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Is Rainwater Harvesting it right for me?

Is Rainwater Harvesting it right for me?

You should expect savings of up to 50% of your mains treated drinking water in houses and up to 80% of water in a business or commercial building by using a Rainwater Harvesting system.

The average house can harvest 64,000 litres of rain water a year from their roof…that’s 213 water butts!

The average person uses around 18,000 litres of water per annum just to flush toilets and wash clothes,

Rainwater harvesting is a logical choice.


How much do you need?

Rainwater harvesting systems can provide water for:

  • Flushing Toilets
  • Washing Machines
  • Dish Washers
  • Watering Gardens
  • Car Washing

Please note that only Direct Fed Rainwater Harvesting systems can provide water for your washing machine and dishwasher.


Sizing a Rainwater Harvesting System

The volume of collectable rainwater (RWHsize) is equal to:

  1. your roof area size multiplied
  2. by the “run off” coefficient multiplied
  3. by the filter efficiency multiplied
  4. by the annual rainfall in your area.

The roof area is calculated by the length times the width.

The “run-off” coefficient is the amount of rainwater that hits the roof that can actually be collected.

A coefficient of “0” means no rainwater can be collected; whilst a coefficient of 1 means 100% can be collected.

Typical coefficients are:

  • Pitched roof = 0.85
  • Flat Roof (smooth surface) = 0.55
  • Flat Roof (non-smooth surface) = 0.45

Filter efficiencies are nearly always supplied by the manufacturers. In the absence of a filter efficiency a figure of 0.8 (80%) should be used.

Monthly and Annual rainfall data can be obtained for free from the Met Office.


Example Sizing

The pitched roof area is 20m x 15m = 300 meters squared

The annual rainfall for the area is 600mm.

Therefore amount of rainfall we can collect is:

300 x 0.85 x 0.80 x 600 = 122,400 litres

122,400 / 1000 = 122.4 meters cubed of storage space as a maximum should be installed

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