welcome to GoY.
first is to make sure the roof is strong enough to take the weight of the shallow soil when wet and then the plants.

'living roofs' tend to use plants that are shallow rooted and succulent, such as sedums.
is this what you were thinking of?

As Eileen says succulents will work well but the only herbs that I can think of that will tolerate the shallow roots and drought conditions will be the thymes.