I just finished reading the "Green Fingered Photographer" post on his green roof and thought I also have one but in another sense. I haven't planted anything on my shed roof but I have trained our Engelmann Ivy over the shed roof as well as both privacy trellis at either end of our deck. In a way it is the "lazy green roof" method. It does however supply the same benefits to us and our visitors of the garden. It cools the shed a bit and that is wonderful because it is metal and you know what happens with the sun. It has a few insects for the birds to eat in the summer as well as berries in the Autumn. It provides a resting space for butterflies and birds.
The colour of the vine in Autumn is nothing less than spectacular. So all in all it works very hard from Spring to winter .. and I am grateful I had the brain cells zapping at me with that idea those few years ago when I went "WOW" wouldn't doing this be a good idea !
Yes .. my rain barrel is very happy in that spot too .. no boiling water on the shady side of the garden in the afternoon .. phew !