Indirect Collection System:

Sometimes it is just impossible to install a storage tank or reservoir directly under roof gutter outfalls. This may be due to a lack of space or that there may be multiple outfalls that cannot all be run straight into the storage vessel.

The solution is run pipe work from all the roof gutters to one common underground sump, then pump from the sump to wherever it is convenient to set the storage container.

The following case study faced exactly this scenario.

The Numbers:

A) Glasshouse Roof Area to be Harvested 6746m²
B) Annual Precipitation Rate 0.626m/year (25”)
C) Annual Potential Yield (A x B) 4223m³
D) Yield from single 1” rainfall event (common)(A x 0.025m) 169m³
E) Total Annual Nursery Useage 12,060m³
F) Cost of water. (E x £0.92/m3) £11,095
G) Annual Yield x m3 cost of mains (D x £0.92/m3) £3885 annually


In practice, not all winter water (Nov-Feb) could be saved as the storage vessel would need be too large to be practicable. However, because spring summer and autumn demand is high, anything falling on the roof in these months could be used.

In practice £2500-3500 annually could be saved. This figure varies from year to year.

The System:

  • Intercept the roof drain pipe and insert a sump. In this case 30m³ was ideal. The bigger the sump the less water floods out of the sump overflow to waste during heavy rainfall events. Divide the sump into two halves . Roof water flows into the 1st, climbs up the dividing weir wall and drops into the second from where it is extracted. This creates a sedimentation system.
  • Install a pump to suck out of the sump and to blow the water to a storage tank via an auto filter. Pump can be submersible if appropriate. Pump size needs to be sufficient to keep up with a moderate in flow . In this case 30m³/hr at 4 bar. Pipe size was 75mm over 250m.
  • Erect the rain water storage tank (328m³) adjacent to the existing mains water storage. Inlet from the sump into the tank. Outlet joins the irrigation pump suction. This means that water can be used from mains or rain water tanks or mixed.


Pump system, Auto Filter, 250m of transfer pipe, tanks and outlet valves and pipework £ 13,500
Concrete tank base, sump, installation £ 8,000
Total £ 21,500.

This gives a capital payback of approx. 6- years, after which there is a net saving on the water bill of £3000 /year.

The secondary benefits of water security and improved water quality start from year 1.

Rainwater Harvesting Indirect Illustration: