Cape Town is located in a winter rainfall area and receives its rain from April to October. During winter it does not rain every day. Sometimes there will be a couple of days of rain and then several days of sunshine, however, it does sometimes rain for a couple of weeks in succession.
A considerably lesser amount of rainwater can be harvested from a tile roof than a metal roof. This is as roof tiles are porous and retain heat much longer than then metal roofs. Once pores in the tiles have absorbed the rainwater and the the tile has cooled to the ambient temperature both tile and metal roofs harvest equal amounts.
As typically no more than 22mm of rain falls during a rain shower, the volume of rainwater that may be harvested can accurately enough be calculated using the table above.