Rain water harvesting is the process of collecting rain water from roof surfaces and using the collected water before it goes back in to the drains. Rain water collected can be used for watering your garden or toilet flushing, as well as ground hosing and even in your washing machine. There are many benefits to harvesting rain water as it reduces the amount of water you use from your mains supply and saves you money. It also helps in areas that are likely to flood as it holds back water from flowing in to the drains, but also in times of drought where it can help to avoid hose pipe bans.

Did you know?

33% of water used in a home is for flushing the toilet
We all use approximately 150 litres of water each per day
50% of water used in homes could be replaced by rainwater

Rain water harvesting is growing across the UK as it contributes to the Code for Sustainable Home in inviting excellent code ratings and can also help businesses to benefit from tax allowances under the Enhanced Capital Allowances (ECA) scheme. The rain water harvesting system needs to be on the Water Technology List before your business can claim 100% first year capital allowance on its cost. The majority of our products are included in the Water Technology List. Current incentives for the general public are restricted to the obvious energy, water and money saving (reducing utility bills ) , however these can result in a very speedy pay back

Calculating Your Rain Water Tank Size

When fitting a system it is important not to oversize the tank. Apart from the obvious additional cost, a well designed system has a regular and consistent overflow, removing fine debris from the water surface and keeping the water quality fresh. Below is a simple calculation to establish a guide to your required tank size.

Available rainfall = annual rainfall for your area (mm) x roof area (sqm) x evaporation factor* x filter loss factor*.

*Assume as 0.9

Expected usage is 45% x 150 litres per person per day. As an example, the average three bed house yield would be:

800 x 120 x 0.9 x 0.9 = 77,760 litres x 0.05 = 3,888 litres

For an average four bed house:

105 x 4 x 365 x 45% = 98,550 litres x 0.05 = 4,927.50 litres

If you calculate 5% of the result then a tank size of 3,888 litres is suitable (or closest size available) for the three bedroom house. The benefits of a modular system are that additional tanks can be joined at a later date with ease.

To see your expected rainfall check the met office website

How do I know what I need?

It is important that you ensure that a rain water harvesting system is the right option for you. You will need to consider what you are going to use the water for and we will work out for you:

  • How much water you can collect?

  • How much water you need to store?

  • The system you need and the cost of the system.

We would always recommend the use of additional other water efficiency measures to give maximum water savings. Additional water efficiency measures may be low flush cisterns, variable flush cistern and displacement products, flow regulator taps and changing your water use practices (showers instead of baths, boiling kettles with only enough water for your immediate needs etc).

Environmental Systems and Services are industry specialists in rain water harvesting and will be happy to advise you on the best option for you.

Frequently Asked Questions

Calculating how much water you can save will depend on what you want to use the water for, where in the country you live and what type of building you are collecting the water from. We can help you to work out what type of system will give you the best results.

The cost of a rain water harvesting system will depend on the size of the system and the installation. We can design bespoke systems according to each customers needs, but it may be a very simple system that best meets your requirements. We can also give you information on who is best to install your system. For simple garden watering systems we can give you the information you need to install yourself!

Fitting a rain water harvesting system to an existing building is possible. Many new buildings are incorporating rain water harvesting as a standard to meet the Code for Sustainable Homes. We are developing systems that are suitable for retrofit and can advise you on the best option for you.

Once a rain water harvesting system is installed, you will be advised on maintenance actions you can take yourself to ensure the system performs as it should. These will be simple actions that require little or no training. We do recommend considering a regular service for the whole system which could be annual or dependent on the system usage to ensure the best performance of your system.

More information about rain water harvesting can be found on the websites of these organisations as trusted sources of advice and information:

  • Environment Agency
  • Defra
  • Rain water Harvesting Association