Green Roof Basics

In the simplest terms, a green roof is plants on top of a roof. Also known as a rooftop garden, a green roof typically consists of the following components: 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, usually made of 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.

Green roofs improve air quality, conserve energy, reduce stormwater runoff and help reduce the urban heat island effect. The garden's plants reflect heat, provide shade and help cool the surrounding air through evapotranspiration, which occurs when plants secrete or "transpire" water through pores in their leaves. The water draws heat as it evaporates, cooling the air in the process. Plants also filter the air, which improves air quality by using excess carbon dioxide to produce oxygen.

There are two basic types of green roofs: intensive and extensive. Intensive green roofs require a minimum of one foot of soil depth to create a more traditional rooftop garden, with large trees, shrubs and other manicured landscapes. They are multi-layer constructions with elaborate irrigation and drainage systems. Intensive green roofs add considerable load to a structure and require intensive maintenance. In contrast, extensive green roofs range from as little as 1 to 5 inches in soil depth, adding less load to a building. Extensive green roof systems also generally require less maintenance than intensive systems. Some green roof designs incorporate both intensive and extensive elements.

The documents attached as PDF files below, offer useful tips for planning a green roof. Here are some general considerations to keep in mind:

Condition of the Roof

In general, the most cost-effective time to construct a rooftop garden is when the roof needs to be replaced or newly constructed. However, some types of green roofs can be installed on existing roofs.

Structural Capacity of the Roof

Prior to designing and constructing your rooftop garden, you must first determine if your roof can support the additional weight of soil and plants. A licensed structural engineer or architect must be hired to conduct a structural analysis. The structural capacity largely will dictate the type of rooftop garden that you can build. If your structural capacity is sufficient, you will need to obtain the proper permits from the Department of Buildings before proceeding.

Access to the Roof

Consider what kind of access to the roof will be needed during and after the rooftop garden is installed.

Cost

In addition to the cost of structural analysis, your construction budget should include any needed structural or safety improvements, irrigation systems, garden materials, maintenance costs, transportation and fees associated with any ongoing professional assistance and permits.

Irrigation

The City of Chicago encourages landscaping with native plants as an environmentally-beneficial garden design. Still, you should plan for the need to supplement rainfall with irrigation. You will need to hire a licensed plumber or irrigation contractor to design and install an irrigation system or new water connection on your roof.

Drainage

A key environmental benefit of rooftop gardens is that they absorb rainwater and reduce runoff to sewer systems. Still, 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.

Selection of Plants and Growing Media

Generally, the growing media should be as deep and have as great a volume as possible within the constraints of the structural capacity, to provide plants with stability in wind and keep the system from becoming too dry. Many plants, especially native varieties, are suitable and attractive options for rooftop gardeners.