There are a lot of folks who would like to garden in an enclosed back yard or in an area that is totally covered with concrete or flagstone. Not having any soil to work with calls for the installation of a raised bed on top of the concrete. Soil depths should be at least 8 inches, but I suggest you install a raised bed with a soil depth 14 to 18 inches. The deeper soils allows for more root development and a wider range of larger plants for the raised bed garden. The deeper soil depths also helps control soil temperature fluctuations which creates a better habitat for plants. Add 1 to 2 inches of compost each spring to keep the soil in good shape.