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 Figure 1 below.

Figure 1: Multiple layers of a green roof

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.

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.