Low Impact Development Manual for Michigan - OSEH - University ...

Irrigation
Extensive vegetated roofs with drought resistant plant
species have to be irrigated only during planting and
installation maintenance over the first two years. After
its establishment, the annual rainfall is sufficient to
sustain the vegetation. In contrast, the requirements are
more involved for intensive vegetated roofs with lawn,
shrubs, or trees. An adequate number of precisely dimensioned
hoses with automatic irrigation units make plant
maintenance during drought periods more manageable.
The water supply for roof gardens with no slope can be
increased through additional dam-up irrigation. Vegetated
roofs can also be irrigated with cistern water.
Fire prevention
As a part of the “hard roof” classification, intensive
vegetated roofs provide preventative fire protection
in the case of sparks and radiating heat. The criteria
that extensive vegetated roofs must meet in order to
be considered fire resistant, are already met by most
vegetated roof systems that are offered by suppliers.
Openings within the vegetated roof (e.g. skylights)
need to be installed with a vegetation free zone (approx.
20 in). On larger roof areas a vegetation free zone (e.g.
gravel strip or concrete slabs) are to be installed at least
every 130 feet.
Vegetation Considerations
Extensive vegetated roofs
Plants for extensive vegetated roofs have to survive
intense solar radiation, wind exposure, drought, low
nutrient supply, freezing temperatures and limited root
area. Suitable plant varieties are those growing in severe
locations with little moisture and nutrient supply, such
as dry meadows. The main varieties are sedum, and
delosperma. The plants are able to store high amounts
of water in the leaves, are stress resistant and recover
easily from periods of drought. Other varieties such as
dianthus species, asteraceae and ornamental grasses are
also suitable for these conditions.
Plugs prior to planting extensive vegetated roof
Source: Mallet’s Creek Library, Ann Arbor, MI
Intensive green roofs
Having an appropriate vegetated roof system and sufficient
growing medium (with higher root penetration
volume, nutrients and water supply) growth of sophisticated
plant varieties on the roof is possible. The selected
plants need to be resistant to intense solar radiation and
strong winds. Vegetation with various plant varieties
such as perennials, herbs, grasses and trees allow
for a natural character on the roof. Having a broader
plant community increases the amount of maintenance
required.
Conventional roof prior to retrofit
Extensive vegetated roof cover retrofit incorporating a patio
for viewing
LID Manual for Michigan – Chapter 7 Page 311