Why Rainwater Is Worth Catching

Every square metre of roof sheds roughly a litre of water for every millimetre of rain. In a typical English county that means 700 to 900 mm a year, so even a modest shed roof is quietly tipping hundreds of litres down the drain annually. That is water your borders, containers and hanging baskets would happily drink, and it arrives free, soft and slightly warm rather than shockingly cold from the mains.

There is a practical edge here too. Mains water is treated to drinking standard, then we pour it on petunias. If your supply is metered, the saving is real, and during a hosepipe ban harvested rainwater is normally still permitted for garden use. For most gardeners, though, the appeal is simpler: a full butt means you can water properly in July without watching the tap.

Rainwater also suits ericaceous plants such as rhododendrons, camellias and blueberries far better than hard tap water, and it will not leave limescale marks on leaves or greenhouse glass.

Choosing the Right Butt for Your Plot

Water butts come in sizes from a slimline 100 litres that tucks against a wall, through the classic 200 to 250 litre workhorse, up to linked systems holding well over a thousand litres. Two 200-litre butts joined together are usually a wiser buy than one large tank, because you can site them where they fit and let the first fill the second.

Look for these details when you buy:

• A tap positioned at the lowest possible point, with enough clearance underneath for a watering can.
• A securely fitting lid that keeps out leaves, daylight and mosquitoes. Dark green or black plastic limits algae far better than pale colours.
• A moulded inlet near the top and a threaded overflow outlet, so linking and draining are straightforward later.
• Recycled plastic construction, which shrugs off frost and costs less than timber barrels.

Standing the butt on a proper plinth is not fussiness. A 200-litre butt weighs around 200 kg when full, so a level base of paving slabs, gravel or a purpose-made stand will stop it leaning and cracking over winter.

Siting It Where Gravity Does the Work

Position matters as much as capacity. A butt raised 30 to 45 cm on a stand gives you a decent flow from the tap into a can, and a metre of height above your watering point is enough for a short hose run. Placing it in the shade reduces algae, and keeping it close to the beds you water most often saves a hundred trips with a full can.

Practical points worth checking before you commit:

• Can you reach the downpipe without a long, kinked run of connecting hose?
• Does the butt block a path, a door or a window you actually use?
• Will the overflow discharge somewhere useful, such as a border or gravel soakaway?

Fitting a Diverter to Your Downpipe

A rain diverter, sometimes sold as a rain trap, is the small device that makes the whole system work. It fits into the downpipe, sends water along a short hose into the butt, and once the butt is full, the surplus simply continues down the pipe to the drain.

First, identify your downpipe: most modern UK guttering is 68 mm round or 65 mm square, while older cast iron is nearer 110 mm. Buy a diverter sized accordingly. Then measure the diverter body against the pipe, usually 10 to 15 cm of pipe needs cutting out, and mark it carefully.

Cut the pipe with a hacksaw, clean the cut edges with a file, and slot the diverter in, refitting the pipe below it. Most units are push-fit with rubber seals, though some designs require a hole cut with a hole saw instead. Fit the diverter as low as the butt height allows, and check the seals with a bucket of water before the next downpour.

Linking Butts and Handling Overflow

To link two butts, use a connector kit fitted near the top of each, so the first fills the second once it reaches that level. Keep the connecting pipe short and with a slight fall, and make sure both butts stand on level ground or the joints will strain.

The overflow is the part most gardeners overlook. Route it to a border, a rain garden, or a gravel-filled soakaway at least a metre from the house wall. A dry shade bed of ferns and hostas will appreciate the occasional soak far more than a soggy patio. If you have a soakaway or a water butt overflow pipe, direct it away from your foundations.

Keeping the Water Clean and Using It Well

Roof runoff carries moss, grit and bird droppings, so a few simple habits keep the water pleasant to use. Fit a mesh leaf guard in the gutter or at the diverter inlet, keep the lid firmly on, and empty and rinse the butt each autumn before the winter refill. A first-flush diverter, which discards the first few litres of each storm, keeps sediment right down.

Use harvested water freely on borders, containers, pots and the greenhouse. It is noticeably warmer than mains water, which suits seedlings and tender plants. Do not drink it, and wash any fruit or vegetables before eating, particularly leafy crops. If a butt sits empty through a hard frost, leave a little headroom when it refills so ice has somewhere to expand, and check the overflow is clear before winter. Set up properly, a simple butt and diverter will serve your garden for fifteen years or more, quietly turning wet days into a summer supply.