A standard O’Neill cylinder provides about 321.6 square kilometers of usable land. For comparison the surface area of Singapore is 716.1 square kilometers. Since a standard O’Neill cylinder contains only a small amount of land, efficient land use will be essential. (Larger cylinders can be built, but they are unlikely in the early stages of space colonization.)
The principal function of a O’Neill cylinder is to house space settlers, since heavy industry and the major portion of agriculture will be located at other space stations. Within an O’Neill cylinder area will be divided between residential and recreational functions. Technically the available land will be divided between buildings and open spaces.
Open space are what they are, a single layer system. A building is at least a two layer system, since every building has some kind of roof. Because building have multiple layers on top of each other, they use land in a more efficient manner than open spaces. Nevertheless open space are important for psychological reasons.
Unfortunately roofs are too often ignored in designing buildings. For most people the only function of a roof is to protect the inhabitants/users of the building against the elements. The result is that most roof tops are bare spaces.
Roofs are essentially open spaces that can assigned useful functions, however. Roof gardens are one such function. These roof gardens could be used as an allotment garden, for instance for the residents of an apartment building. And don’t forget the roofs of office buildings: if one could rent space inside an office building, then why not its roof? Other examples are: sport fields on top of shopping malls, or the children’s playgrounds on the roofs of schools.
Not only do buildings have roofs, they also have walls. Most people don’t know better than that walls are from bricks or concrete, modern technology has made it possible to use walls more efficiently. Green walls are wall that are prepared to grow vegetation on it, and are essentially vertical gardens.
Roof gardens and green walls contribute to better living conditions in several ways. First they counteract the desert-like climate common to most urban areas (because of large surfaces of concrete, cities are dryer and hotter than the surrounding areas). Secondly, they provide oxygen through photosynthesis. Thirdly, they improve the aesthetics of buildings.