Rainwater Harvesting for Gardens: Water Butts, Storage and Use

Rainwater harvesting for gardens means catching roof water in water butts and storage tanks, so you can water plants free of charge and keep them going through dry spells.

A typical roof sheds hundreds of litres in one shower, and most of it runs to the drain. This guide explains how much you can collect, which butt to choose, how to connect it, how to keep the water clean and how to use every litre well. It’s general information, and your local water company, your extension service or the Royal Horticultural Society (RHS) can add advice for your area and any local rules.

It belongs to the Soil, Water and Fertilizer guides and works with Drip Irrigation and Smart Watering, Mulching Mastery and Watering Plants the Right Way.

Rainwater harvesting for gardens at a glance

These are typical values. Rain patterns and local rules vary, so check with your water company before you plumb in a larger system.

How much rainwater you can collect

Multiply roof area in square metres by rainfall in millimetres to get litres, then take about 80% to allow for losses. A shed roof of 4 m² collects about 130 litres from 40 mm of rain, while a house roof section of 30 m² collects about 960 litres from the same rain. A single butt fills quickly, so a second and third butt linked by a short pipe stores the surplus. Plan for about 10 to 15 litres per square metre per week for a vegetable bed in summer.

How to set up a water butt step by step

1. Choose a downpipe. Pick one near the beds you water most, on firm, level ground.
2. Build a stable base. A full butt weighs 200 kg or more, so use paving slabs or a sturdy stand 30 to 40 cm high.
3. Fit a diverter. Cut the downpipe at the right height and fit a diverter kit that sends water in and lets overflow run to the drain.
4. Add a leaf filter and lid. A fine mesh keeps debris and mosquitoes out, and a lid keeps light out to reduce algae.
5. Raise the tap. A tap 30 cm up lets a watering can fit under it, and a hose gets better flow from a raised base.
6. Link more butts. Connect them with an overflow pipe so the first fills, then the next.

How rainwater affects soil and plants

Rainwater lacks chlorine and carries few salts, so it suits blueberry, azalea, rhododendron and camellia, which need acid soil. Hard tap water raises pH and locks iron in hydrangea and camellia, while rainwater reduces that risk. Mulch of 5 cm protects soil and reduces evaporation by up to 50%, compost feeds loam and holds water in sand, and clay holds water and causes rot where loam balances drainage. Tomato needs steady water because swings cause blossom end rot.

Drip lines of 2 litres per hour carry rainwater to the roots and reduce blight on tomato and potato. Lavender and rosemary tolerate drought and need little stored water, while lettuce and courgette need regular drinks. Compost tea made with rainwater feeds tomato and rose lightly, and earthworm improves loam in wet beds. A small pond filled by overflow attracts frog and dragonfly, and a shallow dish helps bee, so water early in the morning to lose less to heat.

A worked example: a small vegetable plot

Say you’ve got a 4 m² shed roof and a 6 m² raised bed of tomato, lettuce and bean. In a 20 mm shower the roof gives about 65 litres, so two linked 200 litre butts fill after a few showers. The bed needs about 60 litres a week in summer, so a full pair of butts lasts 6 to 7 weeks without extra rain. Mulch protects roots and reduces watering by up to half, and compost feeds soil and holds water in sand. Slug attracts to constant wet leaves, so water at the base in the morning. Tomato needs steady water, and lavender tolerates a dry edge, so you’ll keep the thirsty plants closest to the butt. Your water company or extension service can confirm local rainfall and any rules on larger tanks.

Marigold repels whitefly near tomato and basil attracts bee beside it. Blight attacks potato and tomato in damp weather, so water the soil rather than the leaves.

Keeping stored rainwater clean

• Cover everything. A lid and a fine mesh stop mosquitoes laying eggs and leaves rotting inside.
• Fit a first-flush diverter. It sends the dirtiest first litres of a storm to the drain.
• Use within a few weeks. Stagnant water in warm weather can smell and grow algae.
• Clean once a year. Empty the butt, scrub the base and rinse it.
• Don’t drink it. Rainwater from roofs isn’t safe for drinking, and you shouldn’t use it on salad leaves close to harvest without washing them.

Avoid collecting from roofs treated with copper or chemical moss killers, or from a roof with a lot of bird droppings. Your water company or extension service can advise on local safety rules.

Making every litre count

Water in the early morning, aim at the base of plants and soak the soil to 15 cm rather than sprinkling. Add mulch after watering to lock moisture in. Group thirsty plants such as tomato, courgette and hydrangea near the butt, and keep drought-tolerant lavender, rosemary and sedum at a distance. Link the butt to a drip line with a gravity feed, and you’ll water a bed in 30 minutes without carrying a can. For method details see Watering Plants the Right Way and Drip Irrigation and Smart Watering.

Common mistakes

• Using a small butt on a big roof. It overflows in the first shower and runs dry in a week.
• Leaving the butt uncovered. Algae, leaves and mosquitoes follow quickly.
• Placing the butt on soft ground. A full butt can tip or sink.
• Forgetting winter. Disconnect and drain pipes in freezing weather to prevent cracks.
• Watering lightly and often. Deep, less frequent watering builds stronger roots.

Next steps

Measure your roof, choose a butt and fit it before the next rainy season. Combine it with Mulching Mastery and Compost Tea for healthier beds, and read Reading Your Soil to match watering to soil type. The Soil, Water and Fertilizer guides link every step.