rainfall intensity) before allowing rainfall to just flow off. Preliminary results from testing of a 4”-thick (100 mm) green roof in Portland, Oregon indicate that it can absorb a full inch of rainfall during a summer rain event (when the soil starts out fairly dry) before any water runs off of it.
Stormwater Runoff Comparison:
Pollutant Removal.
In cities, rainwater is often contaminated with a wide range of airborne pollutants: from diesel soot and other particulates, to hydrocarbons, VOCs, pesticides, and heavy metals. These pollutants typically settle onto rooftops to be washed into our waterways. A green roof can filter particulates out of the rainwater falling on it. Microorganisms living in association with the vegetation can break down many types of pollutants; other pollutants, such as heavy metals, will bind to soil particles.
Reducing cooling loads
Unlike the vast majority of low-slope roofs, which get extremely hot—up to 175°F (79°C) for black roof surfaces on a sunny day—vegetated surfaces are kept shaded and cool.
While the bare roof can reach temperatures as high as 90°F (32°C), the temperature beneath the planting media on the green roof would not rise higher than 60°F (16°C). With a lower temperature differential across the roof system, cooling loads in the building will be lower—how much lower depends on the levels of insulation there is in the roof system. A December 30, 2000 article from Environmental News Network noted that a 3–7°F (2–4°C) temperature drop can reduce air conditioning requirements by 10%; therefore a green roof could potentially reduce the cooling costs for a single-story building by 20–30%.
Improving outdoor air quality.
Plants filter dust and other pollutants out of the air. Green Roofs for Healthy Cites reports on their Web site that 1,000 ft 2 of green roof can remove approximately 41 pounds of airborne particles per year (200 g/m2). Plants also absorb carbon dioxide and emit oxygen through the process of photosynthesis.
Providing wildlife habitat:
In urban areas, habitat for songbirds, butterflies, and other wildlife can be in very short supply. Even small oases of green in a sea of asphalt and concrete can provide vitally important islands of habitat, enabling wildlife to migrate through or take up residence in our cities. Planted with native vegetation, green roofs could become an important component of habitat-improvement efforts. Experience has shown that songbirds and butterflies will make their way to green roofs even many stories above ground level.
Visual and aesthetic benefits.
Green roofs offer the potential for people to enjoy the outdoors—whether actually outside on a roof, or merely gazing out an office window onto a section of planted roof (perhaps watching a butterfly flit from flower to flower). This sort of experience can help us relax; it may also make us more likely to work to protect our wild heritage. Some scientists believe that humans have an inborn affinity for other forms of life—the so-called biophilia hypothesis, first put forth by Harvard biologist Edward O. Wilson. Biophilia, Wilson argues, can lay the groundwork for an enduring conservation ethic—through which we (humans) will want to protect nature. A corollary to this hypothesis is that helping people see and appreciate nature will increase the likelihood that those people will work to protect their environment.
Enhanced roof membrane durability.
Installing other materials over the roof membrane will protect it from exposure to ultraviolet light, extreme temperature fluctuation, hail, falling branches, and other natural forces that can damage or degrade waterproofing membranes. Green roof component manufacturers and designers claim that a green roof will double or even triple the life of a roof membrane. Manufacturer of the roof membrane used for the green roof at Chicago City Hall, extended the warranty for its PVC membrane by 50% (from 10 years to 15 years) simply because of the green roof.
The Components of the VBA Green Roof
The Green roof system installed on the VBA is made up of the following components: (in order from Roof Deck to Top Soil)
A.) Roof membrane:
Arguably one of the most important components of any roof system is the waterproof membrane that keeps rain out of the building. In this case the membrane is adhered directly to the roof deck (concrete structure) after the concrete is primed (Soprema ElastocolTM) and expansion and isolation joints prepared with Sopra JointsTM. The membrane itself is a 2-ply torch applied Modified Bitumen system.
….Flashing and Drainage:
The “humble” flashings are undeniably the most critical component of the roofing system. Directing Water towards drains and protecting every projection and material interface, flashings may account for a small percentage of the overall project cost….however 75 to 80% of all low-slope roof failures are due to poor flashing and water management details.
The flashings used in the VBA roof system are 24 gauge, heavily galvanized metal flashings. These flashings are fully integrated into the membrane system.
B.) Protection Board:
This 1/8” thick sheet provides a separation between the stationary, adhered membrane and the drainage and insulation layers above. It also provides an air-gap thus neutralizing any potential hydro-static head (water-pressure) up against the membrane.
C.) Drainage/retention layer:
The sub-surface drainage layer installed within the VBA roof is SopradrainTM which includes the use of filter cloth. Drainage is needed in a green roof to remove excess rainwater and prevent saturation of soil, which could suffocate plant roots. The drainage layer can also be designed to retain some water, as is done in a number of the European systems.
The geo-textile filter-fabric is in place to exclude soil and silt. A special aeration layer may be included between the insulation and drainage/retention layer to enable the insulation to dry.
D.) Protection Board:
In this location (between the drainage layer and insulation) the 1/8” board protects the flexible drainage and filter fabric from the constant movement of the rigid insulation board which has a very high coefficient of thermal expansion and contraction.
E.) Insulation.
While not really a true part of a green roof, adequate roof insulation is a key component of any environmentally responsible building, and how that insulation interacts with the other green roof components is very important.
The VBA roof system places the insulation ABOVE the membrane thus protecting the membrane from UV exposure and potential traffic and weather related damage. This is commonly known as a “Protected Membrane” or “Inverted Roof” system. 4” of rigid and highly compressive extruded Polystyrene insulation provides an additional R20 Effective insulation value.
F.) Root barrier:
Because roots of some plants can penetrate bituminous roofing, a green roof relying on built-up roofing must include a separate root barrier above the roof membrane. MicrofabTM root barrier sheeting has been used within the VBA roof system. This enables to planting of heavier, “intensive” vegetation including small trees and shrubs.
G.) Planting media (soil):
To support plantings, a green roof needs to include some sort of soil. To work effectively in the harsh and highly variable environment of a roof (strong sun, wind, high temperatures, and rapidly changing moisture levels), the soil generally has to include highly absorptive, yet lightweight components.
Also contributing to the soils effectiveness is the use of specified fertilizing agents that will NOT contribute to the degradation of the roofing components or integrated drains or flashings.
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