A Green Building Envelope: A Crucial Contribution to Biophilic Cities - Biotechnologies and Biomimetics for Civil Engineering - page 146

Fig. 6.8 Extensive sloped green roof (Berghem, Belgium)

Horizontal green

Extensive green roof
(drought resistant, low growing
speed; sedum, herbs, grass)

Intensive green roof
(higher requirements for water and
nutrients; trees, shrubs)

Fig. 6.9 Basic principles for a green roof strategy

general moss and sedum plants need the thinnest substrate thickness, and grasses
and herbs need the upper limit of 15 cm. The growing medium should provide the
vegetation of sufficient water, nutrients, and oxygen. Maintenance of extensive
green roofs is low, since normally only mosses, sedum, or herbs are growing.
Extensive roofs are limited accessible for people (only for maintenance purposes)
and are less heavy (up to 170 kg/m 2 ) due to the low substrate thickness.
Intensive green roofs can be compared with parks or gardens in terms of plant
diversity and application. They require deeper soil than extensive roofs and regular
maintenance (Kadas 2006 ). This type of green roof, often accessible for people,
requires a high carrying capacity of the structure of the building. The substrate
layer varies from 25 cm or thicker, and must provide sufficient water, nutrients,
and oxygen. The weight of intensive green roofs is usually more than 300 kg/m 2
(Table 6.2 ).
The boundary between intensive and extensive green roofs is vague and
depends on perception. But a common rule of thumb is: non-accessible roofs with
low vegetation are extensive roofs and accessible roof gardens with high vege-
tation are intensive green roofs.