Measuring area diverted to rainwater

The roof area that is diverted to a rainwater tank can include all roof surfaces measured to the outside of the gutters, excluding parapets and trafficable roof terraces and measured in the horizontal plane.

Roof areas diverted to a stormwater tank can include all roof surfaces (including trafficable roof terraces/decks, etc), as well as impervious areas, garden and lawn areas and planter boxes.

If rainwater is mixed with stormwater then the mixed water source is defined as stormwater and there are limits to how it can be used without treatment.

What are impervious areas?

Impervious areas are surfaces that do not allow natural infiltration of rainfall to the underlying soil and include concrete, gravel and timber surfaces.

Diagram showing how to measure area diverted to rainwater or stormwaterDiagram showing how to measure area diverted to rainwater or stormwater


Note: If you have nominated a large roof area (approximately more than 150m²) to carry water to your rainwater or stormwater tank, ensure that the piping used to carry the amount of water collected by the roof area is appropriately sized for the collection area you have nominated. Speak to your hydraulic designer/plumber for more information, as this design guide varies across NSW.