2013 ASHS Annual Conference

Green roofs are gaining popularity as stormwater management tools in densely urban areas across the country. German standards have been adopted across North America; however, these are applied to materials prior to and at the time of installation with no guarantee of roof performance. The purpose of this study was to collect substrate samples from mature green roofs in the Mid-Atlantic region to compare current particle size to the widely adopted German FLL particle size distribution curve limits. Particle size distribution is pivotal to green roof performance because it dictates important characteristics such as substrate water holding capacity, air space, total porosity, and green roof live load. A secondary objective was to conduct freeze-thaw analyses on popular commercially available green roof substrates to determine their durability in the Mid-Atlantic region. Five mature green roofs (3–7 years old) were sampled in Maryland and fifteen replicates per roof were dried prior to sieving. Analyses showed the substrates no longer met FLL standards for particle size distribution, with particle diameters much smaller than recommended. The authors hypothesized the cause to be weathering due to freeze/thaw cycles, so a laboratory experiment was conducted using traditional expanded mineral green roof substrates. After 30 freeze/thaw cycles, particle size distribution was determined and compared to initial distribution of samples from the same batch of material. Significant weathering was identified through loss of larger particles and an increase in small particles. Decreased particle size distribution will increase water holding capacity and decrease air space in the root zone that could lead to decreased stormwater mitigation potential or plant disease. In conclusion, performance-based standards for green roof materials should be developed based on rigorous scientific study to expand the green roof industry’s knowledge and accountability of total system performance and reliability.