Before the green roof installation can begin, a licensed structural
engineer or architect should be hired to determine what, if any,
upgrades need to be made to the existing roof. Due to the weight
of green roofs, some buildings cannot initially support them. Consultants
will determine the weight capacity of the building, type of roof
membrane, current condition of the roof, height above ground, roof
slope and orientation, water supply, and accessibility for installation
and maintenance. Additionally, the consultants will determine what
irrigation and drainage systems are needed.
Overall, the initial cost of installing a green roof usually includes
consultant fees; irrigation and drainage systems; garden materials;
plants; and maintenance, transportation and city permits. The average
cost varies because of the different green roof options. The weight
load of the building and the plantings desired will determine which
type of green roof is necessary. The options include either an extensive
or intensive green roof. The main differences between these two
types of green roofs are the growing media depth and organic contents—which
in turn affects the cost and planting options.
With a growing media depth of three to six inches, extensive green
roofs are less costly and are only ideal for growing drought-tolerant
plants such as sedums and grasses. Little maintenance is required
on an extensive green roof. An intensive green roof contains a media
depth of a foot or more; thus, an intensive green roof can support
plants, ground covers, ornamental grasses, shrubs and trees. Like
any garden, maintenance such as watering and weeding is required.
For both types of green roofs, plant selection must be adjusted
to local environments and conditions.
A common misconception of green roofs is the thought that you can
use any type of soil for the growing media. Soil alone does not
provide the proper characteristics for a green roof to survive.
The growing media must be designed and manufactured to provide the
nutrients, water-holding capacity and drainage characteristics to
promote a sustainable green roof environment. An example of a type
of specifically designed growing media for green roofs is rooflite.
Although the growing media is often the biggest expense of installing
a green roof, it is the most important aspect because the green
roof will not survive without it.
While the initial cost of installing a green roof might be high,
the cost savings realized in the long run are tremendous. The roofing
membrane will have a longer life span because a green roof protects
the roof from ultraviolet rays, severe temperature changes, and
physical and wind damage. A green roof saves energy costs because
of the added insulation, meaning less heat in the winter and air
conditioning in the summer. Additionally, while more maintenance
may be necessary on a green roof, fewer repairs are required overall
because of the protection green roofs provide. Lastly, depending
on the city, often there are reductions in city fees.