Green roof media

ASTM Interantional 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 Granular Drainage Media [2].

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 [3] recommends green roof media specifications which may not provide optimal stormwater management or vegetation in our region. 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 high proportion of compost.[4]

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 content alternative to be substituted if requested. The benefits of 'high organic' media are:

1. Increased water holding capacity, which benefits both vegetation health and stormwater retention,
2. Often lighter weight, even when saturated.

The disadvantages of a 'high organic' planting medium are:

1. Increased phosphorous concentration in runoff water, however this may be mitigated by the reduced runoff volume resulting in a lower loading overall,
2. 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[5]. Stormwater retention was not improved with this increase in depth[6].

Gravel, 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.

A 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.

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

Gravel, 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.

The portion of water precipitated onto a catchment area, which then 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.

The 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.

(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).

Includes the protection of soil from dislocation by water, wind or other agents.