There are a number of reasons to consider including stone areas on a green roof, including factors like the roof's exposure to wind, the regulations you are following in the area where you are installing, and whether the aesthetics were a reason behind the installation of you or your client's green roof.
Let's go into more detail:
Access: as plants such as Sedum are not intended to support frequent foot traffic it would be a good idea to consider stone paths if frequent access is required for maintenance or leisure.
Aesthetic: some roof designs use stone areas to create artistic patterns or even a simple border might add character in some situations.
Fire break: the FLL Guidelines and the GRO Green Roof Code include requirements for the provision of fire breaks within green roof design. Some councils stipulate the inclusion of stone areas for a number of reasons (usually to act as a fire-break).
Wind resistance: in particularly blustery areas a stone border can be used to prevent the chance of wind uplift pressures on the green roof.
Prevent debris: a stone border can effectively stop wind-borne plant parts and debris reaching the edge of the roof and gutters.
So while not all green roofs require stone areas it is often a wise consideration based on the reasoning above. If you are looking to include stone in your green roof we have some quality Scottish pebbles for this purpose, and for roofs needing a stone border where there is no parapet or upstand, you might consider using a GrufeGuard too.