Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 116

environment. The applied growing medium will reduce stormwater runoff,
but alone it might not be suficient for optimal performance, if immediate
stormwater management results are desired or needed. In Germany, fast-
growing plants have been advocated to increase the beneits of a living roof
shortly after construction (Umweltbundesamt 2012). Alternatively, pre-grown
modules or mats might be appropriate.
• Accentplantscanbeimplementedtoincreasethethree-dimensionalaspect
of the roof and enhance seasonal changes. Annuals can be used to ill the
gaps while the permanent plant cover establishes. Their primary role is to
enhance the visual appearance of the roof during the establishment period
(irst 18 months). Thus they may help to increase the acceptance of the roof
and with it encourages interaction and proper maintenance. Accent plants
may include summer-dormant bulbs which lower early in spring, and long-
lived, slower-growing plants that may be established at low densities. When
using bulbs, plant them in dispersed areas so the unvegetated gaps, created
when bulbs are dormant, are small.
• Nativeplantsshouldbeencouragedtopromotebiodiversity.Anyadaptable
plant choice that increases the aesthetic appearance and biodiversity is a good
choice. But it must be recognized that sedum species play an important role in
the “plant toolbox” and in some climate cases may be the only choice where
growing media depth is limited.
• Thelivingroofhabitatisoftennotabletoaccommodateawiderangeof
native plants, as there may be few similar natural environments in the vicinity.
Limited commercial availability of native plants at low unit prices may also
restrict their use. In this case, the surrounding area on the ground should be
integrated into the design of the living roof. If some of the indigenous plants
cannot thrive on the living roof, the surrounding area at ground level may
provide a natural environment for wildlife and plants (Butler et al. 2012;
Snodgrass and Snodgrass 2006). An important aspect of plant selection in
new areas is investigating local habitat templates, and including some local
species at low densities to increase understanding of their performance.
4.5 DRAINAGE LAYERS
Modern drainage layers inluence the stormwater mitigation ability of the roof.
Historically, living roofs did not have drainage layers (Ngan 2004). The living roof
was a single-layered, soil-based system, mostly planted with grass or tufts to act
as waterprooing and rooing insulation material. Today, multiple elements ensure
adequate drainage and prevent standing water on the rooftop in the interest of
public safety. One aspect is through the design of the growing medium; another
is to provide distinct drainage layer under the growing medium to promote free
drainage to the roof outlets and downpipes.
Drainage layer types for a living roof are appropriate depending on design
objectives and on the following: growing medium (i.e., its water-holding capacity),