Why Rainwater Is Worth Catching

Tap water is treated to drinking standard, then we pour it on the borders. Rainwater, by contrast, is soft, free of chlorine and lime, and roughly the temperature of the air around it. That combination suits plants far better than a cold blast from the mains, particularly acid-lovers such as camellias, rhododendrons, blueberries and pieris, which can yellow when watered with hard water over time.

There is a practical side too. A water butt connected to a downpipe means you can keep watering through a dry spell without watching the meter spin, and it gives you a buffer when hosepipe restrictions arrive. In most areas, water drawn from a stored butt is still permitted during a hosepipe ban, though it is always worth checking your supplier's current rules before relying on it. Tucked against a wall, a butt is also quietly useful for rinsing muddy boots, topping up a bird bath or filling a watering can for the greenhouse.

Working Out How Much You Can Collect

The maths is simpler than it looks. Every square metre of roof catches roughly one litre of water for each millimetre of rain. So a modest 30 square metre roof section will send about 300 litres down the downpipe during a single 10mm shower. Across a typical UK year, that same section could yield many thousands of litres, most of which currently disappears into the drain.

You do not need to capture all of it. A sensible approach is to size the butt to the space you have and the watering you actually do:

• Small courtyard or balcony: a slimline 100 to 150 litre butt tucked into a corner.
• Typical suburban garden: a 200 to 250 litre butt, often the best balance of capacity and footprint.
• Large plot or allotment: linked butts totalling 500 to 1,000 litres, or a pair of 250 litre units joined by a connector kit.

If you can, position the butt near where you water most, ideally on a paved or firm surface. A full 200 litre butt weighs over 200kg, so soggy grass is not the place for it.

Fitting a Diverter to the Downpipe

The clever bit is the diverter, a small unit that fits into the downpipe and sends water sideways into the butt. Most work by capturing the flow and letting anything the butt cannot take continue down the pipe as normal, so you never have to dash out in a storm to redirect anything.

Fitting one usually means cutting a short section out of the downpipe with a hacksaw, then clipping the diverter in place with the supplied seals. Measure twice. The diverter should sit high enough that the butt's lid still clears it comfortably, because as the butt fills you will want to lift the lid to check and clean. If your downpipe is round, oval or square, buy the matching collar rather than trying to make a round one fit.

Two details are easy to overlook. First, the overflow: fit the supplied hosetail and run a short length of pipe away from the house, ideally towards a border, soakaway or drain, never against the wall where it can dampen the brickwork or undermine a patio. Second, the stand. Raising the butt by 30 to 50cm gives noticeably better flow from a tap or drip line, and it makes filling a can far less of a stoop.

Keeping the Water Clean and Fresh

Rainwater off a roof carries grit, moss and the odd leaf, so a little housekeeping pays off. A leaf guard or mesh gutter screen at the top of the downpipe keeps the worst of the debris out, and a fine mesh over the butt inlet catches the rest. Dark-coloured butts limit the light that encourages algae, and a well-seated lid keeps it dark and discourages mosquitoes and midges from breeding.

Once a year, usually in early spring before the growing season gets going, empty the butt, rinse it out and check the diverter and overflow for blockages. That is also the moment to check the tap washer and to make sure the lid is secure, particularly if there are young children in the garden, since even a part-full butt is a drowning risk.

In a cold snap, leave a little air space at the top so expanding ice has somewhere to go, and check that any exposed pipework is not sitting in a puddle that will freeze solid around it.

Getting the Most From Your Stored Water

Gravity gives you a gentle flow rather than a powerful jet, which is fine for most jobs. A short hose run to a drip line or soaker hose works well on a raised butt, and watering cans remain the most precise tool of all.

A few habits stretch every litre further:

• Water at the roots in the early morning or evening, not over the leaves at midday.
• Spread a 5cm mulch of bark or compost over borders to slow evaporation.
• Use a can with a fine rose for seedlings and newly sown lawns, where a heavy flow would wash soil away.
• Group thirsty pots together in part shade so one watering session covers more ground.
• Top up the butt from the mains only as a last resort, and use a temporary hose with a backflow preventer if you do.

Rainwater also suits houseplants, ferns and carnivorous plants better than tap water, so keep a lidded can indoors for those. Once the system is in, it tends to become second nature: the butt fills, the plants thrive, and the hosepipe stays coiled in the shed for another summer.