Roof gardens can be divided into two categories: extensive and intensive. Extensive roof gardens, which are more common and less complicated, are typically constructed to a depth of 6” and are frequently designed to meet specific performance goals. Intensive roof gardens, which can be relatively deep, commonly include large plants, trees, and elaborate gardens that require much maintenance. Key benefits of roof gardens include controlling stormwater run-off, mitigating urban heat island effects, and reducing sound reflection and transmission.
Working our way up from the bottom, a standard roof garden (extensive in our example), commonly includes these important layers (not including the substrate and insulation):
- Waterproofing | Roof Membrane: This is crucial to a roof garden. Approved waterproofing layers will vary by manufacturer.
- Root Barrier: Often composed of concrete or cellular glass.
- Drainage: Often includes plastic sheets, granular mineral layers, or fabric and synthetic mats. Gravel or pumice is typically used close to the gutters and scuppers to aid drainage.
- Water Filter: Polyester or polypropylene cloth is used. This layer filters water while keeping the top layers together.
- Growing Medium: The top two layers are soil and vegetation. Selection varies depending on the desired outcome such as run-off control, appearance, accessibility to the public, and drought tolerance. For the soil, it is important to use a growing medium that will not foster weed or pathogen growth.
- Vegetation: To ensure that your plants thrive, use plants that are found in your growing zone or at least are hardy enough to withstand the elements. Also take into consideration the maintenance the plants will require.
Green roofs are heavy, even extensive ones. If installing a roof garden on an existing structure, you will need to have the building evaluated first. Consult a roof contractor who can tell you how much your existing roof can hold and what extra structural support may be required. The Great Lakes WATER Institute of the University of Wisconsin Milwaukee in their green roof project spells out in detail the costs and weight considerations.
- Extensive roof gardens have a soil depth of 1" to 6" and can weigh between 15 and 50 lbs per square foot.
- Intensive roof gardens have a soil depth of more than 6" and can weigh anywhere from 80 to 150 lbs per square foot.
Green roofs are a huge investment, especially financially. However, the benefits outweigh the cost. Not only do green roofs help to combat urban heat island effect, they are energy-efficient and can be quite useful, particularly if they include vegetable gardens and fruit trees.