How to Install a Rooftop Garden

Rooftop gardening is expanding as city-dwellers look for space to grow flowers and vegetables. Many cities encourage rooftop gardens as a way to improve air quality. Simple rooftop gardens consist of plants growing in pots and containers, while more complex rooftop gardens actually transform the entire roof into a green garden.

These "green roof systems" require special layers for drainage and nutrients, making them more difficult to install. However, they also provide extra insulation for the building, lowering cooling and heating costs, extend the life of the roof, and reduce storm water runoff.

Things You'll Need

• Structural engineer or architect
• Building permits
• Flower pots (optional)
• Professional contractor (optional)
• Waterproof membrane (optional)
• Root barrier (optional)
• Insulation layer (optional)
• Drainage layer (optional)
• Filter mat (optional)
• Growing media
• Irrigation system (optional)
• Plants

Instructions

1. Consider your roof's strengths and limitations. Think about your roof access, keeping in mind the supplies you will need to carry for your garden. Hire a structural engineer or architect to assess how much weight your roof can support in different areas and consider any existing leaks or damage. If you are considering replacing an older roof, this is an ideal time to install a rooftop garden. Talk to your contractor about adding waterproof membranes and root-resistant materials to the new roof.

2. Decide what kind of rooftop garden you want. If you simply want to spruce up your rooftop and grow a few flowers or vegetables, your best option is gardening in pots and containers. If you want the look and growing space of a full green roof, decide what kinds of plants you want to grow. Shallow, "extensive" green roofs are lighter and easier to maintain but will not support delicate flowers or vegetables. Deep, "intensive" green roofs are deeper and will support any plants that grow in the ground, but they need a higher weight capacity and more maintenance.

3. Calculate your potential garden's weight. For container gardens, include the weight of each pot, as well as soil and water. Water weighs about 8 pounds per gallon, and potting soil can vary depending on the mixture (find the weight on the bag of potting soil). Extensive green roofs have a soil depth of 1 to 6 inches and weight of 15 to 50 pounds per square foot. Intensive green roofs have a soil depth of 6 to 24 inches and weight of 80 to150 pounds per square foot. Compare your garden's weight to the weight capacity calculated by your structural engineer to see if your roof can support a rooftop garden. If not, you either need to reduce your garden plans or replace your roof.

4. Obtain the necessary permits. Your city permit office will inspect your plans to make sure they comply with city ordinances. Most cities require that rooftop gardens are safe for people, including barriers around the edges and multiple exits. Some cities allow you to diagram the plans yourself, while others require you to hire an architect to draft the garden designs. Call your city hall to find out which rules apply to your rooftop garden and incorporate these requirements into your plans.

5. Construct the garden. If you chose a potted garden, bring your containers to the roof and plant your flowers and vegetables. If you chose a green roof system, you will need professional help to install it. Green roof systems require seven layers to be installed on top of the existing roof, starting with a waterproof membrane, then a root barrier, insulation layer, drainage layer, filter mat, growing media and finally plants. A crane might be necessary to move the materials to the roof, and an irrigation system may need to be set up. Unless you are an experienced contractor, you should not attempt to install a green roof on your own.

Tips & Warnings

• If you live in an area with cold winters, remember to include snow in your weight calculations. Chicago Municipal Code requires that roofs be able to support at least 30 pounds of snow per square foot; add this weight to your garden materials when you consider the total weight of your garden.

• Do not attempt to exceed the weight limit of your roof or calculate weight limits without professional help. Putting too much strain on your roof can be dangerous.

References

• Canada's Office of Urban Agriculture: Rooftop Gardens
• Chicago Department of Education: A Guide to Rooftop Gardening
• Green Roofs for Healthy Cities: Green Roofs Benefits