How to harvest rainwater

How to harvest rainwater

John, Passionate gardener, Cambridge

Guide written by:

John, Passionate gardener, Cambridge

103 guides

Even if you take every possible measure to save water, you'll still have to water your garden from time to time. Harvesting rainwater is a cost-effective, eco-friendly and plant-safe way to provide your plants with the water they need. Read on to find the right water butt or tank to harvest rainwater from your own roof.

Important features

  • Advantages of harvesting rainwater
  • Different rainwater harvesting systems
  • Accessing your water
  • Water consumption
  • Regulations on harvesting rainwater
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Rainwater harvesting systems: the basics

Water butts

A traditional water butt will only take about an hour to install and just about any DIYer should be well equipped to handle the task. Designed to connect to your downpipe via a rainwater diverter, this type of rainwater harvesting system is ideal for home gardens. What's more, there's a model out there to match every budget. You can choose from a range of different designs from the most basic tank to a water butt with style!

Underground water tanks

Underground water tanks are costlier to install and require fairly extensive preparations as they need to be buried in the ground. Some below ground rainwater tanks can even be connected to a special rainwater plumbing system and an overflow for domestic use (provided you conform to UK legislation). The water you collect must can only be accessed using a submersible pump. These tanks are most commonly used to meet the watering needs of larger gardens.

UK rainwater harvesting legislation 

  • There are no restrictions when it comes to using rainwater to water the garden or clean outdoor equipment (for example, your car or garden dining set).
  • However, if you want to use rainwater indoors, you will have to conform to strict regulations. For example, you are allowed to use rainwater to supply a toilet, but you aren't allowed to use it to drink or wash with.
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Underground water tanks

Why harvest rainwater?

  1. First of all, it's good for the planet. Water is a natural resource and while it is of course renewable it is becoming increasingly scarce during certain times of the year. With things like water bans on the rise, it is always a good idea to find alternative ways to save water!
  2. It is free. Mains water is costly to supply and you'll feel the effects of this if you have metered water. A rainwater harvesting system will only cost as much as the tank and installation, and you're sure to make this money back fairly quickly.
  3. Rainwater is great for plants. Tap water is made potable by the addition of different additives, in particular fluoride and chlorine. It is not entirely clear what effect these chemicals have on plants. Almost half of the total amount of water we use up on a daily basis does not need to be treated. In fact, the only situation in which using rainwater in the garden is not advisable is in coastal areas as the salt content of the water can be too high.

Different types of rainwater harvesting systems

Standard water butts

This system is the most straightforward to set up and basically consists in installing a water butt or tank at the base of your downpipe. You may have to remove part of your downpipe for installation or you can divert the water through a rainwater collector. If you want to re-purpose another type of tank, be careful: it may have been used to contain chemicals. You can choose from a wide variety of rainwater harvesting systems which can range from 200 to 10,000 litres in capacity. If you're only looking to supply your garden or vegetable patch with water, a capacity of a few hundred litres (or up to 1000 litres) will do.

Stone-effect water butts

These days, it's possible to find all manner of decorative plastic water butts such as terracotta vases or amphora-inspired designs. Others have been designed to blend into the background with stone or wood effects.

Collapsible water butts

It's also possible to find high-capacity collapsible water butts that can be tucked away under a deck, crawl space or any other hidden spot.

Underground rainwater tanks

The most discreet — but also the most expensive — solution consists in burying a tank underground. It can be a good idea to incorporate this type of system when drawing up plans for a new home or garden, especially if you plan to use rainwater for a variety of purposes (washing clothes, flushing the toilet, cleaning the car, gardening and so on).

Water butt link kits

A ready-made installation water butt link kit will make it much easier to install your water butt. These kits usually comprise a rainwater diverter and a hose to attach to your water butt. Don't forget to install a filter at the top of the downpipe to catch various types of debris including dead leaves or moss and to clean this part on a regular basis. You can also install a leaf trap at the top of the downpipe.

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Gutter accessories

How to access your harvested rainwater


You may decide to set your water butt up on a stand some inches off the ground. In this case, your water will simply be accessed by gravity. A tap can be turned to draw water through a hose or directly into a watering can.


An electric submersible or surface pump can also be used to access water for the garden. Underground rainwater tanks will need to be equipped with a submersible pump to distribute the water stored in the tank.

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Submersible pumps

Evaluating your water needs

You will need to calculate your water needs based on how you plan to use the rainwater. If you just want to water the garden, vegetable patch and perhaps wash your car about once a week, a water butt or tank with a capacity of 200 to 1000 litres will suffice.

If you plan to use the water indoors, you will need a much larger tank (around 2500 to 10,000 litres). A tank capacity of 5000 litres is generally the most common option if you're looking to use rainwater around the house (bearing in mind that the tank must be buried underground in this case).

How much rainwater can I harvest?

If you live in an area with only a moderate amount of rainfall, your roof should provide about 600 litres of water / m2 each year. You can find a more precise estimate by using the following calculation:

  • [(Surface of the roof in m² x Average rainfall in mm) x 0.9] = Litres of harvested rainwater per year

Basically, multiply the surface of your roof (in square metres) by the average amount of year rainfall in your area in millimetres. To account for water loss through evaporation, you should multiply your result by 0.9. This will give you a clearer idea of how much rainwater you can collect over the course of a year.

For example, if your roof measures 100 m2 and your yearly rainfall average is 700 mm, you can harvest 63,000 litres of water in one year (or 63 m3 ) as 100 x 700 x 0.9 = 63,000.

Estimating your water needs

Daily consumption

Monthly consumption

Usage over a set period

Average consumption per person

110 litres / day

3300 litres / month

40,000 litres / year

Watering a lawn

3 litres / m² / day

90 litres / m² / month

270 litres / m² / 3 months

Watering a vegetable garden

6 litres / m² / day

180 litres / m² / month

1080 litres / m² / 6 months

UK regulations on harvesting rainwater

Rainwater is not suitable for consumption. You cannot drink it, use it to cook with or even wash yourself as this water is likely to contain pollutants such as sulphates, nitrates and even pesticides. There are no rules on collecting rainwater from your roof if you plan on using this water outdoors (for example, to wash your car or water the garden).

However, the rules are not the same if you want to use rainwater indoors for your toilet, cleaning the floor or washing clothes (which is allowed as long as the water is treated but is not advisable). What's more, there are strict regulations to ensure that your water sources (i.e. your rainwater and mains water supply) are never mixed.

If you aim to use a rainwater harvesting system for any of these applications, it must be installed by a professional. You must also contact your local planning authority to alert them to your plans.

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Guide written by:

John, Passionate gardener, Cambridge, 103 guides

John, Passionate gardener, Cambridge

When I was young, I was already working in the family garden. Perhaps that is where my interest in plants and gardening came from. So, it was logical for me to study plant biology.   At the request of various publishers I have, over twenty-five years, written many books on the subject of plants and mushrooms (a subject that is close to my heart). They were mostly identification guides at first, but shortly after they were about gardening, thus renewing the first passion of my childhood.   I have also regularly collaborated with several magazines specialising in the field of gardening or more generally in nature. There is no gardener without a garden, I have cultivated mine in a small corner of Cambridge for the last thirty years and this is where I put into practice the methods of cultivation that will I advise you in as well.

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