roof plantery is one of the amazing new techs and one of the best ways of re-useing useless places. it could help you to save more in both sides of your roof, first keeping energies under your roof and less ago-water that rain or snow fall on your roof and second growthing vegitable foods and flowers to use or sale and more oxygen around you for your health. but the most important helps of it, is helping to the earth, against its heat increasing problem.
and here is actualy a very simple and very low coast way for diserving any kind of flat or non-flat roof to a little garden.
at least you need these three parts as a must on a now working water resistant roof that exist above your head if only with low deep root plants.
and i should recomend that dont made heavy thing above the strength range of your roof and don't add heavy material or trees on a roof that wasn't designed for that.
Step 1: A Simple "dimple Sheet" As Drainage Layer
for reserving water far away from roots and making safe area against roots growing , you need a drainage layer. the specific material for this layer is a semi-deep dimple sheets but forget about expensive coast of this material. simply recycle some useless plastic parts for example an old lego puzzle parts, drinks bottles and doors of them (I had kept the plastic coca cola bottles door and asked my sisters to do so and after a month or some more I take them used hear) (if you are going to use the bottles own you may need to cut them from half and it depends to depth of your box) .
then put them all in a box of any kind for keep them together and also in upside fixed position. I used a wooden pallet of tiles (which used in transporting companies) as my box and a 10cm height foam sheet in size of my box for keeping the doors fix in their position.
Step 2: Adding Filter Membrane
filter membrane is acting as a layer between the roots and the reserved water; because the roots of plants is going spoiled in continuous touch of backwater(still-water).
it could be selected from various materials with a same characteristic of being permeable for water and not
permeable for solids and specifically for your garden soil and growing medium.
I use a thick fabric but a polyester(a synthetic textile fiber) sheet is the best.
then fix the layer from around and make sure about the isolation for solids and influent for water.
Step 3: Growing Medium
if you made it away from your roof (or anywhere you want) and fix it In its position. take it there because after this final step its going heavy and difficult to move.
add 5 to 10 cm of gardening soil on the top layer and drain water to going full drain. full drain means draining water until all the dimples going full of water; you can find out this from ago water of system (the volume of water that going off from the system).
and now, it's by your own tastes and you have freedom in selecting vegetables and/or flowers in field of low depth roots plants.
I hope you'll be successful in made increase in ratio of oxygen to carbon dioxide of earth and decrease in your wasted heat to help against the global warming problem.