We have underground water here aside tap water, which is pumped up to rooftop to supply coolers. it's very cheap.
And we have episodes of no rainfall at all for long periods of time, plus short days of flooding crazy rainfalls.
Kids are forbidden to enter the rooftop, after two of kids died there.
One of them dropped down and died (due to walking backwards carelessly) when flying her kite up there.
And some years before than that, a 5 years boy jumped off the rooftop while holding an umbrella in hand, thinking it would slow him down and give him a nice aerial ride. The parents said he watched a TV kids show yesterday where two crazy inspectors dropped off a 10 story building using umbrellas. poor kid was impaled on the sharp cherry tree in the backyard.
After all these, all residents decided to lock the rooftop, and I was chosen as the only key holder.
Anyway... back to our topic:
I am also thinking about using papercrete to build the raised bed.
I've worked with it before, it's a fantastic material, cheap, light, and easily workable.
The only drawback is that it's not waterproof, but that can be solved by elevating it 10cm, and also placing a layer of plastic sheeting under it on the rooftop surface.
I just worry about rot and destruction.
I'm not sure if papercrete does rot and deform gradually when exposed to water, or not.
Especially the variation made out of paper and plaster of Paris, because concrete is only sold in bulk here, they won't sell less it less than 200kg.
But I'm sure wax can be applied on papercrete because of its texture, allowing the wax to stick to it perfectly.
I haven't decided what to grow in that garden, but I will grow mostly annual vegetables, and maybe some grape vine.
Also if I can build a bed that can last for years, I will make a greenhouse and grow various tropical sacred plants like mimosa and caapi.
take a look at here:
http://davesgarden.com/community/forums/t/472203/I like to build one of them, but in the shape of a vast and big box, around 2x6 meters, and 40cm soil depth.