Sustainable Design for the New Millenium (3)

Intensive Greenroofs

Intensive greenroofs look more like a traditional roof garden. They can

incorporate a much wider variety of plants (such as flowering shrubs, vegetables,

and even trees) because the substrate depths can be designed to be as deep as

the designed roof load will support. Depths start at about six inches up to 15 feet.

The main difference between a roof garden and an intensive greenroof is

that a greenroofing system is applied on top of the entire roof deck surface,

allowing unimpeded drainage and a more even weight distribution over the whole

roof.

Architectural accents -- such as waterfalls, ponds, seating areas, and the

like -- can be part of an intensive greenroof system. Such roofs can provide

recreation areas where people can interact with nature and with one another.

These systems can take advantage of otherwise forgotten (and usually ugly)

rooftop space by creating active areas for contemplation and play.

The Advantages of Greenroofs

Loss of greenspace and its inherent natural processes are by-products of

our modern "asphalt jungle." Plants and engineered soil atop a greenroof

enhance the environment through the natural processes of evapotranspiration

and photosynthesis, thereby ameliorating the surrounding ecosystem.

The specific benefits of (and market drivers for) greenroofs run the gamut

from easing environmental stress to creating an eco-friendly corporate image to

reestablishing endangered bird species. The following sections discuss the

advantages of greenroofs in more detail.

Environmental Benefits

Stormwater Management

Greenroofs reduce stormwater volume and slow down water flow, thus

helping to alleviate the pressure on stormwater infrastructure systems.

Many large, older U.S. cities (such as New York, Philadelphia, and San

Francisco) have combined sewer systems where wastewater from storm drains

and sewage pipes is intermingled. During heavy rains, runoff from impervious

surfaces such as rooftops and pavements can cause overflow in already overburdened

systems, resulting in contamination of lakes, rivers, and other

freshwater sources. Exhibit 2 shows the percentage of impervious cover that is

typical of various contemporary land use types.

Greenroofs capture and retain huge amounts of water that otherwise

would go down the storm drains, absorbing anywhere between 50 to 95 percent

of the rain that falls on site. Factors affecting retention rates include the intensity

of the storm, depth of media, and plant mass.

The intelligent use of best management practices (BMPs) includes

greenroofs that intercept and delay rainfall runoff and reduce the peak flow rate.

These practices can result in significant environmental improvements, as well as

long-term savings to building owners and municipalities.

Redrawn from Bruce Ferguson's "Introduction to Stormwater: Concept, Purpose, Design," 1998.

Water Quality Improvement

Greenroofs also filter and cool water runoff. They can help prevent

nitrogen, phosphorus, and toxins from entering streams and waterways. Heavy

metals and nutrients found in stormwater are bound in the engineered soil of the

greenroof instead of being discharged into groundwater or streams and rivers.

Greenroofs can remove over 95 percent of the cadmium, copper, and lead, and

16 percent of the zinc, from rainwater. They can also substantially reduce

nitrogen levels.7

Coastal cities such as Seattle and Portland, Oregon, have experienced

warming of the water in their rivers and bays resulting from discharge of heated

stormwater. This temperature change can greatly affect the health of cold water

fish populations, such as salmon.

Greenroofs can help counteract this effect. They act as a sponge,

absorbing the majority of rain that falls on site. The remaining water that does

eventually run off is filtered and cooled through evapotranspiration made possible

by the plants and engineered soil medium.

Heat Mitigation

In natural landscapes, vegetative canopy biomass greatly lowers air

temperatures. By contrast, the artificial and altered surfaces common in urban

land- and roofscapes greatly raises them. Average city rooftops can easily reach

150 to 175°F in the summer.

In urban areas, tightly sealed surfaces -- such as asphalt and concrete in

parking lots and on rooftops -- soak up heat during the day and then reradiate it

back into the Earth’s atmosphere after sunset as thermal infrared radiation.

This creates an urban "heat island" effect, with the heat that is released at

night forming a dome of higher temperatures over the city. The temperature in

downtown Atlanta, Georgia, for example, often is 10°F warmer than that of the

surrounding outlying areas. Urban heat islands contribute to our growing global

warming problem, and can also affect the local weather by creating unproductive

convective thunderstorms.

Used on a large scale, greenroof infrastructure could help reduce the

urban heat island effect by lowering ambient air temperatures. A 2002 study in

Toronto by Environment Canada estimated that urban temperatures could dip by

1 to 2°C if just six percent of the city’s roof tops were green.8

Chicago has adopted an energy conservation ordinance that includes an

urban heat island reduction provision. The ordinance, which became effective in

June 2002, includes minimum standards for solar reflectance and emissivity as

set by the International ASTM (formerly known as The American Society for

Testing and Materials). The ordinance requires all new and refurbished roofs to

install greenroofs or reflective roofing.

Air Quality Improvement

In urban downtown areas, ventilation is sometimes inhibited by tall

buildings, which reduce wind speed and trap heat in air pockets. Pollutants can

remain suspended for days.

Greenroofs can filter and bind dust particles, and naturally filter airborne

toxins. Smog, sulphur dioxide, carbon dioxide, and other pollutants are absorbed

and filtered through the foliage, naturally cleansing the air. Atmospheric dust is

held until rain washes it off into the greenroof soil substrate.

Greenroofs can also help mitigate the ozone problem in urban areas by

reducing the heat island effect, which contributes to ozone creation. In Atlanta,

the heat island effect doubles the amount of ozone that is produced.

Studies have shown that an increase in ozone levels adversely affects

sufferers of asthma and other breathing conditions. Increasing vegetated areas,

including greenroofs, can greatly improve air quality.9

Erosion and Sedimentation Control

Greenroofs can help protect watersheds and sewer systems. They act as

erosion barriers by reducing stormwater volumes, and assist in the control of

sediment transport and soil erosion. Plants and media properties (friction, root

absorption, and substrate matter) can prevent substances from entering a stream

corridor or other body of water.

Wildlife Habitat Conservation, Creation, and Restoration

Although greenroofs are not intended to be replacements for natural areas

located at ground level, they nevertheless can provide some habitat for wildlife.

In a landscape ecological context, greenroofs create an artificial or man-made

edge, while also serving as a vegetative habitat patch.

These greenroof patches, set within the matrix of a city, can accomplish

several ecological functions. If multiple greenroofs were grouped and designed

as vegetated corridors, some semblance of landscape connectivity could be

achieved.

Such corridors could offer respite for migrating birds and butterflies.

Studies show that birds will travel up to 19 stories, and butterflies up to 20 stories,

above ground in search of food and cover.

Even in densely populated areas, greenroofs can attract beneficial insects,

birds, bees, and butterflies. Such greenspace also can introduce or increase

biodiversity into a highly urbanized setting. In the UK and Switzerland, for