Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 95

These recipes may be carefully guarded. Non-proprietary media may be equally
effective, and new media may be developed. For example, in New Zealand, the
lack of a proprietary market led to signiicant research on local materials, testing
procedures, sourcing and supply, and installation techniques (Fassman and Simcock
2012; Fassman et al. 2010, 2013; Fassman-Beck and Simcock 2013). Media trans-
port cost can be substantial, therefore often the most cost-effective supply will
usually be from the closest source, particularly for the coarse materials that makes
up the bulk of a medium.
4.1.1 Composition
Vegetation-based SCMs such as extensive living roofs and bioretention typically rely
on engineered media to achieve consistent, baseline stormwater management out-
comes. Plant evapotranspiration (ET) provides an additional, but variable, contribu-
tion to stormwater management depending on climate and available moisture.
Engineered media refers to a blend of materials that would not be naturally found
as a unique mixture, while the individual components may be naturally sourced or
manufactured. Physical and hydraulic properties of engineered media, and the
ability to support vegetation, differ substantially from most natural soils. Engineered
media for extensive living roofs should be comprised of 80-95 percent (measured
by volume, noted as percentage v/v) lightweight aggregate (LWA) and 5-20 percent
v/v resilient organic matter. Use of standard aggregates (e.g., sand) leads to much
heavier roofs requiring greater structural support.
In Europe, expanded clay is the most common form of LWA, whereas
expanded shale or expanded slate is used in North America, and expanded perlite
is sourced from Australia. Expanded minerals are manufactured aggregates, and
often used in lightweight concrete as well as other horticultural applications. To
expand the raw mineral, it is ired in a rotating kiln at high temperatures, causing
internal moisture to vaporize and escape. The result is a porous, low density,
high-strength, ceramic-like LWA. Volcanic deposits in North America's Paciic
Northwest, Iceland, France, New Zealand and Australia provide naturally sourced
pumice and some sources of scoria, lava and zeolites that are LWA that do not
require supplemental expansion.
There is growing interest in the use of recycled materials as media compo-
nents. In the UK, researchers have studied the potential to recreate ground-level
habitat using recycled demolition materials for the growing medium (Bates et al.
2009, 2013; Olly et al. 2011). Materials such as recycled brick and crushed con-
crete are not lightweight, but where supplies are abundant, the approach sup-
ports sustainable reuse of materials (Fentiman and Hallas 2006). Other recycled
products investigated for use include recycled rubber chip or tire crumbs, polysty-
rene, recycled plastic beads, and ly ash (clinker) from coal-ired power stations.
Some companies use plastic foams or recycled foam rubber chip as part of a
growing media. Most sands are not lightweight (compared to natural or manu-
factured LWAs); ine sand can reduce permeability if added in signiicant propor-
tion (>5 percent v/v) (Fassman and Simcock 2012).