Adding railings, handrails or balustrades to a flat roof that you intend to use as e.g. a balcony or roof garden / roof terrace can be a great looking and practical finishing touch. Before building the idea firmly into your plans however it is worth remembering first that:
Handrails, railings or balustrades have to be able to stand up to the elements, need to be solid and safe, and should look good. The most popular options are:
Stainless steel is very strong, looks good and is resistant to corrosion. Toughened glass has high thermal resistance and in the unlikely event that it is broken, unlike normal glass it will break into small cube-like pieces that are not as sharp and are less dangerous.
Combining glass panels with stainless steel rails can provide some protection from the wind as well as looking very attractive and modern.