To serve as an educational model demonstrating the benefits of green roofs and the technology imbedded in green roofs, Allegheny County, Pennsylvania decided to install a green roof exhibiting four types of green roof technologies:
- Intensive – 8-12 inches of lightweight engineered growing medium with shrubs and plants requiring that rooting depth.
- Semi-intensive – 6 inches of lightweight engineered growing medium with the ability to grow plants and shrubs with that depth.
- Roll out mat – a pre-grown sedum mat set on 4 inches of lightweight engineered growing medium, providing instant coverage.
- Tray system – pre-grown sedum established in lightweight engineered growing medium trays.
By exhibiting these four green roof technologies, it is noted that the substantial additional weight of the intensive and semi-intensive systems requires close examination of the structural integrity of a roof, whereas the mat and tray systems allow green roof application on roofs with less significant weight restrictions. In addition to the visual display, monitoring systems were installed to prove the benefits of green roofs. The performance of this green roof is amazing and noteworthy to share in the scheme of green technology.
Initially, the project’s focus was on mitigation of stormwater. With Allegheny County in the middle of a federal consent decree for combined sewer overflow (CSO) reduction, this seemed a perfect opportunity to demonstrate the effectiveness of green infrastructure. As noted by Darla Cravotta, Special Projects Coordinator for Allegheny County, the county “received federal assistance through the energy funding and opted to put together a team of thinking partners to develop a strategy for an educational green roof. After intense planning for 8 months, we (Allegheny County) quickly came to the realization that the stormwater drainage area was quite nominal, but the heat reduction could be significant. The sensors showed an impressive range of 50 degrees cooler on the green roof. Heat reduction impacts the ozone and this is certainly yet another benefit of the roof.
During installation, AC (Allegheny County) discovered leaks in the cooling system and with the added layer of insulation, AC saw reductions in the electric bills the first year.
AC invited businesses, non-profits, universities, and others to tour the green roof with a focus on the downtown Pittsburgh area to engage building owners and operators.
The cost to install the green roof due to a variety of issues of an older building (lack of elevator access, needed to rent a crane and close streets to haul the material) may prohibit others from installation.” In addition, it was noted by Eric French of Eisler Landscapes, the contractor for the project, that the overall costs were also driven by Davis Bacon wage rates, long-term maintenance purchases, and very difficult access.
Allegheny County’s office building green roof was designed and built to be an exemplary project and living laboratory to provide information on the thermal and hydrologic benefits of green roofs. The project design and construction was completed entirely with local talent (see team below). Performance monitoring was done using an extensive, near-real-time monitoring system created by Civil & Environmental Consultants, Inc. (CEC). The green roof final design and installation was led by Cuddy Roofing, with Eisler Landscapes, IDC Architects, and CEC acting as subcontractors on the project. Project details are available on Allegheny County’s website.
The monitoring system evaluates the performance of four intensity levels of green roof construction on the west side of the Allegheny County office building, as well as the conventional roof on the east side. The monitoring system consists of an internet-linked network of about 90 sensors monitoring weather, soil moisture, water infiltration, and temperature every 15 minutes throughout: 1) the green roof, 2) highly controlled “microcosm” study areas within the green roof, and 3) “control” roofs that did not receive the green roof treatment. Differences in surface and air temperature, as well as flow of stormwater off the roof, will be compared under various levels of green roof intensity (soil thickness and plant composition) over a six-year period. The project is being led by Darla Cravotta, Manager of Special Projects for Allegheny County, with support from federal economic stimulus funds.
Within a year of planting, the green roof was robustly vegetated, ranging from a Sedum sward on extensive portions of the green roof to Juniper species in the intensive portion. Stormwater reduction has varied seasonally and with the thickness of soil used. Thicker soils attenuated more stormwater and also supported a more diverse palette of plants. A near-real-time summary of the green roof stormwater reduction performance (updated every four hours), as well as links to a Carnegie Mellon University evaluation of heat flux changes made by the green roof, is presented on the Allegheny County Office Building Green Roof Project website.
Economic impacts of the green roof have been very encouraging. When electric utility costs before green roof installation (2009) were compared with post-construction (2010) costs, the numbers dropped by about $90,000, despite the fact that 2010 was hotter than 2009 (the building is heated with steam, so most of the electric costs are associated with summer air conditioning). Carnegie Mellon University students Daisy Wang and Dyanna Becker analyzed temperature data across monitored roof profiles and demonstrated a 75% decrease in summer heat flux through the green roof and into the building relative to the conventional roof.
Reduced temperature extremes are expected to reduce the thermal strain on roof membranes, and thereby more than double the life of the roof. To overcome concerns about potential leaks in the membrane below the green roof, the design includes an electric field vector mapping system grid, which can be used to pinpoint leaks to within one square foot. To provide an additional level of roof leak security, the monitoring system below the roof deck includes humidity sensors, with automatic reporting of elevated humidity levels to key decision makers.
The monitoring systems of the Allegheny County office building’s green roof prove significant benefits, some of which are not readily considered in designing green roofs. This case study’s demonstrated energy reductions need to be emphasized by landscape architects to their clients pondering the installation of a green roof.
Allegheny County Office Building Team
Conceptual Design and Bidding Phase
Allegheny County: Darla Cravotta
Final Design, Installation, and Monitoring Phase
Allegheny County: Darla Cravotta
General Contractor: Cuddy Roofing (a Scalo Company)
Green Roof Landscape Final Design and Installation: Eisler Landscapes / Eric French
Roof Design Detail Drawings: IDC Architects
Monitoring System, Engineered Soil Design: Civil & Environmental Consultants, Inc.
by Lisa Kunst Vavro, RLA, ASLA