When choosing your green roof, there are 2 main options: an extensive green roof or an intensive green roof.
Extensive green roofs are the most commonly found. They are lighter weight, lower maintenance and not generally designed for access. They use shallow-rooted plants and grasses that require very little water and are typically chosen by homeowners or on buildings where weight can be a problem.
Intensive green roofs on the other hand, are designed as conventional gardens with walkways for continuous access, and allow for deeper-rooted plants that can even include shrubs and trees. Intensive green roofs often require structural reinforcement to the existing roof and are higher maintenance than an extensive green roof.