Introduction - Living Roofs in Integrated Urban Water Systems - page 11

sphagnum, coir, wool, jute or felt. Foams and other synthetic moisture retaining
materials are emerging in the marketplace. Retention layers have variable longev-
ity and are likely to become less effective over time as they decompose. Protect-
ing the material from UV exposure, such as by placing it at the base of the
growing medium, should help to extend its useful life.
1.4.4 Drainage layer
A distinct drainage layer under the growing medium promotes free drainage to
the roof outlets (scuppers and downpipes). Separate drainage layers are often
made from synthetic mats or rigid boards, with a bonded (attached) geotextile,
and may also contain a root barrier. Alternatively, granular material such as a
lightweight coarse aggregate (e.g., pumice) or non-reactive recycled materials
may be used. In this case, a separate geotextile should still be placed between
the drainage layer and the growing medium.
While the drainage layer itself does not provide waterprooing, it provides
physical protection from shovels or other gardening implements that might
damage waterprooing during planting or maintenance. Many drainage layers
are made of synthetic mats (egg crate or spider web shaped) providing in excess
of 50 percent of their material volume as airspace. These mats provides air circu-
lation for plant roots, and an evaporation zone to keep roof insulation of inverted
living roof assemblies dry (see Chapter 3 ). A drainage layer prevents free water
on the roof, minimizing potential freezing damage in cold environments.
1.4.5 Geotextile separation
The geotextile helps to hold the growing medium in place, preventing it from
compromising the effectiveness of the drainage layer. The geotextile does not
typically act as a ilter for pollutant capture. A geotextile bonded to a synthetic
drainage layer tends to be more easily installed than a separate geotextile layer
(which would likely be required if an aggregate drainage layer is speciied).
1.4.6 Growing medium
The growing medium is perhaps the most important functional component of a
living roof system in terms of stormwater management. It is sometimes called sub-
strate or engineered media. The growing medium provides the main “sponge” for
rainfall storage on the rooftop. Water captured by the growing medium is subse-
quently returned to the atmosphere via ET, rather than becoming runoff. Even
when the moisture (rainfall) storage capacity is full, encouraging a tortuous low
path through the growing media (rather than across the surface) reduces the veloc-
ity of runoff compared to the rainfall intensity or a conventional roof's runoff.
Over and above stormwater beneits, the growing medium supports plant
vitality physically and nutritionally. The growing medium continually contributes
to energy demand mitigation because it provides thermal mass when wet, and
insulation when dry. In many cases, it can be used to provide ballast against wind
uplift.