Using Rainwater Butts in the Garden

Why rainwater is worth collecting

Rainwater is soft, slightly acidic and free of the chlorine, fluoride and lime that come out of the tap. That matters more than most gardeners realise. Blueberries, camellias, rhododendrons, pieris and most ferns will thank you for it, and even thirsty summer bedding establishes better with a few canfuls of the soft stuff than with a hard mains soaking. It is also, quite simply, free once the kit is up.

The maths is encouraging. Every square metre of roof collects roughly one litre for each millimetre of rainfall. A modest terrace roof of 20 square metres will gather around 100 litres from a 5mm shower — a whole water butt from one afternoon's rain. Across a typical British summer, that is a serious buffer for a dry fortnight in July.

Choosing a butt and siting it properly

Most garden water butts hold between 100 and 250 litres. A 200-litre model is the sensible default: big enough to be useful, small enough to move when empty and to tuck beside a wall. Look for a dark or opaque body, because light encourages algae, and a well-fitting lid. Wooden butts are handsome but heavier and pricier. Second-hand food-grade barrels work beautifully if you fit your own tap and inlet.

Water weighs a kilogram a litre, so a full 200-litre butt is a fifth of a tonne. Everything about the installation follows from that.

• Set it on a firm, level base — paving slabs on a bed of sand, not bare soil.
• Raise it on a proper stand so a watering can clears the tap comfortably.
• Site it near the downpipe it serves, ideally uphill of the beds you water most.
• Keep it close to the house but clear of the damp course and airbricks.
• Secure the lid and think about children and pets; an empty butt is light and can blow over.

Fitting a diverter kit — the step-by-step

A diverter is what separates a tidy installation from a bodge of guttering and string. It sends water into the butt and, once the butt is full, directs the rest back down the downpipe as normal. Measure first: standard downpipes are usually 68mm round or 65 to 80mm square, and diverters are sold to match.

• Hold the butt in place and mark the height for the diverter, allowing a few centimetres of fall from the downpipe to the butt inlet.
• Mark and cut a clean section out of the downpipe with a hacksaw, then smooth the cut edges with a file.
• Fit the diverter body over the gap, checking that it sits above the butt's highest water level, and fix it to the wall with the brackets supplied.
• Drill the inlet hole in the butt using the correct hole saw — usually 25mm or 38mm — near the top but below the rim.
• Run the flexible hose from the diverter to the inlet with a gentle downward slope, and trim it so it does not kink or sit in a loop where water can pool.
• Fit the tap at the lowest practical point and seal every joint with the washers provided.

Test with a hosepipe up the guttering. If the butt fills and the downpipe still runs freely afterwards, you have got it right.

Linking butts and handling the overflow

One butt is rarely enough in a hot spell. Linking kits use a low-level connector between two butts so the water finds the same level in both; keep them on stands of equal height and stand them level, or one will overflow while the other sits half empty. Position the second butt so its own overflow can be directed usefully.

Never let the overflow simply soak the base of a wall. Run it into a soakaway, a shallow rain garden of moisture-loving plants such as astilbe or Siberian iris, or along a border that has been mulched and needs the extra. A soaker hose laid from the tap will trickle water straight to the roots of a hedge or vegetable bed while you get on with something else.

Keeping the water fresh

• Fit a gauze or foam filter to the inlet to catch moss and grit from the roof.
• Keep the lid on at all times — it keeps out leaves, discourages mosquitoes and stops small wildlife falling in.
• Empty and scrub the butt once a year, ideally in early spring, and check the tap washer while you are at it.
• In a hard winter, drain small taps and exposed pipework, or leave the butt part-full with a floating ball to take the expansion of ice.
• If the water starts to smell, it is usually stagnant debris; a good clean and a fresh run of roof water sorts it.

Making every drop count in dry weather

Collected water is precious, so use it where it does the most good. Water in the early morning or evening when evaporation is lowest, deliver it to the soil rather than the foliage, and mulch afterwards with bark or garden compost to hold the moisture in. A newly planted shrub wants four or five litres a week in dry weather, applied slowly so it sinks in rather than running off.

Keep the can by the butt and the butt by the beds, and the habit becomes effortless. Within a season you will notice the difference in your acid-loving plants, you will barely touch the mains tap from May to September, and the rain that currently disappears down the drain will be quietly working for you instead.