Does a green roof need to be irrigated?

This answer depends on the type of green roof installed:

Extensive green roof (3-7 inches of soil) – No, however the roof will need to be watered occasionally during the first year of establishment just like any landscape. But after the first year the plants can sustain themselves, with the only watering exceptions in extreme periods of drought.

Intensive (8 inches + of soil) – Yes. Since an intensive green roof can accommodate a large variety of plants, shrubs and trees, their watering requirements are higher than succulents and herbs. Intensive green roofs generally have an irrigation system installed.