Extensive green roofs are highly technical growing systems that must overcome environmental, physical, and biological challenges in order to function properly. Proper selection of materials for inclusion in the substrate layer is of particular importance. Compost is often included in green roof substrates to increase fertility and water holding capacity. However, compost is a highly variable material whose effects on many other aspects of green roof functionality are poorly understood. A green house experiment was performed to measure the effect of six different composts on the physical properties of substrates, the growth and development of plants grown in those substrates, and the water quality of run off during simulated rain events. Compost selection was found to be an important factor in all of those categories. Conventional substrate materials also represent a significant percentage of the total embodied energy and environmental impact of green roofs. The ecological role and overall sustainability of green roofs would be greatly improved by finding alternatives to the energy-intensive substrate components. Two materials from recycled sources, foamed glass and crushed bulk porcelain, were compared to heat-expanded shale in an outdoor field trial. Total plant coverage in both materials over a two-year observation period was equivalent to coverage in the conventional substrate. Foamed glass was half the density of the expanded shale and porcelain substantially reduced substrate temperature fluctuations.
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