A roof garden can be a green alternative to those who lack space or garden area in an urban neighborhood. Roof gardens can be created in two different ways. The first is with container gardening. Using containers to hold flowers and vegetables allows urban dwellers to get the most out of their space. Another option is roof landscaping. This is where large parts of the roof area are covered with soil and plants. In order to have a safe roof garden, the structure of the building---especially the roof---must be stable and sturdy.
Examine the roof carefully. This is an important step in assuring the safety of the rooftop garden. The roof should be flat or have an accessible flat surface in order to properly maintain the plants. There should be a safe way to access the roof area and a stable surface for walking and caring for the plants. A hose with water access should be readily available and accessible to the roof garden. The support system under the roof should be solid and not have any dips or cracks. Keep in mind that there will be a significant amount of additional weight added to the roof including the topsoil, plants and water saturation. Hire a building inspector or contractor to inspect the roof to assure a safe environment.
Create a garden plan. Determine what type of garden will fit your lifestyle. If there is limited space, container gardening is a viable option. Decide if you want an annual flower garden, a vegetable garden or a combination of the two. A garden that is created directly on top of the roof's surface can include flowering annuals, perennials and vegetables. Because there is no existing topsoil, a rooftop garden will need a liner, topsoil and a retainer to contain the soil and plants. Consider an all-in-one unit such as a green roof tray (see Resources).
Purchase plants. Choose plants that are hardy and flowers that are in bloom. Because it is unlikely that there is shade on the roof, full sun plants such as petunias and sunflowers can be planted. Shade-loving plants, such as impatiens or begonias, can be used if the roof is between tall buildings or shaded by nearby trees. Most vegetables and many flowering plants require full sun for maximum growth.
Plant the garden. Place containers strategically around the roof area. Fill the containers with potting soil almost to the edge and place the plants firmly into the soil. Fill in gaps with potting soil. For non-container gardens, lay down the garden liner or biodegradable roof tray. Place edging around the perimeter of the liner. Fill with topsoil and then the plants. Water thoroughly.
Maintain the garden. Most gardens should be maintained daily or every other day. Fertilize once every other week. Pull weeds from around the plants. Water daily or as needed.