When creating a roof garden, we must follow slightly different rules than we do when making a ground garden. There are several factors we must consider. It is primarily the bearing capacity of the roof structure, prevention from roots and water penetrating the roof structure, inclination of the roof that must not exceed 300, altitude of the attic, etc. Depending on the bearing capacity, you may choose whether you will design your roof garden as merely ornamental or "stepable"(when the bearing capacity is over 300 kg/m2). There are numerous ways of designing roof gardens but the good thing is that they are no more a privilege of royal homes. You can visit Roof-Gardens.com before goinf for a roof garden.