Grantee Research Project Results

2012 Progress Report: Lightweight Green Roof Water Retention System

EPA Grant Number: SU835066
Title: Lightweight Green Roof Water Retention System
Investigators: Spatari, Sabrina , Mickute, Monika , Magee, Michael , Zakutny, Matthew J , Malawski, Kevin , Hoffman, Patrick T , Damis, Maxime , Tran, Annabella
Current Investigators: Spatari, Sabrina , Mickute, Monika , Zakutny, Matthew J
Institution: Drexel University
EPA Project Officer: Page, Angela
Phase: II
Project Period: August 15, 2011 through August 14, 2013 (Extended to December 31, 2015)
Project Period Covered by this Report: August 15, 2011 through August 14,2012
Project Amount: $75,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet - Phase 2 (2011) Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Awards , P3 Challenge Area - Safe and Sustainable Water Resources , P3 Challenge Area - Sustainable and Healthy Communities , Sustainable and Healthy Communities

Objective:

Typical North American vegetated roof systems consist of an expanded clay-based growing medium. The bulk density of these dry mixes ranges from 25 to 56 lb/cf (Wark, 2003). Depending on the structural conditions of an existing structure, installation of a conventional vegetated roof can be cost prohibitive. Providing a reliable modular vegetated roof system weighing less than 10 lb/sf will minimize the affordability concerns for the average homeowner and facilitate the introduction of green roofs on buildings across North America. The widespread application of more vegetated roofs in urban areas will result in benefits, including heat island effect mitigation, carbon sequestration, providing heating and cooling energy savings, and most importantly managing stormwater runoff by reducing impervious surface coverage while delaying peak runoff.

Progress Summary:

Wrap up of Phase I

During Phase I of our research, we were able to combine several lightweight, water absorptive, natural and synthetic materials to develop a unique planting medium, which sustains vegetation and weighs less than 10 lb/sf when fully saturated. This specially engineered Lightweight Green Roof (LGR) ‘soil’ satisfied all of the required ASTM standards demonstrating an appropriate performance without the added weight, or associated costs. During Phase I of the research we have identified key components of the system to achieve desirable performance. Prior to proceeding with Phase II of the project, the research team concentrated their efforts in making this system more sustainable by reducing its carbon footprint. We have decided to replace a German product, DELTA drainage pan, with a US product, J-Drain. Our primary concern was to ensure that a total water retention capacity of the system still satisfies a 1 inch rainstorm event requirement.   DELTA drainage mat has a water retention capacity of 21.98 fl. oz/sf (1.374 lb/sf). J-Drain drainage mat has a slightly lower water retention capacity of 14.08 fl. oz/sf (0.88 lb/sf). Based on ASTM-E2399 and ASTM-E2397 J-Drain mat will satisfy our system requirements by managing 1.01 in. of stormwater and weigh 9.81 lb/sf when fully saturated. Changing the drainage material to lower water retention capacity added another advantage; in order to still manage a 1 in. rainstorm event, we are to increase our cell thickness by 0.2 in. providing more planting medium for the vegetation.

The research team will use the 2012-13 Winter season to investigate other planting medium options, such as replacing vermiculite with peanut shells and replacing currently used virgin EPS with an alternative post-consumed or recycled material. We attempted replacing current micro EPS with recycled 1/8 in. EPS, however this attempt was not successful. EPS is the lightest component of the planting medium and when an 1/8 in. size particle was introduced into the mixture, it did not create a well distributed soil mixture.  There was also an issue with the EPS particles rising to the top when saturated with water.

The research team required additional time to refine the prototype design and planting medium formula to make it durable under ambient testing conditions. The final planting medium will be verified by Turf Diagnostics to meet appropriate ASTM test.

