Humans have grown plants atop structures since antiquity. Besides the decorative benefit, roof plantings may provide food, temperature control, architectural enhancement, and recreational opportunities. Available gardening areas in cities are often seriously lacking, which is likely the key impetus for many roof gardens. The garden may be on the roof of an autonomous building which takes care of its own water and waste. Hydroponics and other alternative methods can expand the possibilities of roof top gardening by reducing, for example, the need for soil or its tremendous weight. Plantings in containers are used extensively in roof top gardens. Planting in containers prevents added stress to the roof's waterproofing. One high-profile example of a building with a roof garden is Chicago City Hall.
For those who live in small apartments with little space, square foot gardening, or (when even less space is available) living walls (vertical gardening) are wonderful solutions. These use much less space than traditional gardening (square foot gardening uses 20% of the space of conventional rows; ten times more produce can be generated from vertical gardens). These also encourage environmentally responsible practices, eliminating tilling, reducing or eliminating pesticides, and weeding, and encouraging the recycling of wastes through compost. In small apartments, a Bokashi compost system is more practical than conventional composting.
The related idea of a living machine is based on the most basic mode of gardening: dumping wastes (compost and sewage, appropriately broken down, usually in some specialized ditch or container) on the soil, and harvesting food which, when processed, generates compost, and when eaten, generates sewage. In most of the world, this kind of very tight closed loop gardening is used, despite certain health risks if necessary precautions are not taken. Compost including human or pet waste should reach thermophilic conditions and age for at least a year before being used. Manure from vegetarian animals is safe without these measures.
Composting itself is a safe process which, when composed of a variety of different materials, is one of the best forms of fertilization available.