Rainwater harvesting at home: a practical guide

Rain falls on your roof for free. Catching it before it disappears down a drain cuts your mains water demand, is better for your plants, and costs very little to set up.

Why collect rainwater?

Mains water has been abstracted from rivers, reservoirs or aquifers, treated to drinking standard, pumped through miles of pipe and metered into your home — all at an energy and financial cost. Using it to water the garden, which needs no treatment at all, is an inefficiency worth solving.

• It's free. Once your collection system is in place, the water costs nothing. In hot summers or drought periods that can mean real savings if you're on a water meter.
• It's better for plants. Rainwater is naturally soft, slightly acidic and free from the chlorine and fluoride added to mains water. Acid-loving plants like blueberries, rhododendrons and tomatoes do especially well on it.
• It eases pressure on local supplies. In periods of high demand, every litre of rainwater used in the garden is a litre of treated mains water saved.
• It reduces surface water runoff. Collecting roof runoff before it reaches drains reduces the risk of local drain overload in heavy rain events.

Install a water butt — step by step

A standard water butt on a downpipe diverter is the simplest and most common setup. You need the butt itself, a diverter kit (usually sold together), a stand to raise the butt for tap access, a hacksaw and a drill.

1. Choose your downpipe. Pick a downpipe from a large roof area — a garage, kitchen extension or main house roof. The bigger the catchment, the faster the butt fills.
2. Position the stand. Place the stand on a firm, level base — paving slabs or compacted gravel are ideal. Raise the butt high enough so the tap clears the ground and a watering can fits comfortably underneath. At least 30 cm (12 in) of clearance is practical.
3. Mark and cut the downpipe. Hold the diverter fitting against the downpipe at the correct height for the butt inlet. Mark the cut line. Cut with a hacksaw — take your time for a clean, square cut.
4. Fit the diverter. Follow the manufacturer's instructions — most diverters clip around the cut section and include a flexible hose that runs to the butt inlet. When the butt is full, water automatically bypasses down the original pipe.
5. Connect the inlet hose. Run the diverter hose to the inlet on the lid of the butt. Fit any included mesh or filter to keep leaves and debris out.
6. Fit the overflow hose. Connect the overflow pipe (usually supplied) to the overflow outlet on the butt and direct it to a drain, soakaway or a second linked butt.
7. Fit the tap. Most butts come with a tap fitting. Attach it with the supplied washer and tighten — hand-tight plus a quarter turn is usually sufficient to prevent leaks without cracking the plastic.
8. Check it works. Wait for rain — or pour a bucket of water into the downpipe above the diverter — and confirm water flows into the butt and the overflow works correctly.

Sizing and placement

The amount of water you can collect depends on two things: your roof area and how much rain falls on it. As a rough guide, 1 mm of rain falling on 1 m² of roof yields approximately 1 litre of collectible water (minus a little to losses). A 50 m² (540 sq ft) garage roof in an area with 600 mm annual rainfall could theoretically fill a 200-litre butt many times over in a year.

• A 200-litre (44-gallon) butt is the standard starter size and suits small to medium gardens.
• For larger gardens or drier climates, consider 500–1,000-litre capacity, either one large butt or two standard butts linked in series.
• Linking two butts is simple: a short hose connects the overflow of the first to the inlet of the second, so the second fills only when the first is full.
• Raise the butt on a stand — gravity-fed taps need height to deliver a useful flow. The higher, the better the pressure.

Keep it clean and safe

A water butt left uncovered and unmanaged will turn into a mosquito nursery and a green soup of algae. Neither is a problem if you take a few simple steps.

• Always keep it covered. A close-fitting lid or mesh cover blocks sunlight (which algae need to grow) and prevents mosquitoes from reaching the water surface to lay eggs. Never leave the lid off permanently.
• Filter the inlet. A fine mesh or leaf guard on the inlet hose keeps leaves, twigs, insects and bird droppings from entering the butt.
• Position away from direct sun. A shaded spot slows algae growth significantly. North-facing walls (in the northern hemisphere) are ideal if you have the choice.
• Clean it annually. Once a year — ideally in late autumn — empty the butt completely, scrub the inside with a brush and rinse. Sediment accumulates at the bottom and can harbour bacteria and block the tap.
• Flush first flushes. The first heavy rain after a dry period washes the most dirt off the roof. If your diverter allows, let the first flush go to waste rather than into the butt.

Using your harvested water

In the garden: Untreated rainwater is ideal for watering vegetables, flowers, lawns and established plants. It is free of chlorine and naturally suited to most garden plants. Water at the base of plants rather than overhead, and avoid applying it directly to the edible surfaces of crops you are about to harvest — wash all homegrown produce before eating, as you would normally.

For indoor and laundry use: Some larger rainwater harvesting systems feed filtered water to toilet cisterns or washing machines, which are genuinely water-intensive uses. However, this requires proper filtration, UV treatment or chemical disinfection, correct pipe labelling to prevent cross-connection with drinking water, and compliance with local plumbing and health regulations. This is a worthwhile option for large systems, but must be done correctly — take professional advice and check your local regulations before connecting harvested rainwater to any indoor plumbing.

For drinking and cooking: Roof-harvested rainwater must not be used for drinking, cooking or any food preparation without multi-stage filtration and disinfection. Even then, local regulations may prohibit or restrict this use. In most domestic situations, mains water or a certified filtration system is the appropriate choice for drinking water.

Larger and underground systems

A water butt is the simplest entry point, but it is not the only option. For households that want more capacity or year-round use, larger systems are available.

• Above-ground tank systems (500–5,000 litres) suit larger gardens and allotments. Some include a pump for pressurised hose watering.
• Underground cisterns (1,000 litres and above) are installed below the garden and fed by buried pipes from multiple downpipes. They stay cool (which slows bacterial growth), take up no visible space and can store enough water to irrigate a large garden through a dry summer. Installation requires excavation and is a significant project, but the payback over years of use is real.
• Larger systems typically include a first-flush diverter, coarse filtration at the inlet, an overflow to a soakaway or drain, and may include a submersible pump. These systems benefit from professional installation to ensure correct overflow management and to avoid any unintended connection to mains or indoor plumbing.

Winter care

In climates where temperatures drop below freezing, water left in a butt can freeze, expand and crack the plastic. Take simple steps before the first hard frost.

• Drain the butt before a hard frost — or at least reduce the water level to well below the overflow fitting.
• Disconnect the diverter so winter rain bypasses the butt entirely, and reconnect it in spring.
• Insulate if you prefer to keep some water: an old blanket or bubble wrap secured around the outside gives some protection against moderate frosts.
• Leave the tap open on an empty butt over winter so any residual water can drain and pressure cannot build from ice.

In mild maritime climates where hard frosts are rare, you may be able to use your butt year-round with no special winter preparation beyond keeping the lid on.

Quick setup checklist

• Choose a downpipe from a large roof area close to where you use water most.
• Position the butt on a stand so a watering can fits under the tap.
• Fit a diverter kit according to the manufacturer's instructions.
• Fit a mesh inlet filter to keep out leaves and debris.
• Connect an overflow hose to a drain or a second linked butt.
• Keep the lid on at all times to prevent mosquitoes and algae.
• Position in shade where possible to slow algae growth.
• Clean the butt out once a year, flushing sediment from the bottom.
• Drain and disconnect before hard frosts in cold climates.
• Check local regulations before using harvested water for any indoor purpose.