The short answer is yes, completely possible.
The step by step way to approach this from a safety point of view is first, have an engineer assess the weight bearing capacity of your roof.
- For an extensive (thin) greenroof you will need approximately 35 lbs per square foot (psf).
- For veggie beds (intensive or thicker) probably closer to 80 psf.
- To learn a littel more about the differences between extensive and intensive greenroofs go here.
The weight load will vary depending on the soil mix you use of course. An interesting presentation on how to estimate loads and the weight load of several different soil mixes can be found here.
The roof membrane
Assuming that you have the capacity, and generally older factory roofs were built way beyond capacity, the next consideration is the roof membrane itself.
- Is it under warranty?
- Will a greenroof void the warranty?
- How old is it?The average roof membrane has a 17 year lifespan.
If the roof membrane is closer to 17 years old, you may want to replace the roof completely before you put a greenroof on it so that you don't have to remove the greenroof in a few years to replace the membrane.An added benefit of this approach is a new roof with a greenroof generally last two or three times as long as a new roof without one.
Finally you get to the greenroof itself.
The greenroof system itself
The system you use to contain the planting medium on the roof is often determined by the manufacturer of the roof membrane due to warranty considerations. If you don't have a warranty and want to proceed anyway, there are a number of relatively inexpensive systems out there that will protect the roof from roots and keep moisture and light from impacting the roof membrane. To learn more about these and get a list of suppliers the the directoryhere.
Once you have a mat, you will need to select a lightweight soil mix and a way of keeping it on the mat. There is something of a learning curve here and it would take pages for me to give the installation process in detail, however, you can find those pages in the resources listed above.
On a personal note, veggie greenroofs are the most productive way you can use a roof top and I encourage you to follow through with this project if at all possible.
For more information:
Read "Are there any cost-cutting shortcuts for planting a green roof on my urban townhouse?" a Q&A answered by Mark Schrieber.