Consider the functions that you want a green roof to fulfill. In addition to being good for the environment, green roofs can provide other functions such as outdoor living space for a home, or an herb garden.
Hire a structural engineer to assess the roof. The assessment will include its load bearing capacity and its condition. The roof may need to be repaired or strengthened before it is suitable for a green roof.
Decide on the type of green roof to install. Depending on the structural assessment and the functions you want the green roof to provide, it will be either extensive or intensive. Extensive green roofs are cheaper to install, lighter weight, and easier to maintain. Intensive green roofs have a thicker layer of growing medium to support a greater variety of plants but they are more expensive and require more maintenance.
Research green roof installers. Unless you have experience with construction and green roof installation, installing a green roof is not a good do-it-yourself project. When selecting a company, consider its experience, cost and the design features it offers.
Obtain the necessary permits. Most cities require permits for installing green roofs, but a few do not. Find out what your city's requirements are and whether it offers any incentives for installing green roofs.
Select your plants. Typically, the best plants for a green roof are native species because they do well in the climate and create habitat for native birds and invertebrates. Intensive green roofs can also be treated as gardens for growing flowers, shrubs, herbs and vegetables.
Install a roof irrigation system. Unless the green roof is extensive with only drought tolerant plants, it will need periodic watering. Your green roof installer might also design and install an irrigation system, or you might have to hire another contractor to do it.
Maintain the roof just like you would a garden. Intensive green roofs need weeding and pruning. Extensive green roofs with native plants need very little maintenance once they are established.