Green roof media

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ASTM International have a number of standards relating to various design considerations for green roofs. These standards provide good technical advice on the testing of systems and components. Of particular note are:

  • E2399 Standard Test Method for Maximum Media Density for Dead Load Analysis of Vegetative (Green) Roof Systems[1], and
  • E2396 Test Method for Saturated Water Permeability of GranularGravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices. Drainage Media [2].

When these tests are completed, the results should be interpreted in relation to the objectives of the green roofA thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.. A product complying with overseas guidelines may not serve the needs of a green roofA thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens. installed in Ontario. In particular, the FLL guide recommends green roofA thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens. media specifications which may not provide optimal stormwater management or vegetation in our region. The green roofA thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens. media used in Ontario can be classed according to proportion of composted biological material. Some existing installations use materials which comply with FLL guidelines, whilst others use a much higher proportion of compost.[3]

In many proprietary systems the default option for planting medium will be a granularGravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices. material with very low organic matter content. However, many companies can arrange for a high organic matter alternative to be substituted if requested.

Considerations for using a 'high organic' planting medium
Benefits Disadvantages
  • Increased water holding capacity, which benefits both vegetation health and stormwater retention
  • Often lighter weight, even when saturated
  • Increased phosphorus concentration in runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. water, however this may be mitigated by the reduced runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. volume resulting in a lower loadingThe total mass of a pollutant entering a waterbody over a defined time period.The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration. overall
  • The lightweight material is more prone to wind erosion(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc)., and this should be accounted for in the initial design, alternatives include erosion controlIncludes the protection of soil from dislocation by water, wind or other agents. blankets, soil tackifiers, or Sedum mats.
Increasing the depth of planting medium from 10 cm to 15 cm has been shown to benefit the vegetation [1]; stormwater retention was not improved with this increase in depth [2]
  1. MacIvor JS, Margolis L, Puncher CL, Carver Matthews BJ. Decoupling factors affecting plant diversity and cover on extensive green roofs. J Environ Manage. 2013;130:297-305. doi:10.1016/j.jenvman.2013.09.014.
  2. Cite error: Invalid <ref> tag; no text was provided for refs named Hill