How do I collect and use rainwater in the garden?
Sizing and siting butts, linking them, keeping the water usable, and where rainwater is genuinely better than tap water.
- Difficulty
- beginner
- Time
- 2 hr
- Read
- 4 min
- Safety
- caution
Short answer
A single 200 litre butt off a house downpipe empties in one dry week. Link several, add butts to the shed and greenhouse, raise them enough to get a can under the tap, and keep them lidded and shaded. Rainwater is essential for lime-hating plants and better for almost everything else.
Roof area is what determines how much you can collect, and it is usually far more than people expect: even a modest shed roof yields hundreds of litres from a single wet week. The constraint is almost always storage, not supply.
Safety
What you'll need
- Water butt with lid and tap
- Butt stand or blocks
- Downpipe diverter kit
- Linking kit (optional)
- Watering can
Step by step
- Work out your roof area.Every millimetre of rain on a square metre of roof yields about a litre. A 30 square metre roof and 10mm of rain is roughly 300 litres, which shows how quickly a single butt fills and overflows.
- Collect from every roof you have.House, garage, shed and greenhouse. The shed and greenhouse butts are the useful ones because they are already where the watering happens.
- Raise the butt properly.High enough to get a full can under the tap, on a purpose-made stand or solid concrete blocks on firm level ground. A full butt is extremely heavy and a collapsing stand is dangerous.
- Fit a diverter, not a cut pipe.A proper downpipe diverter fills the butt and then sends the excess down the drain automatically. Cutting the downpipe means the butt overflows against the wall in every storm.
- Link butts together.A linking kit joins two or more butts near the base so they fill and empty as one. This is the cheapest way to multiply storage.
- Keep it lidded and shaded.A lid keeps children, animals, leaves and mosquito larvae out. Shade keeps algae down. A butt in full sun goes green and smells within weeks.
- Keep leaves out at source.A gutter guard or a leaf filter in the diverter prevents the sludge that eventually blocks the tap and fouls the water.
- Use it on the right plants.Rainwater is slightly acidic and free of the lime in hard tap water, which makes it essential for camellias, rhododendrons, blueberries and anything in ericaceous compost.
- Use mains water for seedlings.Butt water carries the fungi that cause damping off. It is fine for established plants and a poor choice for seed trays.
- Clean it out annually.Tip and rinse in late winter. Sludge in the base blocks taps and makes the water anaerobic and smelly.
- Empty or protect it in hard frost.A full butt can split if the contents freeze solid. Draw it down before a prolonged freeze where you can.
Tips
- One millimetre of rain on one square metre of roof is about one litre. Do the sum for your shed — it is more than you think.
- Butts on the shed and greenhouse get used. A butt at the far corner of the house does not.
- Rainwater for ericaceous plants, mains water for seedlings.
Common mistakes
- Fitting one butt and considering the job done — A single 200 litre butt is about one week of summer watering for a modest garden. Linked butts or several around the plot are what make rainwater a real supply.
- Standing a butt on soft ground or loose blocks — A full butt weighs over 200kg. As the ground settles it leans and eventually goes over, which is dangerous as well as messy.
If it doesn't work
Water smells and looks green
Cause: Sunlight and organic matter — Fix: Fit or replace the lid, shade the butt, and clean the sludge out.
Butt overflows against the wall
Cause: No diverter, or a blocked one — Fix: Fit a proper diverter kit that returns excess to the downpipe.
Tap blocked
Cause: Sludge and leaf debris in the base — Fix: Empty and rinse the butt, and add a leaf filter at the diverter.
Empty within days of a dry spell
Cause: Insufficient storage — Fix: Link additional butts and collect from every roof available.
Questions people ask
How much water can I actually collect?
About one litre per square metre of roof per millimetre of rain. A 6x4 shed roof in a 10mm shower yields around 200 litres, which fills a butt in one afternoon of rain.
Is rainwater better than tap water?
For lime-hating plants, considerably — it has none of the lime that hard tap water carries. For everything else it is at least as good, and it is free. The exception is seedlings, where mains water is safer.
Does a water butt need cleaning?
Once a year. Tip it, rinse the sludge out and refit the lid. Sludge blocks the tap and makes the water anaerobic and smelly.