Green Roofs - Living Architecture Monitor - Summer 2016 - 21

concepts, challenges and benefits. The core of the guideline covers the integration of sustainability and performance measures for green roof systems defined in 5.3.2: 1) dead loads, 2) moisture retention capacity, 3) system longevity, 4) assurances of foliage cover and survival, and 5) assurances regarding roofing/ waterproofing compatible with the green roof; however these concepts are quantified in section 7 Technical Requirements. One of the limitations of the previous ASTM guidelines is the lack of a systems guideline to address some integration and discussion regarding tolerances for growing substrate and drainage in context with vegetation. The new ASTM guideline covers upper and lower limits of substrate particle sizes, proportions of granular materials, organic content and the like. Although much is covered in the guidelines, there are opportunities for future updates including: a broader discussion regarding regional context, meadow-based systems, and further expansion of performance guidelines. Vegetative systems in the new ASTM guideline are loosely defined to perhaps capture diverse approaches to extensive green roofs in North America. Some green roofs have distinctive designs with formal lines or planting designs imbedded. Some green roofs have no geometric design and plants are intended to migrate freely across the system with no clear formal appearance. So the guideline states that an extensive green roof "features plants that can be sustained in shallow media layers (pg. 2, ASTM E2777-14)". This is a good aim to achieve for any green roof. In the FLL Guidelines the aim is specific to ecosystem services including partial recovery of natural systems lost to building development. The FLL guideline states its purpose for extensive green roofs, "...accelerate the natural self-generating process and to use the natural vegetation development to establish a long-lasting population (pg. 8, FLL 2008)". The FLL goes on further to mention the use of "regionally native and naturalized vegetation." I think the approach in Europe is a good one because it seems the right thing to do for long term sustainability of the green roof and is a way to improve habitat corridor continuity and reduce habitat fragmentation. Over the next decade, our understanding of green roofs as ecosystems will likely continue, and this knowledge would be a valuable addition to HIGHLIGHTS OF THE ASTM E2777 - 14 STANDARD GUIDE FOR VEGETATED (GREEN) ROOF SYSTEMS System topic ASTM Previous Guide or Standard ASTM E2777 - 14 Standard Guide for Vegetated (Green) Roof Systems Planting Media E 2399-05 and E 2397-05 determine dead or live weight of media. Section 7.3 provides technical guidance for planting media including mineral properties, nutrient retention, longevity, water retention, permeability, drainage, organic content, density, depth, capillary potential and more. Drainage E 2396-05 and E 2396-05 determine test methods for drainage media and geocomposite layers. Section 7.7 provides technical guidance for granular and geotextile classes of drainage layers including: materials selection, discharge facilitation, hydraulic transmissivity, compressive strength, water retention, and discussion of additives such as polymer gels. Environmental Impacts None Includes discussion of how green roofs improve air quality, carbon sequestration, manage storm water, noise/acoustic attenuation, urban heat island mitigation, energy, habitat creation, social and economic impacts. guidelines to address North America's diverse climates. Regarding meadow-based green roofs (grasses, herbs; annual and perennial vegetation), the FLL Guidelines have a separate designation for them as semi-intensive systems. The semi-intensive category is not formally recognized in the ASTM E2777 - 14. For some, there is a clear distinction between an extensive green roof designed to be lightweight with shallow growing media. Extensive green roofs are typically three to five inches thick with succulent vegetation exclusive to the shallowest applications. This is not to say that some grasses or forbs could persist in some climates without irrigation, but grasses and wildflower systems that are diverse, typically need at least six inches of substrate and with up to ten inch depth to support a much greater range of vegetation. The treatment and articulation of semi-intensive systems may be needed, because they have different soil and moisture requirements compared to succulent-based systems. The ASTM guideline does discuss herbaceous vegetation, but with limited content. Intensive green roofs are typically articulated as roof gardens with woody plants and need regular maintenance. Meadow and prairie-based systems can be designed to be low maintenance and considerations for these systems would be an important addition. Finally, one of the highlights of the new ASTM guideline is that it includes tolerances for defining performance expectations for growing medium particle size, drainage properties and such. If meadow-based systems become treated independently in a future issue, tolerances and performance aspects could be further articulated READ THIS ISSUE ONLINE AT LIVINGARCHITECTUREMONITOR.COM for meadow-based systems, especially designs that consider varied and non-uniform soil depths to create biodiverse habitats. The FLL green roof guidelines have been reissued several times over the past thirty years, including input from those involved with research, industry and maintenance. The FLL guidelines have become a universal starting point for many green roofs outside of the temperate climates of Europe. Green roof substrates in some climates may need to become more porous to allow for intense storms or less porous to retain water or be considerate of other local factors. Over time, with research and development North American green roof guidelines may become more regionally responsive regarding climate or function. Because green roofs are being used to grow food, treat grey and black water and function as habitat for rare or endangered species, new forms of guidance may be needed. Until then, the ASTM E2777 - 14 is a solid foundation and point to begin. Many experts from the industry, research community and design professions contributed and volunteered hours over the more than seven years in its making. Regional experts will be needed to help shape and inform future updates to the guideline. Bruce Dvorak is an Associate Professor in the Department of Landscape Architecture and Urban Planning at Texas A&M University, coordinator of the Master of Landscape Architecture Program and cofounder of the Interdisciplinary Green Roof Research Group at Texas A&M University: https://tamugreenroof. wordpress.com/ LIVING ARCHITECTURE MONITOR / SUMMER 2016 / 21 https://tamugreenroof.wordpress.com/ http://www.LIVINGARCHITECTUREMONITOR.COM

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