Green Roofs - Living Architecture Monitor - Spring 2020 - 24

VEGETATION LAYER / VEGETATED MAT

GREEN
ROOF
SYSTEM
LAYERS

GROWING MEDIUM
FILTER FABRIC / RETENTION FLEECE
DRAINAGE LAYER / GEOTEXTILE
ROOT BARRIER

Image courtesy C Rugh

is because as the column height (= media depth) increases, the force
of gravity on the water column, also called hydraulic head, leads to
greater water loss at the upper levels of the media profile beginning
immediately after a storm. In other words, the deeper the media
profile, the drier the uppermost levels will be.
Though the growing medium layer may hold the largest fraction of stored water in the green roof assembly, virtually no green
roofs achieve all their water holding capacity from media alone.
Other components - e.g. drainage layer, fleece fabric, and selected
plant types - contribute extended and enhanced water detention
performance. Drainage layer materials can be coarse aggregates,
plastic sheets with molded wells, or open-flow geocomposites
with sorptive fleece fabrics. Use of a coarse aggregate drainage
layer provides an additional sorptive layer; though also has a concomitant increase in overall loading weight. Molded-plastic drain
sheets have built-in "cups" to collect percolated water, though
lose storage capacity as cups fill with fine particulates or roots.
Woven geotextiles and non-woven fleece fabrics possess water
holding capacity and restrict hydraulic flow to increase media
moisture content and delay runoff. Horticultural mineral wool
media products possess an extremely high water-holding capacity
relative to dry weight, so enhance storage and media profile moisture content. The obvious defining characteristic and most visible
feature of a green roof is the vegetation, which on extensive
green roofs is generally a mix of low-profile succulents, though
may include virtually any local climate adapted plant species in
a properly designed green roof assembly. The green roof plant
community stores water in above and below ground tissues and
transpires water vapor to "recharge" storage capacity between rain
events. Many plants have specific rooting depth and soil moisture
requirements, which influence design of growing media formulation, profile depth, and use of other component layers; each of
which impacts water storage capacity, assembly load weight, and
runoff characteristics.
LIVING ARCHITECTURE MONITOR / SPRING 2020 / 24

By ignoring the complete green roof assembly, prescriptionbased criteria do not adequately represent the benefits of other
non-aggregate or alternative media components. This may
bias toward overbuilt, heavier green roofs; which not only are
more expensive to initially build, but in many cases require
substantially more maintenance support. Failure to consider
storage-efficient, lighter weight assemblies may disqualify
older or expansive buildings for more effective green roof
implementation in critical urban watershed districts. The
primary goal for municipal green roof policy funding should
be to increase coverage with a flexible toolbox of demonstrated
stormwater beneficial designs rather than a "one-depth-fits-all"
approach.
On the other hand, performance-based policies focus
directly on the municipal stormwater management goal by
recognizing that each component of the green roof assembly
helps to achieve the program goals. Unfortunately, failure to
consider the entire green roof profile as part of the stormwater
treatment blocks lighter weight, ecologically advantageous,
lower maintenance, lower cost, and site compatible green roof
designs.
Municipal green roof incentive programs have provided a
critical service for urban communities and the essential stormwater treatment infrastructure their tax dollars support. Many
government programs have been creative and visionary in their
development and promotion of smart green infrastructure policies, including performance-based green roof criteria. Publicly
funded programs should adopt the most comprehensive and
versatile design requirements to fully realize the benefits of
these proven technologies.
Dr. Clayton Rugh, Director, XeroFlor America. clayton@xeroflora.com
Visit LivingArchitectureAcademy.com for Green Roof and Stormwater
course.
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Green Roofs - Living Architecture Monitor - Spring 2020

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