Why rooftop rainwater is worth catching

Every time it rains, your roof sheds hundreds of litres of perfectly good water into the drain. Meanwhile, most of us reach for the tap to fill a watering can — water that has been filtered, treated and pumped to drinking standard, only to be poured over a flowerbed. Plants actually prefer rainwater. It is soft, slightly acidic and free of the chlorine and lime that leave white marks on pots and can build up in soil over the years. If you are on a water meter, every litre you catch is a litre you do not pay for, and during a summer hosepipe ban a full water butt is quietly reassuring.

There is a second benefit that is easy to overlook. A butt that fills and then overflows slowly into a border or soakaway takes pressure off drains during heavy showers, which matters more and more as our downpours get more intense.

Working out how much you can collect

The maths is genuinely simple. One millimetre of rain falling on one square metre of roof yields roughly one litre. A modest semi-detached roof might present fifty to sixty square metres to the sky, so a decent ten-millimetre shower delivers five or six hundred litres — far more than a single butt will hold. That is the key point: your storage, not your roof, is the limiting factor.

Before you buy anything, take a walk around the house on a wet day and see which downpipes run hardest. Choose the one nearest the beds, pots and greenhouse you water most often, then measure it. Most British downpipes are either 68mm round or 110mm square, and diverter kits are made to fit each size. A few minutes with a tape measure saves a frustrating return to the shop.

Diverters and downpipe connections

A rain diverter is the neat solution. It fits into the downpipe, usually with a small cut made with a hacksaw, and sends water along a short length of hose into your butt. Once the butt is full, the surplus simply continues down the drain as it always did. Look for one with a winter bypass so you can send everything straight to the drain during the wettest months.

• Gravity or passive diverters are the simplest and cheapest, relying on the butt sitting lower than the pipe.
• First-flush diverters discard the first few litres of a shower, which carry the bulk of roof grit, bird mess and moss.
• Filter baskets sit in the diverter or the butt inlet, catching leaves and twigs before they sink and rot.

Fit the diverter on a straight run if you can. Bends and joints make sealing awkward and tend to weep in a downpour.

Choosing and siting your barrels

Butts come in sizes from about 100 to 250 litres, and you can link two or three together at the base with a connector kit so they fill and draw down as one. Whatever you choose, remember that water is heavy — a full 200-litre butt weighs well over 200 kilograms, so it needs a firm, level base of paving slabs or a purpose-made stand. Never balance one on loose bricks.

Height also governs pressure. Roughly speaking, every metre the water surface sits above the outlet gives you about 0.1 bar. That is a gentle flow, fine for filling a can but barely enough for a sprinkler. If you want to run a soaker hose, raise the butt on a sturdy stand and keep the run short. Finally, plan the overflow deliberately: a length of pipe leading into a planted border or a small soakaway, never against the base of the house.

Filtration, covers and keeping water fresh

Stored rainwater goes stale if it sits open to light and debris. Fit gutter guards to keep moss out, use a filter basket at the inlet, and choose a dark, lidded butt rather than a translucent one, since light encourages algae. Stretch fine mesh over the inlet to stop mosquitoes and midges breeding in the summer. Give the whole system a clean each autumn: empty it, scoop out the silt, rinse, and check the seals and connectors before the winter rains arrive.

Using rainwater in the garden

Rainwater is ideal for ericaceous plants such as rhododendrons, camellias and blueberries, for tomatoes in grow-bags, and for anything in a container, where tap water gradually raises the pH. Water at the roots in the early morning rather than over the leaves, and mulch afterwards to hold the moisture in. A watering can is the simplest tool, though a short soaker hose or drip line fed from a raised butt will quietly irrigate a border while you get on with something else.

Keep it strictly for the garden. Stored rainwater is not treated and should never be used for drinking, cooking or washing produce you will eat raw, and standing open water needs covering if small children or pets are around. Treat it as a free, generous resource for the soil and the plants, and it will repay you every dry week of the summer.