Green Roofs - Living Architecture Monitor - Spring 2020 - 23

water storage of many green roof assemblies, so the prescriptivebased approach assumes the media layer depth serves as a proxy for
the stormwater management capacity of the system.
Most green roofs are a "deep-dish pizza" of functional components: vegetation, growing medium "engineered soil", filter
fabric / capillary fleece, drainage layer, and root barrier, each of
which are essential for plant survival and long-term roof system
performance. Of these components, the growing medium layer
has the broadest range of options and most significant impact
on stormwater management performance. Growing medium is
a fabricated material, in that it is not made solely from raw soil
materials; rather is a mix of processed and/or earthen products
such as heat-expanded porous shale/clay/slate (ESCS), composted
organic matter, and sand. Green roof growing media are usually
coarse aggregate mixes, though may also include fabricated sheets
or layers, such as horticultural mineral wool. Growing media
composition and media depth should be designed to accommodate the rooting profile and physiological needs of the selected
plant palette across a range of material types and functional
parameters: type of aggregate mineral (i.e. manufactured heat-expanded minerals, recycled materials, or naturally-occurring earth
products), aggregate particle size, type of organic matter (OM),
OM material fragment size, ratio of media components (e.g. 80%
aggregate: 20% OM v/v), inclusion of special admixture ingredients (e.g. clay, lime, pelletized fertilizer, etc.). Growing media vary
PERFORMANCE - In performance-based policies, the complete
by capillarity (= wicking), porosity (= aeration), and sorptivity (=
green roof assembly must achieve a saturated water holding
wettability); all of which impact the material density, water holdcapacity equivalent to a target rainfall retention quantity (e.g. 1"
ing capacity and horticultural compatibility. As these properties
rain event). Since the primary goal of these programs is to support
vary dynamically among individual green roof media components,
stormwater treatment infrastructure function, performance-based
it is recommended that incentive program applications require
criteria directly address this goal to offer additional advantages for
test-certified metrics for the water holding characteristics of each
broader implementation.
proposed medium rather than assign an assumed water content
to the media layer. It is important to note that water content at
PRESCRIPTIVE - Prescriptive policies often state that a proposed
green roof must have a stated growing medium depth (e.g. 4" media greater media depths does not equal mathematic multiplication of
a unit water holding capacity: i.e. six inches of media does NOT
profile) to be eligible for program support or permit approval.
result in six times the saturated water content of 1-inch depth. This
Growing medium is often the most significant component for
the surface, and heavier rains pond in the depression, allowing much
of it to soak into the ground (retention). Only the largest volume of
storm flow surcharges the rain garden and is released downstream.
The limited weight bearing capacity of most buildings as well as
maintenance and water quality issues makes rooftop "lakes" impractical, so detention on green roofs is a more technically feasible and
cost-effective strategy. Use of green roofs as an effective stormwater
management BMP uses their sponge-like properties to soak up
and delay runoff to roof drains, though some systems have longerterm detention elements as part of the integrated assembly, often
referred to as blue-green roofs. While an individual green roof
cannot offset all the stormwater impacts within a local watershed,
each successive installation further reduces runoff volume and flow
intensity that would otherwise contribute to CSO events.
Municipalities have established incentives for green roofs with
government programs offering financial incentives, zoning and
permitting advantages, or legislative mandates for new buildings.
(https://greenroofs.org/policy-document). Incentive programs
have a variety of policy criteria, such as: particular roof size class,
priority sewerage district location, new or retrofit construction
projects, and other possible project or BMP performance requirements. Green roof policies fall under one of two different strategies
to achieve their municipal stormwater management goals:

"IT IS RECOMMENDED THAT INCENTIVE PROGRAM APPLICATIONS REQUIRE
TEST-CERTIFIED METRICS FOR THE WATER HOLDING CHARACTERISTICS
OF EACH PROPOSED MEDIUM RATHER THAN ASSIGN AN ASSUMED WATER
CONTENT TO THE MEDIA LAYER."

- CLAYTON RUGH

LIVINGARCHITECTUREMONITOR.COM

LIVING ARCHITECTURE MONITOR / SPRING 2020 / 23

https://www.greenroofs.org/policy-document http://www.LIVINGARCHITECTUREMONITOR.COM

Green Roofs - Living Architecture Monitor - Spring 2020

Table of Contents for the Digital Edition of Green Roofs - Living Architecture Monitor - Spring 2020