Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 104

the living roof should not fully freeze). High permeability and inhibiting satura-
tion is achieved with a predominantly coarse-grained medium with large pore
space and low organic content. Air-illed porosity is a measurement method
recommended in the FLL (2008), but experience in developing growing media
using locally available materials in Auckland recognized the inter-relationships
of particle size distribution, organic content fraction, permeability and air-illed
porosity, thus limiting the need to perform all tests speciically (Fassman et al.
2010).
The FLL (2008) requires each candidate growing medium to show compliance
with a minimum “water permeability” according to living roof assembly. Media
for intensive living roofs must provide at least 0.0005 cm/s, extensive media
installed with a separate drainage layer must pass water at a rate of at least
0.001 cm/s, while media for extensive living roofs without a separate drainage
layer must provide at least 0.1 cm/s capacity. These velocities are deined to
prevent ponding subject to typical German conditions (Koehler, personal commu-
nication, 2010), and are recommended to be adjusted for the local climate
(Krupka 1992). Using ASTM or FLL test procedures, Auckland has established a
minimum threshold of 0.05 cm/s for extensive living roofs with a separate drain-
age layer, while maintaining the FLL guidance for those without a separate drain-
age layer (although this coniguration wasn't speciically assessed locally)
(Fassman-Beck and Simcock 2013). The former target was established based on a
combination of testing 32 locally-derived media blends, as well as consideration
of rainfall intensities for a range of events. North Carolina's draft locally-derived
objective is 0.06 cm/s, also derived from ASTM or FLL testing methodologies, and
considering local rainfall intensities (NC DENR draft ).
Double-ring or single-ring iniltrometer methods (e.g., ASTM D3385-09,
Standard Test Method for Iniltration Rate of Soils in Field Using Double-Ring Inil-
trometer) are not considered appropriate for testing living roof media. This
method was developed for in-situ assessment of natural soils, requiring the test
apparatus (the two rings) to be inserted to depths of 50-150 mm. Natural soils
with typically ine textures mean only a narrow gap is created between a ring and
soil during insertion that is easily sealed using compression (i.e., gently pressing
the surrounding soil against the rings). Comparatively, extensive living roof media
are typically very shallow and the coarse aggregates make it dificult to prevent
direct preferential low between the ring and living roof media. As a practical
consideration, once the living roof is constructed, it is very dificult to amend
media should the results of an in-situ test be infavorable. In a ield assessment of
three living roofs, iniltrometer measurements were shown to be signiicantly
greater than laboratory-assessed measurements using FLL (2008) methodology
(Fassman and Simcock 2012).
4.1.3 Installation depth to provide minimum stormwater capture
The depth of media required to satisfy minimum moisture (rainfall) storage
requirements depends primarily on the growing media's water-holding capacity