You need to know the structural capacity of your roof. You do this by reviewing your architectural drawing set which should have a page dedicated to roof structure listing the dead load and live load of the building. If you do not have access to these drawings, you will need to hire a structural engineer to come on site and assess your roof. This assessment can cost anywhere on average from $3,000 to $8,000. 

You also need to assess the quality of your waterproofing. Is your waterproofing new or old? How many years left do you have in the warranty? Does the roof have leaks that need to be repaired. A green roof should be installed on top of a relatively new waterproofing system so that the green roof does not need to be removed to repair or replace the waterproofing.