An important thing is not to forget the drainage core of the drainage sheets or layers with a nonwoven (may be woven but non woven is cheaper) filter fabric.
The ready made drainage products already have filter on them. But you have to wrap filter on the drainage core. Otherwise the soil, gravel or etc may go inside the drainage layer and block the drainage or at least decrease the amount of water to be drained.
Drainage sheets are used to remove excess water.
On the drainage layers you may use a water retention layer depending on where you live.
For more information on green roof solutions check our roof garden section for all posts of this subject.