Roofwater Harvesting

Roofwater harvesting (or rainwater harvesting) is the use of rainfall runoff from roofs for water supply. It typically involves the diversion of roof downpipes in to above ground storage tanks where it is filtered and disinfected for use in non-potable applications such as toilet flushing and landscape irrigation.

Roofwater is a high quality water source suitable for most end uses with minimal treatment. Treatment usually starts upstream of storage with screening and first flush devices employed to prevent debris and initial dirtied runoff from entering the harvesting system. After storage, roofwater is typically filtered using a simple micro strainer. Disinfection may also be required for end uses such as irrigation and washing where there is potential for human exposure.

Most roofs are suitable for roofwater harvesting and the natural elevation assists in keeping systems above ground to minimise pumping requirements.

The control requirements of a roofwater harvesting system vary with the complexity of the system. A small control panel for the distribution pumps is often sufficient to operate the entire system and track water savings. If available, a mains water connection may be used to top-up or back-up supply.

Key engineering considerations for roofwater harvesting systems are summarised further below and in the attached brochure.

Roofwater needs to be diverted in to the harvesting system. This requires access to adequate and suitable roof areas and the ability to divert roofwater away from downpipes and in to storage.

Key considerations:

Exposure of roof areas to contaminants

Roofwater diversion point and mechanism

Storage is needed to balance irregular rainfall with typically regular demand for water. Roofwater is screened before entering storage tanks to remove debris such as leaves. Depending on the application, it is also typically passed through a filtration and disinfection system before use.

Key considerations:

Storage tank location, capacity and material

Need for pre-screening and first flush devices

Level of filtration and disinfection needed to meet end use requirements

Delivery pumps and a dedicated non-potable water delivery line are typically needed to supply stored water to the application point. If a mains water top-up or back-up supply is needed it is typically built in to the distribution system.

Key considerations:

Delivery pump type and activation mechanism

Need for and location of mains water top-up or back-up
The level of control required for a roofwater supply system varies depending on the end uses being serviced. Most systems are controlled by a small programmable logic controller (PLC) that uses pressure sensors in the distribution network to activate the delivery pumps.

Key considerations:

Control capability and required level of system automation

Type and number of sensors and control hardware