  A model is presented for the gravity-driven flow of rainwater descending
through the soil layer of a green roof, treated as a porous medium on a flat
permeable surface representing an efficient drainage layer. A fully saturated
zone is shown to occur. It is typically a thin layer, relative to the total
soil thickness, and lies at the bottom of the soil layer. This provides a
bottom boundary condition for the partially saturated upper zone. It is shown
that after the onset of rainfall, well-defined fronts of water can descend
through the soil layer. Also the rainwater flow is relatively quick compared
with the moisture uptake by the roots of the plants in the roof. In separate
models the exchanges of water are described between the (smaller-scale) porous
granules of soil, the roots and the rainwater in the inter-granule pores.
