Rainwater harvesting: introduction

What is rainwater harvesting?

Rainwater harvesting is simply the collection of water for domestic or commercial use that would otherwise go down the drain. Various techniques have been practised for thousands of years to collect and store water, especially in areas of low rainfall. Today, rainwater harvesting systems can be installed in both new and existing buildings and can range from a quick, cheap water butt to a more complex system. Rainwater can be used for the garden, washing the car, showering, flushing the toilet, the washing machine, and even drinking if treated with UV light.

The simplest way to collect rainwater is in a water butt, with a diverter in the downpipe

The simplest way to collect rainwater is in a water butt, with a diverter in the downpipe.

Water butt

The simplest collection method is a rainwater butt connected via a diverter such as a Rainsava (see below) to a downpipe from the roof – water collected can be used on the garden, using a watering can.

Rain harvesting system

In a typical rainwater system water is collected from the roof and taken via pipes to a storage tank. A filter removes leaves and other debris and a settlement tank allows small particles to sink to the bottom; floating debris is skimmed off the surface via an overflow pipe, and clean water extracted from just below the surface. Water can be pumped directly from the tank to appliances or to your loft header tank. A float switch in the tank will automatically top up with mains water if the level is too low.

Drinking water

Rainwater can be used for drinking if you install a UV sterilisation unit; the unit is switched on permanently and uses c. 40 watts. This unit will need two filters before the water reaches it – 25 and 5 microns, otherwise micro-organisms can ‘hide’ behind particles.

rainfall figs

Table 1: above are the average annual rainfall figures for the last 30 years; use these figures in conjunction with table 2 below to find the volume of water you can expect to collect.
Source: Met Office.

What are the benefits of rainwater harvesting?

  • rainwater collection removes the need for the energy and chemicals used to produce pure drinking water – unnecessary if all we’re going to do is clean the car with it or flush it down the toilet
  • it also reduces the need for the pumping of mains water, and the energy use, pollution and CO2 emissions that go with it
  • it reduces demand on rivers and groundwater: the WWF recently reported that over-extraction by water companies is damaging Britain’s wetlands and trout rivers. The problem is most acute in the south-east, where population is increasing and millions more houses will soon be built. 86% of an average household’s water needs can be met by collecting rainwater, without further purification (33% for washing, 25% for toilet flushing, 22% for car washing and 6% for the garden)
  • other benefits: rainwater is soft, and leaves no limescale; washing clothes in soft water requires less detergent and so reduces water pollution from these compounds; plants love rainwater; it doesn’t contain chlorine, which is thought to be carcinogenic; large-scale collection of rainwater can reduce run-off and therefore the risk of flooding

Rainsava: can be inserted into your downpipe (round or square) to divert rainwater into a water butt; available online or from garden centres.

What can I do?

First, cut your water use: average UK per capita domestic water use is around 55m³/year (160 litres / day). This can easily be cut to less than 30m³/year by installing low-flush toilets (or better still, compost loos), fixing dripping taps, washing the car less often, and having showers instead of baths (especially if you share with a friend). Then find the rainfall figures for your area (see table 1) and your roof area, and check table 2 to see how much water (in m³) you can expect to collect per year. For an average property with average rainfall, you should be able to get around 100m³. Water is metered at between £1-2 per m³ (this will go up – prices have already risen 50% since privatisation). If you’re not metered, your water supplier is obliged to install one for free (currently around 25% of UK households are metered).

Maybe rainwater harvesting is something you want to do regardless of length of payback time, but in certain circumstances, it could be quite a cost-effective thing to do. If you can house a 1.5m³ storage tank in a garage or cellar, you won’t need a submersible pump, and you could buy your kit for around £1000. Then if you are handy with plumbing you could install yourself. Otherwise your tank will be underground and costs will be around £1500 for the system and £1000 for installation.

Some routine maintenance is required, such as cleaning the filters three times a year, keeping gutters clear, and checking everything once a year to see if it is working properly. Decide if you want the water from your tank to be pumped to your loft tank or direct to your appliances (cheaper) – seek advice from suppliers / installers.

expected volumes

Table 2: expected volumes of water collected in cubic metres can be found by checking your average rainfall (see table 1 above) against your roof area.
Source: Environment Agency.

There are no UK regulations concerning rainwater use for toilets, washing machines and gardens, though the back-up from the mains must be in accord with standard regulations.

If you are far from mains water, or don’t fancy chlorine or fluorine in your drinking water, a UV unit costs around £500, and you have to change the bulb each year (c. £40).


The specialist(s) below will respond to queries on this topic. Please comment in the box at the bottom of the page.

Terry Nash is a Director of the UK Rainwater Management Association, a trade organisation that promotes the use of water re-use to help avoid future flood and drought risks.

We'd love to hear your comments, tips and advice on this topic, and if you post a query, we'll try to get a specialist in our network to answer it for you.