Grantee Research Project Results
Final Report: Lightweight Green Roof Systems
EPA Grant Number: SU834730Title: Lightweight Green Roof Systems
Investigators: Mickute, Monika , Zakutny, Matthew J , Malawski, Kevin , Magee, Michael , Spatari, Sabrina
Institution: Drexel University
EPA Project Officer: Page, Angela
Phase: I
Project Period: August 15, 2010 through August 14, 2011
Project Amount: $9,952
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2010) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Sustainable and Healthy Communities , P3 Challenge Area - Safe and Sustainable Water Resources , P3 Awards , Sustainable and Healthy Communities
Objective:
To develop and test a lightweight modular green roof system to retrofit existing buildings and eliminate the need for structural reinforcement, which would otherwise be needed to accommodate the system. The resultant prototypes will weigh less than 10 lb/sf when fully saturated. The Lightweight Green Roof (LGR) system will retain a minimum of 1 inch of precipitation, decrease heat gain, insulate the underlying roof, sequester CO2 from the atmosphere, and make progress to reduce the heat island effect. The proposed system is expected to satisfy the expectations of the City of Philadelphia’s Green City, Clean Waters initiative. The LGR system will also qualify for a LEED SS Credit 7.2, requiring to “install and maintain a vegetated roof that covers at least 50% of the roof area” (USGBC, 2009).
Summary/Accomplishments (Outputs/Outcomes):
During Phase I research, a final planting medium was developed by substituting soil with a combination of natural and synthetic lightweight materials. This final LGR soil mixture underwent extensive testing, including the required ASTM standards for green roof systems, rain simulation, and thermal conductivity.
ASTM Standards
Required ASTM tests for the green roof systems were completed in-house with the following results:
ASTM E2397 - Standard Practices for Determination of Dead Loads and Live Loads associated with Green Roof Systems. The total dead load is the system’s weight at a fully saturated condition, including the ASTM specified 2 lbs/sf vegetation, and was measured at 8.27 lb/sf. The live load, which includes the transient water in the system travelling down the drainage layers while the rainstorm continues, was measured at 0.78 lb/sf. The maximum combined dead and live loads exerted on the structure from the application of the LGR do not exceed 9.04 lbs/sf. This standard was also used to establish system water capture volume in inches. According to the test results, the LGR system will manage 1-inch storm events.
ASTM-E2398 - 05 Standard Test Method for Water Capture and Media Retention of Geocomposite Drain Layers for Green Roof Systems was performed to establish the amount of water captured by the DELTA and COLBOND water capture systems. The total water captured by these layers is 2.25 lb/sf. This value was referenced in ASTM E2397 when computing the water capture volume and dead loads.
ASTM E2399 - Standard Test Method for Maximum Media Density for Dead Load Analysis of Green Roof Systems was used to evaluate the planting medium physical properties associated with the “soil” performance. This standard determined a maximum media density (MMD) of 22.43 lb/cf, dry media density (Ddry) of 6.33 lb/cf, a maximum media water retention (MMWR) of 26% volume, and a water permeability (KMMD) of 0.13 inches per minute. All of the values produced with the ASTM E2399 are comparable to the traditional vegetative roof soil behavior as suggested by the Cantor’s Green Roofs in Sustainable Landscape Design.
ASTM D422 - Standard Test Method for Particle-Size Analysis of Soils was performed to evaluate the grain size distribution of the LGR planting medium. The final percent passing values are comparable to the recommendations listed in Cantor’s Green Roofs in Sustainable Landscape Design. The percent passing values were used to compute the Coefficient of Curvature (Cc) and Coefficient of Uniformity (Cu), which determine the quality of soil gradation. Cc and Cu were computed to be 2.92 and 5.9 respectively, both of which fall within the required ranges of the well graded soil mixture.
Nutrition Test
To enhance the nutrition of the LGR system, multiple traditional and non-traditional methods were explored. Wood ash, recycled coffee grounds, crushed clamshells, and zeolites pre-soaked with ammonia were added to the LGR ‘soil’ mixture to provide longterm nutrition. These ingredients allowed us to develop a soil system with the N-P-K of 5-170-240. To resolve the initial lack of Nitrogen and stimulate the plant growth within the first year, Osmocote Pro 12-14 M slow-release nutrition was incorporated into the system. Long-term Nitrogen supply solutions will be explored in Phase II research.
Plant Viability Testing
A variety of sedum stonecrop plants were selected for the initial LGR system. Specific sedums were chosen according to the North Creek Nurseries and Emory Knoll Farms recommendations. A test cell with newly developed LGR mixture and fresh sedums was constructed and kept indoors under a regulated eight hour artificial lighting schedule. It was watered once every 1 to 2 weeks with tap water. After four months the plants remained looking healthy and continued to grow at the same rate as the control samples planted in the conventional potting soil.
Heat Testing
Four different roof samples were constructed and subjected to identical exposure conditions. After 4 hours of exposure to a set of five 75 W heat light bulbs at a distance of 16 inches, the maximum LGR system temperature reached 91.6°F. This resulted in a favorable temperature difference of 38.4°F when compared to the surface temperature of the traditional asphalt shingle roof. The LGR system thermal resistance (R-value) was computed to be R-6.5, while standard asphalt shingles have a value of R-0.44 only.
Rainfall Simulation Test
A rainfall simulator was built to produce a heavy rain of 0.30 inches per minute (3.15 cubic feet per hour) to observe peak weight, live drainage rate, and live water absorption rate. After 2 hours and 50 minutes of rain, peak weight reached 9.74 lb/sf, and a steady drainage occurred after about 2 hours and 25 minutes. Even though the storm simulation created was 3 times as large as the typical “heavy storm” (Federal Aviation Administration, 2007), the LGR system performed adequately. The rain simulation apparatus will be calibrated for the phase II to represent the actual heavy storm of 0.105 inches per minute.
Wind Test
A 12”x24” prototype was placed in a small-scale wind tunnel and tested to withstand 30 mph wind. No uplift, material disturbance or vegetation dislocations were observed. A large-scale (up to 150 mph winds) test is proposed for the phase II research.
Conclusions:
Research conducted on the LGR system during Phase I shows promising results for the LGR system tested. Initial longevity and nutrition tests have shown no negative impacts on plants, although no organic matter was introduced into the LGR system. Several modular assembly options have been investigated which will be finalized in phase II of the project. This revolutionary do-it-yourself system will facilitate individual homeowners to install the LGR system on their properties. Based on the preliminary data, if 25% of Philadelphia’s residential stock installs the LGR system on their roofs, stormwater runoff would be reduced by 39 million gallons per year, reducing the frequency of combined sewer overflow (CSO) events. With an insulating value of R-6.5, homeowners can save approximately $176 annually in heating and cooling costs for every 1000 square feet of LGR. With an estimated cost of $10 -$12 per square foot, this system will be cost competitive.
Supplemental Keywords:
building envelope, insulation, water retention, carbon reduction, carbon sequestration, heat island effect, sustainable water management, storm water management, green roof, energy conservationRelevant Websites:
Drexel Smart House Exit
Roofmeadow Exit
The Philadelphia Water Department
Green Roof Plants Exit
Green City, Clean Waters Exit
P3 Phase II:
Lightweight Green Roof Water Retention SystemThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.