While most buildings have mechanical rooms, typically in the basement, tall buildings require dedicated floors throughout the structure for this purpose, for a variety of reasons discussed below. Because they use up valuable floor area (just like elevator shafts), engineers try to minimize the number of mechanical floors while allowing for sufficient redundancy in the services they provide. As a rule of thumb, skyscrapers require a mechanical floor for every 10 tenant floors (10%) although this percentage can vary widely (see examples below). In some buildings they are clustered in groups that divide the building into blocks, in others they are spread evenly through the structure, while in others still they are mostly concentrated at the top.