There are a couple of posts on the subject, maybe more.
On 1 November 2011, Pimped Pavement Anthology Post, I dealt with the subject.
It may require some research in terms of concepts. There are green roofs. Steve Martino, in Arizona, on his blog, Weeds and Walls, http://stevemartino.blogspot.com/2011/12/arizona-green-roof.html, approaches it installing one with cacti and irrigation system.
There is a whole culture, ideas, concepts dealing with green roofs, mostly in template climate contexts. In the tropics, where I practice, that is not the case. The one on this clip has no irrigation at all. So far, there are between 10/20 species, some of which you could identify. But time will tell how many will survive the hot/dry season.
In brief, Nature sets the example, the way to go. On this concrete roof garage, falling leaves accumulate, clogging the drainage. The water helps them leaves to decompose with time, sun, heat and wind. creating humus.
That explains the huge size of the Elephant ears, giving the impression of being on steroids. Compare the leaves on the roof, with those in land in the same side of the garden. The soil is 95% sand. Their size is almost 3 times as big.
The end is near. Vertical gardening is another subject for discussion. There is Paul Blanc, a French known for this kind of installation. I have mentioned it previously. Most of the time aesthetics predominate over irrigation, nutrients and maintenance issues. What is vertical, what is a hanging garden?
It seems a simple issue. But it is not. I read about some future vertical forest recently, to be installed in balconies and roofs of high rise structures in Lebanon. The photo shop effect was nice, but the premise is totally absurd unless you also mention maintenance. Will the trees in huge pots will be irrigated?
What about decay? Trees have their way of getting rid of branches no longer useful to photosinthesize or transport nutrients. Maintenance gardeners will also need training in escalating and not afraid of heights to reach some of the branches .
I have 'planted' quite a few on this roof. species. Planting implies digging or just spreading seeds. Here, they were placed on the concrete and covered with the available humus, leaves and organic matter. Most seeds are Cosmos and other vines in the inventory.
The work is done. Now it is a matter of waiting patiently until the dry season arrives. Then I propose to write a post on the surviving species, without irrigation.
One final note. To create a garden on a roof, not only you can not be afraid of heights. You need some stamina to carry the 15' ladder to perform your chores.
To complicate matters...let the record show...this garden is on an abandoned property next door to the most documented garden in the Caribbean..Therefore it also falls under the concept guerrilla gardening, or stretching it some what, pavement pimping, since I requested no permit to do what I do best...dissent...my way. Nature rules...one just have to keep them eyes opened. with a little understanding of the issues.
that is that