Applying Simple Techniques To Develop Smarter Green Roof Substrates

Sourced from Science Trends

Urban sprawl and dense urbanization are creating problems with how we manage our stormwater.  Large areas of impermeable surfaces (i.e. paved roadways, paths, buildings, and loss of green spaces) combined with more intense and frequent storms that are expected to occur in the coming decades, has forced city developers and landscape architects to seek new solutions. One of those solutions, the green roof is elegant in its simplicity and visual aesthetic once it has been established.

Simply put, a green roof is a system comprising multiple layers to simulate natural soils.  This idea is not new; it dates back decades to traditional sod roofs in Scandinavian countries. The modern green roof has been adapted to fit our more complex world, incorporating new materials and new plant species to improve its efficiency and performance. The roofing layers consist of: (1) a vegetative (plant) layer, (2) artificial (or sometimes natural) soils, and (3) protective layers to prohibit mineral losses via runoff, plant root movement, and damage to the underlying roof.

Green roofs aid in the interception of stormwater, slowing the time that that water will enter stormwater management systems (i.e. sewers), and stopping some of that water from entering in the first place. This reduces the strain on management systems and can prevent localized flooding. The intercepted water is then retained in the green roof soil to be used by growing plants, converting CO2 to oxygen, reducing the Urban Heat Island, cooling buildings as the water evaporates taking the excess heat with it, and creating a green space for pollinators or community gardens.

However, with innovative solutions come unknowns that need to be considered to prevent further negative impacts on the environment. One of these is the movement and loss of nutrients from roofs that have been (over-)fertilized to a point where the substrate becomes saturated or their maximum adsorption capacity becomes exceeded in the short term. Nutrient runoff can pollute our surface waters, such as when phosphorus enters our waterways, resulting in algal blooms, the “lake dying” effect, and deadly decreases in dissolved, biological oxygen concentrations.

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