Rainwater Harvesting

What is it?
The capture of rainwater on-site and storing it for the use of flushing toilets, cooling tower make-up, and irrigation of external planting and green roofs/walls. Rainwater is typically captured from the roof, filtered and stored in a tank at ground or basement level, or buried underground. The water is not of a potable standard and therefore is not used to serve sinks.
Rainwater harvesting and grey water recycling systems can be combined to reduce even further the amount of potable water consumed for flushing and irrigation.

Advantages / disadvantages

Reduces the amount of potable water used.  This also reduces CO2 emissions as treating water to potable standard  and pumping it are highly energy intensive.

Strong sustainability message.

Effectiveness depends on the available roof area for rainwater capture and the storage tank size.

Most effective when used to supply systems with continuous demand (i.e. toilet flushing and cooling towers) rather than seasonal demand (irrigation).

Water efficiency

The volume of water saved depends on the size of the roof available for collection, the ability to store it hygienically until  it is needed and connection to consistent and high demand for non-potable water.

Running costs

Rainwater harvesting reduces water supply and waste water  bills which will offset additional maintenance costs providing  water collection and flushing / irrigation demand is high.

Systems will often incur small pumping costs.

Retrofit / improvement opportunities  

Rainwater harvesting is difficult to retrofit unless installed  during a major refurbishment. The additional pipework from  roof collection points to a central water store and space  required to accommodate the storage tank are key  considerations for retrofitting rainwater harvesting.

Applicable buildings

Most effective in low rise building with a large roof space to floor area ratio.


Floor plate implications


Occupant comfort

Occupant perceptions can be negative if discoloured water is provided to toilet pans.

Maintenance implications

Periodic cleaning and replacement of water filters is required.


A large tank is situated at ground or basement level .

Questions to ask

  • Is there a rainwater harvesting system in place?

More information

References required