Planning considerations - Living Roofs in Integrated Urban Water Systems - page 59

The location and number of gutters and drains ensures eficient discharge of
the stormwater from the roof, which is a safety issue. Although the living roof
will produce less runoff than from a conventional roof, surface drainage will still
be required to prevent drowning plants and to avoid ponding. Drainage
calculations are typically found in building codes with minimum rules for
compliance. In the retroit scenario, additional drainage may be required,
depending on the design, but additional drains should be avoided. This is
because the coordination of additional drains may be problematic for the spaces
immediately below the roof (where drains travel down a building through ceilings
and walls). Dropping ceiling heights may not be feasible because of headroom
issues, and existing walls may not be wide enough to accommodate additional
drainpipes. If additional drainage is required, gutters may be a more practicable
solution, as they remain exterior to the building. In the new project scenario, roof
drain allocation is signiicantly less problematic, as the location of drains can be
made to correspond strategically with the architectural elements below. For more
information on the design of drainage channels and gutters see Section 4.6.3 .
The location and method of drainage inluences the aesthetic appearance of
the new and retroit living roof surface. A gutter will appear as a linear structure,
either through or around the planting. Drains will develop a pattern on the roof
surface with visible inspection chamber lids. Both should be integrated into the
living roof surface design of both soft and hard landscapes. Gutters should be
integrated into the paved maintenance or gravel strip areas to aesthetically blend
in with similarly-colored materials. Drains with inspection chamber lids should be
screened with planting. The structural and mechanical engineers together with
the architect, landscape architect and roof installer have to design the roof in
such a way that the increased load is carried by the roof in an extreme rain event,
and be able to remove the excess water quickly. As an alternative on a new roof,
multiple drainage points with alternating roof slopes can remove the stormwater
and therefore balance out the load.
Generally, clients will favor lower drain counts to minimize installation and
maintenance cost. Each penetration of the roof's waterprooing membrane is a
potential weak point. Sealing the interface between the perforations/protrusions
( Figure 3.4 ) and the roof provides the most dificult waterprooing challenge.
Altogether minimizing the number (while complying with relevant building codes)
and determining the appropriate placement of roof drains can be addressed by
the design consultant team together with roof installers.
The number and location of perforations through the roof on a new roof and
retroit, such as air conditioning vents or an elevator machine room, require
address, as their accessibility needs to be ensured. Since perforations are in most
cases ixed on retroits, their locations can have implications on the roof's overall
aesthetics. Both the hard and soft surface design of the living roof may have to
be designed to accommodate roof perforations.