A big driver for green roofing these days is flood mitigation by control of surface water runoff, and so it should be given the serious effects of flooding in recent years.
Can green roofs really make a difference? Certainly, although I will always argue that they are not a substitute for responsible environmental planning. Green roofs will never provide a solution by themselves and should always be considered together with other responsible building and landscaping measures.
However should a green roof be designed as part of surface water runoff control, there are several principles that will make help make it even more effective.
Firstly consider the rainfall pattern of the area you are in and how this will affect the plants, which also play a big role in water management. Dry areas need more depth in general.
In wetter areas, your goal is to detain water during peak flow times and then let it off gradually so that it will be ready for the next downpour. In dry areas you want to retain that water for as long as possible to last until the next drought. This difference can be brought about by thoughtful substrate engineering.
Increasing substrate depth up to about 200mm greatly helps performance in both cases, but above that has less effect as water in lower levels can’t evaporate.
Plants also play an important part, with succulents like sedum so valuable for retaining water during droughts, and others such as many wildflowers which will release water more quickly into the atmosphere.
Plants can always be selected according to the area’s demands, but having a mix with some species that transpire more and others which store water will give a good all round performance in most conditions.