When it comes to designing a roof terrace there are several things you have to consider, the first of which is preparation. The most important part of creating the terrace, preparation involves logistics, this means deciding how are you going to get construction materials, plants etc, all the way up to your roof. You will have to resolve yourself to have workers constantly walking up and down your home, during the construction process. Then there is purpose. Why do you want a roof terrace? How will you use it in conjunction with the rest of the room? Will it be a quiet place to read and admire the cityscape? Or will you use it as a hub for parties? Before you embark on the immensely complicated task of building the terrace, carefully consider whether you need one and what are you going to use it for.
Once you have decided why you need a roof terrace, it is now important to consider the protection. On the ground, trees do a good job of protecting you from both sun and wind, but up on the roof top you will run the risk of overexposure to sun, wind and rain if you are not careful. Always consider tented pavilions and pergolas when building your roof terrace, if you are bringing plant life onto the roof, be sure to insert some automated irrigation system that will keep plants fresh and alive against excessive exposure. Privacy is an important part of the process, you do not want the neighbors peeking in, nor do you want to have unattractive sights from your rooftop, in such cases it is always best to use fence panels and boxwood hedges to block out undesirable views.
Source : http://windowboxplants.tumblr.com/post/82891301224/steps-to-take-when-creating-a-roof-terrace