Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 120

Flat roofs require at least 2 percent pitch to promote positive drainage. In
practice, a 2 percent pitch may be dificult to achieve; construction methods
must avoid localized slope defects that may ultimately result in localized ponding.
In addition to detailed assessment of roof surface levels, conirming the sealant
rings around the drainage points are installed lush with the roof's substructure
ensures that water can drain away without obstruction.
If the living roof is graded into mounds for aesthetic reasons, or on steeply
pitched roofs, drainage is even more rapid, and there is less water availability for
plants on the ridgeline compared to the bottom of the slope. Creating mounds
enhances plant growth and survival by providing areas with greater water reten-
tion and may reduce ET of plants in hollows by increasing shelter from wind (i.e.,
increasing the width of the boundary layer). When selecting plants for steep
sloped or undulating living roofs, plants with lower water needs should be
planted at the ridgeline, while plants with higher water needs should be planted
at the bottom of the slope. Further water retention can be achieved by water
retention techniques described under Section 4.2 .