There is a great variety of green roofs. They all have in common that they are built on joist constructions. All green roofs have certain functions in common, such as managing runoff water. To be able to construct a well functioning green roof, a thorough site analysis is required to identify the conditions for vegetation and construction that will exist. Plant choices and the type of care that is possible have to be taken into account. Solar cells can be used in combination with green roofs and must be mounted on the underlying structure if it is not possible to use a ballast construction.