Green Roofs - A Primer Explains Benefits and Types
1. GREEN ROOFS – A Primer
By Heather Ellis, FLC Environmental Center
‐Fall 2007‐
OVERVIEW
Green Roofs are essentially thin layers of living vegetation installed on top of conventional flat or sloping
roofs. Green roofs protect conventional roof waterproofing systems while adding a wide range of
ecological and aesthetic benefits. They are a powerful tool in combating the adverse impacts of land
development and loss of open space. A well‐planted roof absorbs solar radiation and CO2, decreases
storm‐water runoff, provides valuable insulation, and numerous other benefits (economic,
environmental, and aesthetic).
Green roofs can take a thousand different forms depending on location, but their structure is pretty
basic: a waterproof layer over a standard roof, topped with drainage materials, soil, and finally, the
plantings, which vary from region to region. (New, modular systems allow one to create a green roof
from premade blocks, greatly simplifying construction). Your green roof may be a thick layer of grass, or
a garden with food, to a forest‐like rooftop.
BASIC TYPES OF GREEN ROOFS
Types of green roofs fit into two basic categories: (1) intensive or (2) extensive. This categorization is
based on soil depth and also determines a variety of factors including weight, cost, maintenance, plant
selection and function.
Intensive green roofs require a minimum of one foot of soil depth and generally require more effort for
the maintenance of plants and proper watering schedules. Intensive green roofs involve a greater
diversity of plants, including trees and shrubs. These roofs emphasize the use of space and therefore
raise higher aesthetic expectation than more functional extensive green roofs. There are certain
degrees of intensiveness in the approach to rooftop greening. Intensive green roofs are generally more
expensive than extensive green roofs.
Extensive green roofs are roofs that call for a more passive approach than intensive. The vegetation
used within an extensive roof is extremely resilient, providing a lush carpet‐like foliage. Plants for an
extensive green roof are those that require only a few inches of growing medium, from 2.5 to 6 inches.
Extensive green roofs can be established on a very thin layer of “soil” (most use specially formulated
composts): even a thin layer of Rockwool laid directly onto a watertight roof can support a planting of
Sedum species and mosses. Sedum species are plants with a waxy leaf that retains moisture. An
extensive green roof will only require minimal maintenance and irrigation will only need to be addressed
in drought conditions. Extensive green roofs are designed to be virtually self‐sustaining.
2. Table 1. Characteristics of Intensive and Extensive Green Roofs.
Characteristics of Intensive and Extensive Green Roofs
Intensive Extensive
Characteristic
Green Roof Green Roof
Requires minimum of Requires only 1 to 5 inches of
SOIL
one foot of soil depth soil depth
Accommodates large Capable of including many
VEGETATION trees, shrubs, and well‐ kinds of vegetative ground
maintained gardens cover and grasses
Adds only 12‐50 pounds per
Adds 80‐150 pounds per
square foot depending on
LOAD square foot of building
soil characteristics and the
load to structure
type of substrate
Regular access
Usually not designed for
ACCESS accommodated and
public accessibility
encouraged
Annual maintenance walks
Significant maintenance
MAINTENANCE should be performed until
required
plants fill in
Includes complex
Irrigation and drainage
DRAINAGE irrigation and drainage
systems are simple
systems
Questions:
What is the soil depth required of different native species?
What is cost savings in terms of energy from the solar absorption and insulation effects of a green roof?
Is there enough CO2 absorption to qualify as a carbon offset?
By how much can they extend the life of a roof?
BENEFITS of GREEN ROOFS
Green roofs have numerous benefits, from ecological to aesthetic to environmental benefits. Green
roofs are an attractive roofing option that can reduce urban heat islands by providing shade and through
evapotranspiration, the release of water from plants to the surrounding air. Green roofs can also absorb
air pollution, collect airborne particulates, and store carbon. They protect the underlying roof material
by eliminating exposure to the sun’s ultraviolet (UV) radiation and extreme daily temperature
fluctuations and can thus extend the roof’s life. Green roofs also serve as living environments that
provide habitats for birds and other small animals. They can also be an area for food production: for