Green Roof

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Construction of Villanova's Green Roof took place in 3 days in the summer of 2006. The design was a retrofit of a small portion of Villanova's Center for Engineering Education Research (CEER) roof. The green roof is located on a second storey terrace above the "Holy Grounds" coffee shop. It is highly visible because it is located outside of the main stairwell and serves multiple purposes.

  • By eliminating the first half inch of rain falling on that portion of the roof for any storm event.

  • By providing research opportunities for students and possible quantification of the environmental advantages of a green roof.

  • By providing a much more aesthetically appealing roof than its predecessor.

Underlying the green roof is a traditional roof which was refinished in order to ensure maximum life of the green roof. The next layer is an additional waterproofing layer which is also resistant to root penetration. On top of this layer is an insulation layer which is a thick mat designed to keep the potential for a large temperature gradient between the surface of the green roof and the original roof layer. The insulation is overlain with a geosynthetic layer that serves several purposes including drainage pathways, water storage areas, and aeration. This layer which looks much like an egg crate has holes at the high points to allow excessive amounts of water to drain. A filter fabric lies on top of it and allows water to pass through but keeps the fine sediments from the growth media from occupying the voids in the drainage layer. The media is a mixture of baked minerals which look like gravel and make it hard for weeds or anything aside from the highly specialized Sedum species to grow.

The environmental advantages that a green roof can provide are the reduction of stormwater runoff coming from the developed site. Reducing the amount of runoff that runs directly to piped systems lowers peak flows in downstream areas which can reduce erosion of the channels and the resultant sediment pollution. This site alone will not solve this problem but if similar runoff reduction systems are utilized in other developing areas, then the cumulative effects will help alleviate the situation.

In addition to preserving the environment downstream of the site, green roofs are also environmentally friendly and cost efficient to the building occupant. The green roof shields the originally sealed roof from the very intense UV radiation from the sun. The multiple layers of the green roof also act as a buffer and do not allow temperatures at the original roof surface to fluctuate drastically. These two factors can extend the life of the roof greatly. The insulating effects of the green roof can also allow the building to retain its heat in the winter and keep the radiant roof temperatures from heating the building in the summer.

The green roof is a very practical BMP in highly urbanized areas where there may not be room for an infiltration BMP in the ground. Its footprint on the landscape is no more than the building itself because it stores the water on the roof and is used by the plants or evaporates. This site should provide useful insight with respect to the actual capacity of a green roof in the field. The green roof is incorporated within the “Best Management Practice Demonstration Park" on Villanova’s campus. It adds to the list of BMPs in use on the University’s campus and will be monitored and evaluated to determine its effectiveness.

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Figure 1: Multiple Layers of a Green Roof. (Source: Earth Action Mentor)

Villanova University's green roof covers approximately 530 ft2 above the Holy Grounds coffeehouse in CEER. The site was designed to capture and retain the first half inch of every precipitation event, thereby reducing downstream stormwater volumes, stream bank erosion, and non-point source pollution. The green roof will also protect the underlying roof material by eliminating exposure to the sun’s ultraviolet (UV) radiation and extreme daily temperature fluctuations. A cross section of the layers present in a typical green roof can be seen in the Figure 1.

The green roof was designed as a BMP but is unlike most because it is not a necessary part of the buildings comprehensive stormwater reduction system. Most green roofs are installed out of necessity by law. While this green roof will act as a BMP and reduce the amount of runoff coming off of the building, its primary function is to serve as a research site. The extensive instrumentation installed on the site will provide the necessary data for further understanding of the capacities of green roofs in general. The instrumentation includes two temperature sensors and a rain gage on the roof as well as a flow meter located in the downspout.

The two temperature sensors are installed on the roof. The first was installed on the original roof surface and the second is located on the surface of the growth media. These two sensors serve to illustrate the roof's ability to insulate the building from the radiation of the sun during hot months and the ability to keep warmth in the building in colder months. The surface sensor will represent the roof temperature and the buried sensor will show the insulating ability of the green roof layers. Another advantage of the green roof is that it is able to protect the waterproofing layers of the roof from the suns UV radiation. Typically traditional rooves need to be replaced every 10-20 years. Cases of green roofs lasting more than four times as long have been documented and time will tell with this site. 

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Figure 2: Growth medium contained within perforated aluminum perimeter and downspout inlet in rocks.

The rain gage and flow meter are used together in order to determine the runoff reduction capacity of the green roof. The rain gage measures the depth of water that falls on the green roof site. The volume of raining hitting the roof can be calculated since the contributing area is known. This water hits the surface and soaks into the plants and medium where some is consumed by the plants through transpiration. The rest of the water will continue down through the medium and through the filter fabric where it enters storage area. The water fills the storage voids and any in excess of the storage capacity flows out of the green roof area and through the perforated aluminum perimeter where it runs through a rock bed and to the downspout which can be seen above in Figure 2. The water that overflows into the downspout is measured by the flow meter.

Recent studies comparing green roofs to traditional non-vegetated roofs have indicated that green roofs are a source for nutrients. In order to determine if Villanova’s green roof is a relatively significant source for runoff nutrients, the water resources lab team began testing the green roof for water quality in the summer of 2012. A composite storm sample and a first flush sample are taken to test the overflow, as well as a precipitation sample which serves as the control. Performance is evaluated against that of other urban and suburban land use types, including a wooded area, a lawn, and a parking lot. Future comparisons will look at the green roof’s performance as compared to other SCMS in the campus stormwater park.

Testing parameters include nitrites (NO2), nitrates (NOX), orthophosphate (PO4), Total Kjeldahl Nitrogen (TKN), Total Kjeldahl Phosphorus (TKP), chlorides (Cl-), Total Suspended Solids (TSS), and Total Dissolved Solids (TDS). Beginning in January 2014, ammonia (NH3) was added as a testing parameter. Preliminary data graphs show that the green roof’s source potential is dependent on the type of nutrient being evaluated. For example, nitrite levels are generally lower than orthophosphate levels when the overflow (GR OUT-1 and GR OUT-2), precipitation (GR P), and first flush samples from other sites (FFW, FFG, and FF02) are evaluated on a storm-by-storm basis.

Plots
Storm Plot of Rain vs Overflow
Storm Plot of Rain vs Overflow
Storm Plot of Temperature
Storm Plot of Temperature
Roof before Construction
Roof before Construction
Life of Green Roof
Extending the life of the underlying roof as well as the life of the green roof
The black "egg crate" layer is the drainage layer but also allows for additional water retention in dry periods for the plants.
The black "egg crate" layer is the drainage layer but also allows for additional water retention in dry periods for the plants.
Using a crane, the growth media is delivered to the roof in large bags.
Using a crane, the growth media is delivered to the roof in large bags.
Plugs of various Sedum plants are taken from their trays.
Plugs of various Sedum plants are taken from their trays.
Tarred Roof
Roof was tarred and resealed with a rubber lining
Insulation Mat
The insulation mat was then put down over the refurbished roof
The filter fabric keeps the growth media (small, soil-like minerals) and drainage layer separate.
The filter fabric keeps the growth media (small, soil-like minerals) and drainage layer separate.
The media is spread evenly over the filter fabric and underlying layers.
The media is spread evenly over the filter fabric and underlying layers.
Plugs are evenly distributed throughout the media and are watered.
Plugs are evenly distributed throughout the media and are watered.