Chronica Horticulturae volume 49 number 2 ... - Acta Horticulturae

A
Figure 3. Three types of green roofs: (A)
extensive, (B) intensive, (C) semi-intensive.
Figure 4. Construction of green roofs. (A) View of protective membrane before placing on
substrate. (Pavillon De Koninck, Université Laval, Québec, Canada.) (Photo by Marie-Anne
Boivin, Soprema.) (B) 100 mm of a volcanic rock based mineral growing medium was blown
on the Maple Leaf Storage Building, Vancouver, Canada. (Photo by Blair Bennet, Soprema.)
B
C
the principal cause of premature failure. A
green roof protects the roof from ultraviolet
rays and studies have shown that they can
extend the life span of membranes by 20 years
or more.
TYPES OF GREEN ROOFS
Generally speaking, green roofs can be divided
into one of three types (Fig. 3). Extensive green
roofs (Fig. 3A) with minimal media depth
require little or no maintenance since they are
comprised of plants chosen for their broad tolerance
such as hardy succulents, or drought
resistant perennials and grasses. Intensive green
roofs (Fig. 3B) resemble traditional gardens and
are composed of a larger variety of plants, such
as shrubs and trees. They can even include
walkways, benches, lighting and bogs. The carrying
capacity of the roof is very important.
Semi-intensive green roofs (Fig. 3C) aim to satisfy
both esthetic requirements and accessibility.
The primary objective is a visual effect and the
plants selected generally require irrigation during
drought, fertilizing, and regular maintenance.
Installation
A green roof system can be installed on flat
roofs or those having a slight slope (Fig. 4). For
slopes of more than 30%, it is necessary to
apply precise methods to avoid soil erosion. The
system generally consists of a waterproof draining
membrane, a drainage panel, a root barrier
and a substrate layer. A capillary mat, which
increases water retention in the substrate, and
insulation layers, can be added to these components.
Several types of materials can be used in
the medium (volcanic pumice, slag, expanded
clay, peat, perlite, sand, and recycled materials).
Substrate depth can vary from 5 to 50 cm,
according to the type of green roof, local climatic
conditions, species desired and the carrying
capacity of the roof.
Costs
The installation of a green roof represents an
important investment. Depending on the type
of green roof, costs can vary from CAN$150 to
450+ /m 2 . Costs can be justified by protecting
the roofing membrane and can have a positive
impact on worker productivity for those having
access to roof gardens. This adds equally to the
overall value of the building, while reflecting
the social and/or environmental commitment of
a company.
Load-bearing Capacity
The main limiting factor in installing a green
roof is the load-bearing capacity of the building
(Fig. 5). When a building is made of cement,
steel or wood, it should be able to support the
weight of the proposed installation, which can
be double even triple when overflowing with
rainwater or melting snow. A substrate depth of
5 to 15 cm can increase the weight of the roof
by 64 to 160 kg per square meter. A structural
engineer must be consulted for all projects. The
roof can be flat or slightly inclined (maximum
35°). It is recommended to build terraces with a
minimum slope of 2% to reduce the depth of
the drainage layer and thus the weight of the
structure.
Plant Selection
The types of plants that can be used for green
roofs vary according to the type of design
required (Fig. 6). In extensive gardens, all plants
selected must be very hardy. Most hardy alpine
plants represent a good choice (e.g. Aster,
Arabis, Allium, Achillea, Dianthus). A large
number of Sedum species are regularly used.
Besides tolerating drought, they often offer
Figure 5. Semi-intensive green roof subirrigated
and planted with perennials in
150 mm of a light growing medium. The
weight of green roof had to be less
than 110 kg per square meter at water
saturation, because it is a retrofit.
(Maison de la Culture Côte-des-Neiges,
Montréal, Canada.) (Photo by Marie-
Anne Boivin, Soprema.)
ISHS • 6