Why rainwater is worth the trouble

If you keep livestock, you'll know that the water bill creeps up quietly. A handful of ewes and a couple of pigs won't trouble the meter much, but once you're watering a few cattle, a small flock and a paddock of poultry through a dry summer, mains water starts to look like a serious overhead. Harvesting rain off your own roofs is one of the simplest wins on a smallholding: it cuts mains use, saves money if you're metered, and gives you a buffer when pressure drops or a hosepipe ban lands in the middle of a heatwave.

There's a practical benefit too. Mains water in much of the UK is hard, and hard water scales up drinkers, ball valves and pipework. Rainwater is soft, so it keeps troughs cleaner, mixes better with mineral licks and veterinary treatments, and generally sits more kindly in a drinker that's refilling constantly. Animals don't need treated water — they need clean, cool, reliable water, and a well-built rainwater system delivers exactly that.

Sizing the system to your stock

Start with demand, then work backwards to supply. As a rough guide, daily drinking requirements per animal in temperate UK conditions are:

• Cattle: 30–50 litres for a suckler cow, 60–100 for a dairy cow in milk
• Horses and ponies: 25–45 litres
• Sheep: 4–6 litres, rising to 8–10 in hot weather or for ewes in milk
• Goats: 5–8 litres
• Pigs: 10–20 litres depending on size and whether they're wet-fed
• Poultry: roughly 0.25–0.5 litres per bird

Add it up and multiply by a safety factor of 1.3 to allow for spillage, cleaning and hot spells. Then calculate your catchment. A square metre of roof yields about 0.9 litres for every millimetre of rain once you've allowed for splash and first-flush losses. So a 6m by 10m barn — 60 square metres — in a 800mm rainfall area gives you roughly 60 × 800 × 0.9, or about 43,000 litres a year. That's comfortably enough for a dozen ewes, a few goats and a pony, with change.

Guttering, downpipes and first flush

Most agricultural buildings have no guttering at all, which is why the first job is fitting it. For a typical shed run, 112mm half-round uPVC is cheap, easy to work and handles the flow from up to about 60 square metres of roof. Fit a gutter guard or mesh strip along the top to keep leaves and moss out, and use running outlets rather than stop-end outlets wherever you can, so debris has somewhere to go.

Then comes the detail that separates a sweet-smelling system from a stagnant one: the first-flush diverter. The first rain off a roof carries bird droppings, dust and roof grit. A simple diverter chamber, sized at roughly 20–25 litres per 100 square metres of roof, dumps that initial flow and then lets the cleaner water through. Pair it with a leaf trap or inline strainer before the tank and you'll halve your maintenance.

Tanks, pumps and plumbing to the trough

For storage, second-hand IBCs at 1,000 litres are the smallholder's standby, but they're bulky for what they hold and can go green if light gets in. Opaque 2,500–5,000 litre static tanks take up less ground per litre and are worth the extra outlay. Site them on a firm, level plinth of compacted hardcore or concrete, ideally in shade, and fit a sealed lid with a mesh vent.

Gravity is the elegant option, but do the sums before you rely on it. Every metre of head gives you only about 0.1 bar, and a trough float valve needs closer to 0.5 bar to refill briskly. Unless you can raise the tank five metres above the trough, fit a small 12V submersible or a 230V pressure set with a modest pressure vessel. Run 25mm MDPE to the troughs, buried 450–600mm deep, with a stopcock and a drain-down valve at the lowest point.

Keeping the water clean and the system running

Water quality is mostly about what you don't put in. Avoid roofs with asbestos cement sheeting, lead flashings or bitumen felt, and never harvest from a roof that's been recently treated. Once a year, drain the tank, scoop out the sediment and check the strainers. Keep the tank sealed against mosquitoes and rodents, and screen every inlet and overflow. If the water sits cloudy, a simple sediment trap or a coarse inline filter before the trough will sort it.

Watch the troughs themselves. A trough in full sun grows algae fast; positioning it in shade and checking the float valve weekly keeps it sweet. Fencing the trough off so animals drink through a nose-operated valve reduces fouling dramatically, especially with cattle.

Winter, overflow and the long view

Overflow is the bit people forget until the first storm. Route it to a soakaway, a rain garden or a vegetated ditch — never into a foul sewer, and check with your local authority before discharging near a watercourse. In winter, drain exposed pipework before the first hard frost or lag it thoroughly, and consider a frost-resistant trough with the ball valve below ground level.

One legal note worth knowing: rainwater falling on your own roofs isn't classed as abstraction in England, so you don't need a licence to collect it. Pumping from a stream or borehole is a different matter, with a 20 cubic metre per day threshold for most users.

Build it in stages — guttering and a couple of IBCs this year, more storage and a pump next — and you'll soon wonder why you ever ran a hose down the field.