Info in an interesting message from Joseph TOLLEDOT :

A boxed garden complete with instructions – follow the pages:

http://www.siyeh.com/projects/garden/index.htm

Rooftop Garden

In the summer of 2003 I built a vegetable garden on the roof of my Chicago apartment. I have no gardening experience other than this, and I was (am) winging it. So far I’ve been pretty successful at getting everything I’ve planted to grow and have already switched over part of the garden after growing lettuce and peas through the spring.

Building the boxes:
The box is a 4′x8′ piece of 1/4″ plywood with 2 10″ pieces ripped off lengthwise to make the front and back of the box. The sides are additional pieces of plywood I had lying around. I also used 2×4’s on the inside of the box, running front to back, to give it some strength. I built this to fit on the brick wall as opposed to placing it on the flat roof – the brick wall can support this with no problem, and I’m not sure that the roof would be able to (especially when the dirt is wet). Don’t underestimate how heavy dirt can get! After carrying the wet bags of dirt up to the roof I definitely don’t.

(continued with practical advice and nice pictures)