Phase II

Value engineering and lifecycle cost analysis of the system is an on-going process as we refine our system. To determine a baseline cost and demand for the system, researchers presented the purpose and benefits of green roof to local community in Wilmington, DE. After the presentation attendees were asked to fill out a survey which evaluated their initial understanding of vegetated roof systems, their interest in the system, and most importantly if they would install one on their home and how much would they be willing to pay for it. A total of 40 individuals of various ages and professions filled out this survey.. The results indicate that the majority of audience (more than 80%) had moderate to low understanding of vegetated roofs. None of the attendees reside in a home with a vegetated roof, but 53% would consider installing one on their home. Only 3% would not consider installing a vegetated roof, while the remainder of attendees still would like more information before making such a decision. Based on conversations with the audience after the presentation, a deciding factor would be payback time of the system specifically for their properties. Our goal was to establish a demand as well as a target cost for a vegetated green roof in a residential market. Only 43% of attendees answered question #5 (5. How much would you be willing to pay (max) per square foot of a green roof?). Of those who answered this question, 47% are willing to pay between $5 and $10 per square foot. Currently, our estimated raw materials for the system arrive at $7.75 per square foot. This estimate is based on purchasing materials in small quantities required for the large-scale prototypes during Phase II of the project. Through the 2012-13 Winter season, the research team will perform value engineering for the LGR system in order to reduce the total materials cost to a goal of $5 per square foot, allowing for a total system market cost of $9 to $10 per square foot.

The single 1 ft by 2 ft test cell assembled in March of 2011.

This cell contained a special extensive green roof plant mixture suitable for the mid-Atlantic region. Plant species and varieties in this mixture include: Sedum acre, oreganum, aizoon, pulchellum, album, reflexum, ellacombianum, sexangulare, floriferum, seiboldii, hispanicum, spurium, stoloniferum, rupestre, kamtschaticum, acre 'Octoberfest', telephium, hybridum 'Czar's Gold', and Phedimus takesimensis. Today, this cell still remains on a test site roof in Wilmington Delaware, office building of BCAD. At least four species of plants remain in the cell, predominantly containing hybridum 'Czar's Gold' and small amounts of reflexum, sexangulare, and hispanicum. After discussing performance of this test cell with a green roof expert, Claudia West, from North Creek Nurseries in Landenberg, PA, it appears that we may have shocked our plants during the assembly of the test cell. Transferring the plant plugs from soil based medium to our LGR planting medium causes a significant shock to the roots of the vegetation causing them to perish.

To remediate this issue, the LGR team will acquire a new fully grown sample plant mix. The research team then will develop an inventory of stock plants, which will be started in the LGR planting medium.  The stock plants that grow well in the LGR planting medium will provide clippings to start the large-scale prototype cells in Spring 2013. This way vegetation will not perish due to shock allowing us to monitor its performance and longevity in real time. Even though the official wind test is not scheduled until fall 2014, the sample cell has been placed on a test site roof in Wilmington DE. This roof is 12 feet above ground, has a single 1/8 inch per foot slope and has no parapet walls, as shown in Figure 1

 

Figure 1: LGR Test Cell

The research team had decided to leave this test cell on the roof during the recent superstorm Sandy.  The center of the storm passed through about 10-15 miles north of this site, and according to a local weather station located a couple miles north of BCAD (US202 & Naamans Rd), the top wind gust recorded was 41mph, with top sustained winds of around 23mph. The test cell remained in the exact same location on the roof after the storm as it did before the storm.

