How deep does soil need to be to grow plants correctly?

Depth of soil for roof gardening requirements varies with the type of plants you want to grow. Grass needs at least 10cms of soil, but preferably 15cm. Sedums can manage with less. Basically, whatever depth of soil plants require on the ground is the same in a roof top environment.

Other points to consider are the weight and stress that soil and plants places on your roof. Check that your roof is structurally sufficiently strong to carry the additional loading of soil, plants and water held within the soil.

In a roof top environment plants will be exposed to higher sunshine, higher winds and hence higher evapo-transpiration rates. So hardy wind tolerant, drought tolerant plants will perform best.