A study of physical and hydraulic properties of green roof substrates including waste materials

Green roofs are becoming increasingly popular in the urban landscape, where levels of air pollution from numerous sources of emissions have rapidly risen compared to rural areas. Green roofs are designed to retain precipitated water, while filtering out pollutants, provide insulation, reduce roof construction costs, be lightweight, contribute to the reduction of temperatures in cities and create habitats for wildlife. When designing green roofs, substrate composition is an important consideration for water retention, plant growth and water quality of runoff. To increase further the sustainability of the green roof systems in terms of material usage, waste materials could be introduced to replace natural soils in the sustrate layer. Current standards however do not include such materials, which appears to be a missed opportunity. In this context, this paper focuses on a number of typical materials used in the green roof substrates and potential suitable alternatives to these, coming from waste streams. For the mixes, commercial growing medium/substrate (control mix) and some common natural or manufactured materials were used, trying to create appropriate substrate mixes in terms of physical and hydraulic properties. These materials were then replaced by waste materials of similar characteristics (e.g., crushed clay brick, paper sludge, and waste glass cullet aggregate, to replace respectively LECA expanded clay, peat and fines such as clay size particles, and sand). For nutrient retention, some small proportion of zeolite was also used in some of the mixes. To assess the suitability of the different mixes with waste materials as extensive green roof substrates, in comparison with commonly used substrate materials, a number of physical hydraulic property tests were performed, as well as germination tests, to attest whether the mixes with the waste materials were adequate for plant growth. The results showed that the studied mixes of different components were overall suitable as green roof substrates and supported plant growth. This encourages further research on the topic (including environmental safety testing, and microbial community studies) towards potential future inclusion of such mixes in green roof design standards.

Please embargo until June 2025