Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 112

Figure 4.2
Wet versus dry climate-
tolerant plants
Wet climate
Dry climate
planted. If stormwater attenuation requirements can be met with a partial living
roof, the conventional roof would be best placed on the “hot” aspect (for a
pitched roof) and the cooler aspect covered with a living roof. In some cases
there may be a balance between the extent of roof greening to meet stormwater
mitigation objectives and maintaining a year-round healthy appearance if only a
portion of the roof is greened. Deciding which side the living roof occupies may
not be quite so simple; factors such as visibility and viewsheds, among others,
should also be considered.
4.3.2 Plant structure - root system and biomass
The authors are not aware of any jurisdictions where differential stormwater
credit for living roofs is allocated based on plant types or characteristics. However,
aspects of a plant's architecture are nonetheless important to the overall success
of the living roof project.
Three design priorities are related to plant root structure and biomass:
1 Protect the membrane from root invasion and the integrity of drainage ele-
ments from root blockage. Bamboo roots are renowned as aggressive. Trees
such as birch in North American and pohutukawa ( Metrosideros ) in New
Zealand have been observed to freely establish on living roofs and should be
removed immediately.
2 Avoid lammable plant biomass, in particular large grasses, including annual and
biennial plants that leave high biomass when they die, unless maintenance
removes the excess and/or dead material. Extensive living roofs in Shefield, UK,
combining “pictorial meadows” and wildlife habitat are managed with an annual
winter harvest or mow. This mirrors maintenance of the 90-year meadows estab-
lished on water reservoirs in Woolishofen in Zurich, Switzerland. In Taupo, New
Zealand, a grass sod living roof was irrigated to prevent it from becoming a ire
hazard, which exacerbated biomass production in the short term.