While there is no standard classification for rooftop gardens, they can be divided into two basic types:
- Intensive Living Roofs – these incorporate plants from between 1 to 15 feet high, including shrubs and trees. They require deep levels of soil to support them and a weight-loading roof. They support a high level of plant and wildlife diversity, but require ongoing maintenance and extensive irrigation. They are not suitable for most domestic buildings
- Extensive Living Roofs – these incorporate low-lying plants from 2 to 6 inches high. They require only a few inches of soil to support them, and only need a low weight-loading roof. They are low maintenance and can be used for any kind of roof, including sheds, garages, houses, balconies, extensions and outhouses, and also commercial buildings.
Both types of green roofs can be used for flat or pitched roof construction. Flat roofs are the most common and the easiest to establish and maintain, but green roofs can have a pitch up to 45 degrees. With sloped roofs, there are design issues affecting drainage and soil loss that need to be carefully considered.
The environmental benefits of a Rooftop Garden include:
Storm-water management, reduction of air pollution, improved air quality, reduction of the envelope of hot air that hovers over cities due to heat reflective material and lack of vegetation (Urban Heat Island Effect), decreased noise pollution, and when enough buildings with rooftop gardens are in an area, the respiring plants can even create a microclimate that can cool a city.
Rooftop gardens consist of: an insulation layer, a waterproof membrane to protect the building from leaks, a root barrier to prevent roots from penetrating the waterproof membrane, a drainage layer made from lightweight gravel, clay, or plastic, a geotextile or filter mat that allows water to soak through but prevents erosion of fine soil particles, a growing medium, plants, and sometimes a wind blanket.
Rooftop Gardens reduce energy costs and roof upkeep. They also insulate and protect buildings from harsh weather conditions and the ultra violet radiation and will also reduce energy consumption and increase the durability of the roof.
Another benefit to installing a rooftop garden is the therapeutic value to hospitals and care facilities. These facilities sometimes use gardening as therapy by offering fresh air, comfort, and a beautiful landscape.
There are two basic types of rooftop gardens:
Intensive: A heavier option, closely mimics a lush natural garden
- Soil Depth: 6 to 24 inches or more
- Weight Load: 80 to 150 pounds per sqft
- Costs: $15 to $25 per sqft
- Allows for a greater variety of plants, shrubs, and even trees
- Requires regular maintenance
Extensive: A lighter option, requires less maintenance and can tolerate harsh weather conditions
- Soil Depth: 1 to 6 inches
- Weight Load: 15 to 50 pounds per sqft
- Costs: $8 to $20 per sqft
- Limited variety of plant options
- Requires less maintenance
Although the initial costs of installing a rooftop garden are greater than a conventional roof system, the long-term benefits and the energy savings outweigh the initial investment. The costs of your green roof will also depend upon the type of roof installed, climate, and the selection of plants.
Some factors to consider with the initial cost of installing a rooftop garden
- Permits: Zoning and building codes vary for every city
- Consultant Fees: Contractor, Designer, Landscaper, and Structural Analysis fees
- Garden Materials: Growing mediums, plants, fertilizers, substrate containers, and pavers
- Plants: Cultivating costs are high because green roof technology is fairly new
- Irrigation System: Drip, Sprinkler, and Drainage Systems
- Maintenance: Weeding and watering extensive roofs, long-term intensive roofs
Irrigation and Drainage
The City of Los Angeles encourages landscaping with native plants as an environmentally beneficial garden design. Still, you should plan for the need to supplement rainfall with irrigation. A key environmental benefit of rooftop gardens is that they absorb rainwater and reduce runoff to sewer systems. Any excess water not absorbed within the garden must be effectively drained from the rooftop. Most gardens should be able to use the existing rooftop drainage system with only minor modifications.
Generally rooftop gardens are designed for flat and/or slightly sloping roofs, although systems are being introduced that can be installed on a roof with a 6:12 pitch.