Green roof types, plants, and substrates
Green roofs can be classified into extensive and intensive types,
based on the depth of substrate. Substrate depth of extensive green
roofs is often less than 20 cm; thus, the weight capacity of the green
roof and the selection of vegetation are limited. In contrast, substrate
depth of an intensive green roof can be more than 20 cm,
which can support woody plants. However, owing to heavy loading
and the careful maintenance work required for intensive green
roofs, their use is limited compared to extensive green roofs. In
Taiwan, there is no standard definition or design for green roofs. The
only official recommendation is from New Taipei City, where German
FLL guidelines are recommended. Major concerns in selecting
the type of green roof for use in Taiwan include drainage issues and
possible damage to roof surfaces. Building owners are concerned
that litter may clog drainage pipes and cause flooding on roofs and
plants may root into roof construction materials and result in water
leakage in ceilings. Thus, most existing green roofs are of the extensive
type and a few use potted plants instead of planting vegetation
in beds. The main advantage to using potted plants is that they can
easily be removed and replaced, but the cost is higher per area than
an extensive green roof. Some newly built buildings have roof gardens
for recreational purposes, with trees planted on a small part
of the roof area. Strictly speaking, this type might not be identified
as a green roof because of the small surface area covered by
vegetation. For example, a green roof is required to cover 20–60%
of the roof surface in Toronto according to the Green Roof Bylaw
and 50% is indicated by the Clean Energy Stimulus and Investment
Assurance Act in the USA.
Taiwan’s tropical and subtropical climate promotes extensive
biological diversity, and a great variety of plant species can be used
on green roofs. Table 3 shows the vegetation types commonly used
in green roofs in Taiwan. Plant selection and preservation has a
substantial influence on green roof performance (Rowe, 2011). For
example, vegetation with greater plant depth and root density can
store more water (Dunnett et al., 2008). Runoff water quality also
varies with the type of vegetation (Monterusso et al., 2004).