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Rooftop rainwater filter calculator

The rainwater filter calculator sizes a rooftop rainwater harvesting filter. From the roof area, design rainfall intensity and roof type it gives the peak runoff in litres per second, the flow per downpipe, the filter capacity to select and the yearly harvest potential, using peak flow = C x I x A / 3,600.

Use the long-term average for your location.

Peak flow2.36 L/s

Peak flow
8.5 m³/h
Flow per downpipe
1.18 L/s
Filter rating per downpipe, at least
1.5 L/s
Annual harvest potential
170 kL (1,70,000 litres)

Q (L/s) = C x I x A / 3,600. Per downpipe = Q / n. Annual harvest (litres) = A x annual rainfall x C.

Use the design intensity your local code or rainfall data gives; 50 mm/h is a starting point. Indicative only.

Send this to an engineer

Your inputs and results go into the quote form, so an engineer can check them against your operating conditions.

How the rainwater filter calculator works

A rooftop rainwater filter must pass the peak flow from the roof during a heavy shower. The calculator works out that peak flow, divides it across the downpipes and estimates the volume the roof can harvest in a year.

  • Peak flow: Q (litres a second) = C x I x A / 3,600, where C is the runoff coefficient of the roof, I is the design rainfall intensity in millimetres an hour and A is the roof area in square metres.
  • Runoff coefficient: 0.85 for RCC and metal roofs, 0.75 for tiled roofs and 0.70 for paved areas. Select the closest match or enter a value.
  • Flow per downpipe: the peak flow divided by the number of downpipes. The filter is rated for at least this flow, and the calculator rounds up to the next 0.5 litres a second.
  • Annual harvest potential: roof area x annual rainfall x C, in litres.

Worked example

A 200 square metre RCC roof with a design intensity of 50 mm an hour gives 0.85 x 50 x 200 / 3,600 = 2.36 litres a second, or 8.5 cubic metres an hour. With two downpipes, each carries about 1.18 litres a second, so each filter is rated for at least 1.5 litres a second. With 1,000 mm of rain a year, the roof can harvest about 170,000 litres (170 kilolitres).

Reading the result

The design intensity has the largest effect on the answer, so it should be taken from local short-duration rainfall data for the city. SIMO Screens makes each filter to the peak flow and the pipe sizes of the installation, so the calculator result and the downpipe sizes are the basis of a quotation.

The rooftop rainwater harvesting filter guide explains filter types and installation, and the rainwater harvesting filter page describes the construction.

Questions

Common questions

How do I work out the rainwater harvesting filter size?

Size the filter for the peak flow from the roof it serves: runoff coefficient x rainfall intensity (mm/h) x roof area (m2) / 3,600 gives the flow in litres per second. The inlet is then matched to the downpipe size.

What runoff coefficient should I use?

Hard roofs such as RCC and metal shed most of the rain, so 0.8 to 0.9 is typical and the calculator uses 0.85. Tiled roofs and paved areas are lower, at about 0.75 and 0.70.

Why use a wedge wire filter for rainwater?

The V-slot sheds leaves and grit instead of trapping them, so the screen cleans itself with the flow and needs far less attention than a mesh filter.

Can one filter serve several downpipes?

Yes, if it is sized for their combined peak flow and the pipes are brought to one inlet. On large roofs, a separate filter on each downpipe is usually simpler to install.

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