Green roof media

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The green roof 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[1].

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:

  • Standard Test Method for Maximum Media Density for Dead Load Analysis of Vegetative (Green) Roof SystemsE2399, and
  • Test Method for Saturated Water Permeability of Granular Drainage Media E2396.

When these tests are completed, the results should be interpreted in relation to the objectives of the green roof. A product complying with overseas guidelines may not serve the needs of a green roof installed in Ontario. In particular, the FLL guide recommends green roof media specifications which may not provide optimal stormwater management or vegetation in our region. In many proprietary systems the default option for planting medium will be a granular 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', compost based planting medium
Benefits Disadvantages
  • Increased water holding capacity, which benefits both vegetation health and stormwater retention
  • Lighter weight, even when saturated
  • Increased phosphorus concentration in runoff water, however this may be mitigated by the reduced runoff volume resulting in a lower loading overall
  • The lightweight material is more prone to wind erosion, and this should be accounted for in the initial design, alternatives include erosion control 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 [2]; stormwater retention was not improved with this increase in depth [3]

  1. Hill, J., Drake, J., and Sleep, B. (2016). “Comparisons of extensive green roof media in Southern Ontario.” Ecological Engineering, Elsevier B.V., 94, 418–426.
  2. 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.
  3. Hill, J., Drake, A. P. J., Sleep, B., and Margolis, L. (2017). “Influences of Four Extensive Green Roof Design Variables on Stormwater Hydrology.” Journal of Hydrologic Engineering.