Why collect rain at all
Rain that falls on a roof normally runs into a drain. Catching some of it in a container gives you free water for the garden, which is useful in dry spells when mains water might be restricted and when plants need it most. Rainwater also has no added treatment chemicals, which some gardeners prefer for sensitive plants, and it is naturally soft, so it does not leave limescale in a watering can the way hard water can.
The benefits to the wider system are modest but real. Garden watering can account for a large part of a household’s summer use, and using stored rain for it can reduce demand on mains supplies. Water butts can also hold back a little water during heavy rain, though they are small compared with the volumes involved in a storm and they are not a flood solution. Overall, rainwater harvesting is one useful part of sensible water management, not a way to become independent of the mains.
What a basic system involves
The usual setup for a home is a water butt: a lidded container that collects water from a roof through a downpipe.
- A diverter kit fits into the downpipe and sends water into the butt. When the butt is full, the water carries on down the drain as before.
- A stand or base raises the butt so you can fit a watering can under the tap. Water is heavy, so the base must be firm and level, and a full butt is not something to move.
- A lid keeps out leaves, insects and light, and stops children and animals falling in. A secure lid is a safety matter as well as a hygiene one.
- An overflow pipe lets excess water escape safely, away from walls and foundations.
- A tap near the bottom, though sediment settles at the base, so a tap slightly above the floor of the butt is sometimes preferred.
Sheds, garages and outbuildings often have guttering that can also feed a butt. Where space allows, linking two butts together multiplies the storage.
How much capacity do you need
Common water butts hold somewhere in the region of 100 to 300 litres, and bigger tanks exist. The right size depends on your roof area, your rainfall and how much gardening you do. A bigger butt stores more during wet periods but takes up more room and is heavier. If you have a small garden or mostly container plants, a smaller one may be enough. If you grow vegetables or have a large lawn and beds, you may find you run dry quickly and wish for more storage. Mulching and choosing plants suited to your local conditions reduce how much you need.
Keeping the water in good condition
Stored rainwater is not sterile. Roofs collect bird droppings, leaves and dust, and water that sits can develop algae or bacteria, especially in warm weather and sunlight.
- Keep the lid on and use an opaque butt to limit algae.
- Clear the gutters and fit a leaf filter or mesh where possible.
- Clean the butt occasionally, following the maker’s advice.
- Use water within a reasonable time rather than leaving it for months.
- Be cautious about roofs with moss treatments or certain old materials, and avoid using water collected from roofs that have been treated with chemicals on edible plants unless you are sure it is safe.
Some people worry about the first flush of rain after a dry spell because it washes off the most dirt. A first-flush diverter handles that, though it is optional for ordinary garden use.
What stored rainwater should not be used for
Garden butt water should be treated as non-drinking water.
- Drinking, cooking and brushing teeth: not suitable unless it has been properly filtered and treated by a system designed for that. A home water filter is not a substitute for that kind of treatment and does not make unknown water safe.
- Bathing and washing: not recommended, particularly for children, babies or anyone with a weakened immune system or open wounds.
- Filling paddling pools or drinking bowls for pets: avoid, as the water may contain bacteria.
- Connecting to household plumbing: indoor use, such as flushing toilets or feeding a washing machine, needs a purpose-designed system, which is installed in line with local regulations. Never connect a rainwater tank to the drinking supply, because this can contaminate mains water. A qualified plumber can advise on what is allowed where you live.
- Edible plants: using rainwater on vegetables is common, but it is sensible to water the soil rather than the leaves of crops you will eat raw, and to wash produce before eating. If you have any doubts about your roof, use mains water on leafy crops.
Rules about collecting and using rainwater differ between countries and regions, and some places have restrictions, so it is worth checking local guidance.
Garden-friendly habits that go with it
Using stored water well matters as much as collecting it. Water in the evening or early morning, aim at the roots, and use a watering can or a hose with a trigger rather than a sprinkler. Beyond the lawn, a pond or other water feature for wildlife can also be topped up with rainwater, which is gentler on aquatic life than tap water, a point that links to wildlife ponds. If you are working out how much you can save overall, the broader picture of household water use is a good place to start, since garden use is only one part.
Is it worth it?
A water butt is inexpensive compared with most home improvements, and it is generally easy to set up without specialist help. It does take some care to keep clean and safe. Whether it saves money depends on how you are billed: it makes the most financial sense if you pay by meter and water a lot, and less so if you pay a flat charge. Even then, many gardeners find it useful because rain is available when restrictions might apply.