The research team has submitted a request for a no-cost project extension for the Lightweight Green Roof Water Retention System project. This project requires year-round large scale testing to evaluate real-time seasonal performance and longevity of the system. Based on the schedule proposed in the Phase II proposal, our research team had planned to install the large-scale prototypes in Spring 2012. However, a number of technical and infrastructural challenges were encountered, which delayed the setup and testing of the prototype system for the following reasons:

• The team faced a delay in laboratory testing due to some sudden and on-going construction activities in Drexel University Campus laboratories; however, the team may have subsequently ensured a suitable space to run the proposed experiments. We are working with Drexel Biology department to get permission to use a large and currently vacant green house space as our laboratory. This space will allow us to grow and prepare vegetation acclimated to the LGR growing medium, assemble and install a roof top sample and continuously improve upon our system based on real-time performance. This space will allow us to generate enough vegetation to fulfill all three large-scale prototypes in Spring 2013.
• The team has considered proceeding with the large-scale prototype installation this fall, however after some additional research and consultations with North Creek Nurseries, it was discovered for optimal growth and performance it is best to install a green roof in mid to late April.  Stock plants must be started in our LGR medium and receive natural sunlight throughout their entire life to reduce plant shock during the transition to large-scale installations. This will allow as more time to investigate proper installation procedures and collect more accurate system performance data. 

Research and the LGR system design optimization is a continuous process under the laboratory conditions. The large scale testing has been delayed by a year; however, project scope remains the same as proposed the Phase II project report. 

Future Activities:

The initial target was to develop a lightweight green roof system weighing less than 10 lb/sf, which will manage a minimum of 1 in. of stormwater. The ASTM tests acted as a metric for us to gauge our success. Our system performed exceptionally well, weighing just over 9 lb/sf when fully saturated and managing just over 1 in. of rain water. In addition, permeability, grain size distribution, and nutrition levels are comparable to those of the traditional planting medium. We are still working in altering the final mixture design to reduce LGR systems carbon footprint and to present a more sustainable product. Currently LGR planting medium contains 90% of mined materials, and our goal is to reduce it to 50% of mined material. The insulating value and rooftop surface membrane temperature reduction were more than satisfactory. The cost of our system is estimated at $10-12 per square foot, which remains competitive to an average cost of $14-20 per square foot for a conventional vegetative roof installation. In addition, our conducted survey among local homeowners has informed us that an average cost of $10 per square foot is appropriate for a vegetated roof system. We are in process of value engineering the LGR system to make it more competitive in the market and more attractive among homeowners.

Due to the unexpected delay, the LGR project remains under the budget with anticipated large-scale prototype installation in Spring 2013. Prior to the large-scale prototype, a weather station will be constructed and installed at the test site. The LGR planting mixture will be refined, independently verified by Turf Diagnostics, and the stock vegetation will be grown in the green house at Drexel University.

References:

• Cantor, S. (2008). Green Roofs in Sustainable Landscape Design. New York: W.W. North & Co.
• Crockett, C. (2010). Parcel Based Billing for Stormwater. Philadelphia: Philadelphia Water Department.
• DeNardo, J. (2005). Stormwater Mitigation and Surface Temperature reduction by Green Roofs. ASABE , Vol. 48 (4): 1491-1496.
• Groll, T. L. (2011, February 28). Office of Watersheds, Philadelphia Water Department. (M. Mickute, Interviewer)
• Miller,C. (2011, March 10). President, Roofmeadow, Inc. (M. Magee, Interviewer)
• National Acadamy of Engineering. (2010). Grand Challenges for Engineering. Retrieved February 8, 2011, from National Acadamy of Engineering:  http://www.engineeringchallenges.org/
• Robert D. Holtz, W. D. (1981). An Introduction to Geotechnical Engineering, 1st edition. Upper Saddle River, NJ: Prentice-Hall, Inc.
• Wark, C. G. (2003). Green Roof Specifications and Standards. The Construction Specifier , pp. Vol. 56, No 8.

Journal Articles:

No journal articles submitted with this report: View all 2 publications for this project

Supplemental Keywords:

Building envelope, insulation, water retention, carbon reduction, carbon sequestration, heat island effect, sustainable water management, storm water management, green roof, energy conservation

Progress and Final Reports:

Original Abstract

2013 Progress Report

2014 Progress Report

2015

Final Report

P3 Phase I:

Lightweight Green Roof Systems  | Final Report