Because of their sheer size, roofs will catch a significant amount of water. The roof catchment area for single storey homes is usually greater than the floor area of the building if there are eaves.
As a guide to collection capacity, consider that each 1mm of rain = 1 Litre (L) of water per square metre (m2) of roof area, then allow a 15% wastage factor.
Another rule of thumb is that the smaller the catchment area the larger the tank because when it rains there is a need to capture as much as possible of the run-off.