How much rain your roof catches, and how to get it to the roots without a pump

A water butt is only useful if it fills, and most fill faster than their owners expect. The arithmetic is simple enough to do on the back of an envelope, and it tells you whether one butt is enough or whether you need three.

The one number

One millimetre of rain falling on one square metre gives one litre of water. That is the whole conversion.

Measure the roof’s plan area, not the sloping surface. Rain falls vertically, so a pitched roof catches the footprint it covers, not the length of the slope. If only one side of a roof feeds the downpipe, use that half.

Then allow for losses. Some water splashes past the gutter, some is lost to the first flush and some overflows. A factor of 0.8 is a reasonable estimate for a tiled roof.

So: plan area in square metres × annual rainfall in millimetres × 0.8 = litres a year.

What your roof catches

A 6 by 3 metre shed roof is 18 square metres. At 800 mm of rain a year, with 80% collection, that is about 11,500 litres. A 200 litre butt would fill and overflow many times over.

A small shed roof in dry south-east England still catches roughly 2,500 to 3,500 litres a year, according to the RHS. That is enough to justify a butt even in the driest part of the country.

The catch is storage, not supply. A 200 litre butt is about two weeks of summer watering for a small plot. If you want to cover a month, you need 400 litres or more, or a smaller area to water.

Sizing and siting

Put the butt on a firm, level stand. A full 200 litre butt weighs 200 kg, and you need height under the tap for a watering can.

Fit a filter and a close-fitting lid. The filter keeps leaves and grit out of the tap; the lid keeps light, insects and plant debris out of the water.

Link two butts with a connector kit so the second fills when the first is full. Put the overflow where it will not undermine a wall or flood a neighbour.

Getting pressure

A butt raised one metre gives about 0.1 bar. That is enough to run a soaker hose slowly, and enough to fill a can. It is not enough for a standard drip system, which usually wants 0.5 bar or more.

The options are:

• a submersible or external pump, which gives proper pressure but needs power
• low-pressure drippers and soaker hoses designed for gravity systems
• a watering can, which is slower but never blocks

If you use a pump, put it on a timer and check the line for leaks. If you use gravity, keep the runs short and the drippers clean.

Ollas

An olla is a buried clay pot with a narrow neck. You fill it through the neck, and water seeps through the porous walls into the surrounding soil. Roots gather around it and take water as they need it.

They work well in beds, raised beds and large containers. Bury the pot so the neck sits just above the soil, and fill it when the soil around it starts to dry. In hot weather that may be every few days.

Ollas do not suit every crop. They are best for tomatoes, courgettes, beans and other thirsty plants with a defined root area. They are less useful for a whole bed of salad, where a soak and a mulch do the job more cheaply.

Maintenance

Empty and clean the butt once a year. Rinse out the sludge, check the tap and the filter, and make sure the overflow is clear. Keep the lid on.

Stored rainwater is for the garden, not for drinking or washing produce that will be eaten raw. Use it on the soil, not the leaves.

When a ban arrives

Hosepipe bans normally cover mains water. Water from a butt is not mains water, so it is usually allowed, but check your company’s wording before connecting a hose. Hosepipe bans and allotments has the rules.

For the watering itself, see when to water vegetables.