Green Roofs

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The City Hall and Police Building Green Roof Project

City Hall Green Roof May 2015 Image 1City Hall Green Roof May 2015 Image 3City Hall Green Roof May 2015 Image 4City Hall Green Roof May 2015 Image 2

What is a Green Roof?

A green roof consists of vegetation and soil, or a growing medium, planted over a waterproofing membrane. Additional layers, such as a root barrier and drainage and irrigation systems may also be included.

Roof of City Hall prior to work commencing on the green roof project
The picture above shows the old City Hall roof before work began.     The picture above shows the replacement of the roof membrane on City Hall.  Picture taken 5/5/2008.                                 

The image above shows the progress of the green roof of the Police Building on 19th May, 2008.  The planting is now well underway in this image.  This photo was taken from the top of the parking garage adjacent to the Downtown Mall.
Final City Hall Green Roof - Over 18,000 plants have been planted and the roof is now complete.  The plants will take a while to become fully established, but immediately the roof will help reduce stormwater runoff and help to reduce cooling costs within the building for the summer months.  Picture taken 30 May 2008.  You can get a view of the whole roof from the top floor of the Market Street Parking Garage.
Green Roof 6/09
One Year Later - this photo was taken just one year later, on June 23, 2009.  The plants have grown in and are blanketing the roof with color.

The term "green roof" does not just refer to roofs which support vegetation. The term may also be used to describe roofs that utilize some form of "green" technology, such as solar panels. Green roofs are also referred to as eco-roofs, vegetated roofs, living roofs, and green roofs.

In Europe, they are widely used for their stormwater management and energy savings potential, as well as their aesthetic benefits.  The technology isn't entirely new - many early American settlers used sod to insulate their walls and prairie grass to cover their roofs.

What is the difference between a green roof and a roof garden?

A roof garden usually consists of containerized plantings of various sizes placed on top of a roof. On a green roof, all the various layers are applied on top of the entire roof deck surface, allowing unimpeded drainage and more even weight distribution.  The vegetation is planted directly into the soil, not in planters or containers.

How Do They Work?

Green roofs control storm water runoff by retaining rainwater in the vegetated layer.  The solar energy that evaporates water from the soil and vegetation is heat that never reaches the building, thus reducing the building's cooling load.  The protection from ultraviolet radiation afforded by the vegetated layer is expected to extend the life of the roof membrane by many years, possibly even doubling the life of the roof membrane.

There are two types of green roofs: intensive and extensive.  Generally speaking, "extensive" green roofs contain soil that is less than 6 inches deep.  These roofs can sustain a range of plants - such as sedums, herbs, meadow grasses, and perennials.  The choice of plant depends on the local climate, and what is native to that area.  Deeper "intensive" green roof systems can sustain complex landscapes - including small trees, even ponds and fountains.  As a result, buildings crowned with "intensive" green roofs have to support much more weight. 

The Role of Plants

Through the daily dew and evaporation cycle, plants are able to cool cities during hot summer months. 

During the process of evapotranspiration, plants effectively "breathe" or transpire water which is evaporated by the heat gained from solar energy.  They also use sunlight energy to convert carbon dioxide into food for themselves through photosynthesis.    The plants' waste product is oxygen.

One m2 (10.76 ft2) of foliage coverage can evaporate over 700 litres of water in a year!

Extensive green roofs require minimal maintenance, especially after the plants reach maturity, normally after 2 years.  Green roof plants are relatively drought resistant and typically do not need irrigating.  Instead of using just soil, green roofs usually use a specific growing medium, usually a lightweight mineral material.  For extensive green roofs, this generally contains about 70% shale, 20% sand and 10% organic compost.  The plants get most of their nutrients from this medium, rainwater and air, so these roof systems usually do not require any fertilizer.  

The Benefits of a Green Roof


As impermeable surfaces like buildings and pavement replace open space and vegetation, green roofs can play an increasingly important role in storm water management. During rainstorms, green roofs act as a sponge, absorbing much of the water that would otherwise run off.  Green roofs can reduce the demand on sewer systems by retaining as much as 50 - 70% of annual rainfall precipitation on the roof.1

Green roofs also filter pollution from rainwater. This is achieved by the natural filtering processes of the bacteria and fungi within the plant root systems. As a result, the non-point source pollutants, nitrogen and phosphorus, are broken down and detoxified.  Excess nutrients can be very harmful to our local rivers and wildlife.  CLICK HERE for more information.

The beneficial processes increase over time as rooftop plants and root systems mature. 

Heat Island Mitigation

The 'Urban Heat Island Effect' is the difference in temperature between a city and the surrounding countryside. It is mainly due to the expanse of hard and reflective surfaces, such as roofs, which absorb solar radiation and re-radiate it as heat.

On hot summer days, the surface temperature of a vegetated rooftop can be cooler than the air temperature, whereas the surface of a traditional rooftop can be up to 90°F (50°C) warmer. On a large scale, green roofs can help to reduce urban heat islands by providing shade, and through evapotranspiration, the release of water from plants to the surrounding air. They also absorb air pollution, collect airborne particulates, and store carbon.  

According to a study by University of Michigan, a green roof has some capacity to reduce pollution through the uptake of NOx (nitrous oxides) which contribute to the formation of ground level ozone.  For further information on the Heat Island Effect, please CLICK HERE. 

Long-term costs and energy savings

Extensive green roofs cost more than traditional roofs because they generally require more material and labor for installation.  As the demand for rooftop gardens increases in the U.S., up-front costs will likely decrease.

The plants on the roof protect the underlying roof material by eliminating exposure to the sun's ultraviolet (UV) radiation and extreme daily temperature fluctuations, which causes wear and tear and damage to a conventional roof.  It is suggested that the mean green roof cost (compared to the conventional roof) is 25 - 29% less over 40 years, with the investment breaking even over a period of 20 years.2

Our vegetated layer will be too thin to have any real insulation value, but the green roof will act as a heat sink - slowly absorbing and holding energy from sunlight and releasing it when the ambient air cools.  Once the plants are fully mature, evapotranspiration will further reduce the cooling demands within the building.  The new roofing under the vegetated layer will be well insulated to help the building retain its energy during the heating season.

Sedum budsCharlottesville City Hall Green Roof

The roofs of the City Hall and Police buildings on the Downtime Mall have been replaced with extensive green roofs. 

The total vegetated area is 9,250 sq. ft - with a total of 18,540 plants distributed across this area.   This entailed using 2,312 cubic feet of soil - enough to cover the entire green roof to a depth of 3 inches.



Green roofs typically contain low-maintenance, drought-resistant plants such as sedum. Sedum plants are hardy and attractive, and once established they can withstand high temperatures and drought and mature into carpet-like coverage. 

The plants chosen for the City Hall green roof are a variety of sedums (pictured above), which when established will result in a flowering season from May through to September, in shades of white, yellow and pink. 

The image above shows a cross-section of the roofing material that is being used for the City Hall green roof.  The roof system contains a root barrier, which protects the base roof membrane from damage, and the water retention fabric helps to ensure that the roof retains rainwater. 

1-2 Green Roof Valuation: A Probabilistic Economic Analysis of Environmental Benefits