Rainwater harvesting for gardens: from rain barrel to underground tank

Free water from the sky

The UK receives between 600 and 1,200 mm of rainfall a year depending on where you live. A typical semi-detached house with 70 m² of roof area catches 42,000 to 84,000 litres of rain annually. That is enough to water an average garden for the entire growing season — and right now most of it goes straight into the storm drain.

Collecting rainwater makes so much sense you wonder why everyone does not do it. Rainwater is better for your plants (no lime, no chlorine), it saves drinking water, it cuts your water bill and it reduces the strain on drains during heavy downpours. The investment pays for itself within a few years.

How much water can you collect?

The calculation is straightforward: roof area in m² multiplied by annual rainfall in metres multiplied by 0.9 (a loss factor for evaporation and splash). A roof of 70 m² in an area with 800 mm rainfall produces: 70 × 0.8 × 0.9 = 50,400 litres per year. That is roughly 138 litres a day on average.

Summer brings less rain and higher water demand. Winter is the reverse. Storage capacity is the pivot point: the more you can buffer, the less you rely on mains water during dry spells.

Collection options

The classic rain barrel

The simplest solution. A 200–300 litre barrel sits beside the downpipe and catches water directly. Advantages: cheap (£25–65), easy to install, no excavation. Disadvantages: limited capacity, overflows in heavy rain, empties quickly in summer.

Tip: connect several barrels in series. Three 300-litre barrels give you 900 litres of buffer. Link them with an overflow hose fitted at three-quarter height.

IBC container

An IBC (Intermediate Bulk Container) is an industrial vessel holding 1,000 litres in a steel cage. Second-hand they cost £20–50 — a bargain per litre of storage. Downside: they are ugly. Build a screen or slatted fence around them, or paint the inner tank dark to inhibit algae growth.

Decorative rain barrel

Rain barrels shaped like an amphora, a wooden cask or a stone column. Prettier but pricier (£80–250) and typically smaller (150–250 litres). Suitable if the barrel will be visible in a front garden.

Underground rainwater tank

The serious option. A plastic or concrete tank holding 1,500 to 10,000 litres, buried in your garden. Advantages: huge capacity, invisible, water stays cool and algae-free. Disadvantages: excavation, higher cost (£700–2,500 including installation), and you need a pump to bring the water up.

For new builds, an underground tank is well worth considering. In existing gardens, you need to get a digger into the plot, which is not always feasible.

Connecting to the downpipe

Most barrels and tanks are fed via a rainwater diverter or leaf filter fitted into the downpipe. Here is how it works:

1. Cut a section out of the downpipe at the desired height
2. Fit the leaf separator or rain diverter — this filters coarse debris and channels water to the barrel
3. Connect the overflow to the drain, a soakaway crate or a rain garden

A leaf separator costs £12–35 and prevents leaves and twigs fouling your barrel. Clean it twice a year.

Overflow: where does excess water go?

Many people forget this. When the barrel is full, water pours over the rim and creates a muddy patch beside the house. Always connect an overflow that routes surplus water to:

• The storm drain (the easiest option)
• A soakaway crate in the garden (water percolates slowly into the ground)
• A rain garden or bioswale (see below)
• A second barrel in series

Pumps for water pressure

Water from a rain barrel runs by gravity: raise the barrel on a stand and attach a hose to the tap at the bottom. That works for a watering can, but not for a sprinkler or drip irrigation.

For pressure you need a garden pump. A submersible pump (£40–120) sits inside the barrel; a suction pump (£70–200) stands beside it. For an underground tank you need a domestic water pump (£120–350) with automatic on/off switching.

Combining with drip irrigation

Drip irrigation fed by rainwater is the holy grail of sustainable gardening. Connect your pump to a timer and a drip-line system, and your garden waters itself with free rainwater. Always fit a filter between the pump and the drip system — fine grit blocks the drip emitters.

Rain gardens and bioswales

A rain garden is a sunken area planted with species that tolerate both wet and dry spells. When it rains, water flows into the depression, percolates slowly into the soil and feeds the plants. A bioswale does the same thing in a linear channel.

Good plants for a rain garden

• Iris pseudacorus (yellow flag iris) — handles both drought and temporary flooding
• Lythrum salicaria (purple loosestrife) — stunning purple flower spikes
• Carex (sedge) — evergreen ornamental grass
• Eupatorium (Joe Pye weed) — a magnet for butterflies
• Cornus sericea (red-osier dogwood) — bright red winter stems as a bonus

Swales and infiltration trenches

A swale is a shallow, wide channel (30–50 cm deep) filled with gravel or rubble, topped with a layer of soil and planting. During heavy rain, the swale acts as a buffer: water soaks slowly into the ground rather than running across the surface.

In new housing developments, swales are increasingly used as an alternative to piped drainage. In existing gardens, you can dig a small swale alongside your patio or path to catch runoff from hard surfaces.

Permeable paving

Every square metre of impervious paving you replace with permeable surfacing gains catchment capacity. Options:

• Gravel grids: plastic grids filled with gravel that let water pass through
• Grass-concrete blocks: paving with holes where grass grows through
• Porous block paving: specially made pavers that allow water infiltration
• Wide joints: standard pavers with generous joints filled with grit

Permeability ranges from 50 to 300 litres per m² per hour — more than enough for the heaviest downpours.

Green roofs

A sedum roof on your shed, garage or carport retains 15–35 litres per m². Some of the water evaporates; the rest drains slowly to your downpipe. An added benefit: insulation in summer and winter.

A sedum roof costs £35–70/m² installed. The structure must handle the extra weight (50–100 kg/m² when saturated) — always have this assessed by a structural engineer.

Grants and incentives

Many water companies in England and Wales offer rebates for rainwater harvesting equipment. Thames Water, for instance, has offered subsidised water butts in the past. Some local authorities provide grants for disconnecting downpipes from the storm sewer.

Check your water company's website and your council's sustainability or flooding pages for current offers. The RHS and environmental charities also run periodic rain barrel campaigns with discounted prices.

Cost-benefit analysis

The saving depends on your garden size and water usage. A household with a 100 m² garden that waters twice a week in summer uses roughly 10,000 litres per season. At metered rates of around £3.50/m³ that is £35. But if you run drip irrigation entirely on rainwater, the annual saving rises to £40–80. The real gain lies in avoiding hosepipe bans and reducing your environmental footprint.

Maintenance

Filters and leaf separators

Check and clean your leaf separator twice a year: in spring and after leaf fall in November. A blocked filter lets water run straight down the pipe instead of into your barrel.

Mosquito prevention

Standing water attracts mosquitoes. Stop your rain barrel becoming a breeding ground:

• Always cover the barrel with a lid or fine mesh
• Drop a small piece of copper in the water (mosquito larvae cannot tolerate copper ions)
• Use biological larvicide (Bti) if the barrel must remain open

Winterising

In areas with hard frosts: drain rain barrels before winter or leave them a quarter full with a polystyrene float inside to absorb expansion. Underground tanks do not need draining — they sit below the frost line.

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