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Green roofs extend roof membrane life and reduce waste to landfills. However, green roof costs must be reduced if their benefits are to accrue more widely. Use of recycled materials may reduce costs and also keep those materials out of landfills. Some work has been done on use of local recycled materials for green roof substrates, but none describe the characteristics, proportions and results of using an entire suite of blended recycled materials in admixtures (i.e., mixtures of very different materials) such as crumb rubber (CR), crushed used brick (CB) and compost (CPT) in concert with greens grade sand (#10), biochar (B) and topsoil inoculum (T). How does plant growth and performance on recycled substrates compare with a typical expanded shale and clay (ESC) substrate?
Two main techniques for measuring performance of green roof treatments are plant biomass and plant coverage. Both measure plant growth and performance, while coverage specifically addresses FLL (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau Guidelines for the Planning, Construction and Maintenance of Green Roofing) standards. This research project tested native plants on recycled substrate admixtures to assess the correlation between these two measures. It was found that the biomass and cover were strongly correlated at 0.665 (P <.0001). Recycled substrate admixtures were found to be heavier, hold less water and had significantly less biomass and cover than the proprietary green roof substrate used as a control, but their native plantings still held the substrate in place. It is suggested that compost is a key ingredient for biomass production and future recycled green roof substrate might use recycled crumb rubber, biochar and compost for lighter substrate loading.