Physical and chemical parameters of various waste materials for living roof systems: A critical review

Abstract

Living roof systems are attractive nature-based solutions that effectively use underutilised roof space to help mitigate urban environmental, economic, and social challenges. Although living roofs offer many thermal and water conservation benefits, as well as ecological habitats, designs typically use substrates manufactured from materials not always locally available and consist of mined or processed materials, which can command large carbon footprints and create other negative environmental impacts. Therefore, waste materials are being explored as suitable alternative substrates that will enhance a more circular economy in times of increasing natural resources scarcity, whilst also supporting numerous sustainable development goals (SDGs). This review comprehensively assessed the critical physical and chemical parameters of a range of waste materials required for application in living roof systems and compared the data to the German FLL Guidelines for living roof systems. It also assessed the ability of the waste materials to support plant life under different climates. Waste materials were categorised according to their source, both in terms of location and industry, to enable understanding of the different categories and regions where suitable waste resources are generated. Analysis showed that waste materials trialled in living roof systems were more frequently sourced from construction, agriculture/forestry, and manufacturing/processing industries compared to industrial waste and mining industries, likely due to the prevalence of these industry types across more regions while industrial and mining materials are more geographically restricted and have a perceived contamination potential. Studies reported that while organic matter (OM) increased water retention, plant growth, and bulk density, some trials found that crushed concrete and crushed brick with much lower organic content had sufficient water retention for supporting plant growth. Most trials recommended that OM should be limited in living roof substrates to prevent excessive growth, nutrient leaching, and substrate compaction. Porosity was found to strongly influence water retention and plant survival in dry periods whereas water holding capacity (WHC) reflected how much water is held and made available to vegetation. In substrates with 5–20% OM, WHC was greatest but this varied depending on the type of organic material used. Vegetation grown in substrates containing crushed brick and/or construction and demolition waste were less affected by drought-like conditions due to better WHC. Of all the waste sectors, interestingly the fishing/seafood sector was rarely considered as a source of living roof materials yet it produces large volumes of waste resources destined for landfill. Analysis of waste seashells revealed that they have properties beneficial for supporting living roof systems, such as water retention and high porosity, so could offer a new stream of waste resources for living roof systems.