"A green roof system is an extension of the existing
roof which involves a high quality water proofing and root repellant
system, a drainage system, filter cloth, a lightweight growing medium
and plants.
Green roof systems may be modular, with drainage
layers, filter cloth, growing media and plants already prepared in
movable, interlocking grids, or, each component of the system may be
installed separately.
Green roof development involves the creation of
"contained" green space on top of a human-made structure. This green
space could be below, at or above grade, but in all cases the plants are
not planted in the "ground'. Green roofs can provide a wide range of
public and private benefits."
-- From the Green Roof Healthy Cities website
