Why Rainwater Is Worth Catching

On a smallholding, mains water is usually the most expensive way to water anything. Sprinkling a vegetable bed, topping up a paddock trough or filling a knapsack sprayer from the tap adds up quietly across a season, and if a hosepipe ban arrives in a dry July, the mains supply may not be available at all. Rainwater, by contrast, is soft, free of the chlorine and lime that leave white marks on greenhouse glass, and generally better for plants and for anything you are trying to keep biologically active.

A modest roof is a serious catchment. As a rough rule, every square metre of roof collects about one litre of water for each millimetre of rainfall. A 40 square metre shed roof in a modest 700mm rainfall year is theoretically catching around 28,000 litres, even allowing for splash, evaporation and the first flush running to waste. Very little of that needs to reach a drain.

The Humble Water Butt

Start where the downpipe already is. A downpipe diverter kit is cheap, fits into a section of pipe cut out with a hacksaw, and lets you feed a butt without cutting the downpipe off completely when the butt is full. Buy the largest butt you can sensibly site. A 100-litre butt sounds generous until you water a polytunnel once; 250 litres is a more realistic minimum, and linking two or three together with a short length of pipe at the base increases storage without another diverter.

• Stand the butt on a firm, level base — a paved slab or a frame of sleepers. A full 250-litre butt weighs over a quarter of a tonne.
• Raise it 30–45cm so a watering can fits under the tap and gravity gives a usable trickle.
• Fit a tap with a hosepipe thread, not just a bare spout, and add an inline tap on the diverter so you can isolate the butt for cleaning.
• Choose a dark, opaque butt. Clear or pale containers grow green water as soon as the sun reaches them.

Guttering, Guards and First Flush

What comes off a roof is not clean. Moss, bird droppings, grit and the first minutes of a heavy shower carry the worst of it. Fit a leaf guard or a length of mesh in the gutter outlet, and consider a first-flush diverter, which sends the opening few litres to waste before the rest flows to the tank. It is a small piece of kit that dramatically cuts the silt and organic load arriving in storage.

Check gutter falls before you blame the tank. A run that has sagged or lost its fall will overflow at the bracket rather than reach the downpipe, and a surprising amount of harvested water is simply lost to a blocked shoe. Clean gutters in late spring and again after leaf fall.

Keeping Stored Water Clean and Safe

Stagnant water in a warm, open container is a mosquito nursery and an algae farm. The fix is mostly about exclusion and a bit of routine.

• Cover every tank. A tight lid, a sheet of ply with a weight on it, or fine mesh over the inlet stops debris and deters egg-laying insects.
• Screen the outlets. Fine stainless mesh over the inlet and around any overflow keeps larvae and tadpoles out and stops the tap blocking.
• Keep it dark. Algae needs light. Paint pale IBCs, wrap them in black damp-proof membrane, or box them in.
• Empty and scrub annually. In late winter, drain a butt, scoop out the silt, rinse with a stiff brush and clean water, and let it refill. No detergent, and no bleach residue that will later go on the veg.
• Don't store for drinking. Roof runoff is for plants, livestock troughs and washing down. Treat anything destined for human consumption separately and properly.

Bigger Storage for Livestock and Polytunnels

For serious capacity, a 1,000-litre IBC on a pallet base is the standard smallholder answer, often bought second-hand. Ask what was in it before — food-grade drums and IBCs that held harmless liquids are fine; anything that held oils, pesticides or unknown chemicals is not. Rinse thoroughly and store out of the sun.

Gravity feed works well if the tank sits above the point of use. A 1,000-litre tank a metre up gives roughly 0.1 bar at the tap — enough for a hose and trough float valve, not enough for a pressure washer. For irrigation lines, a small 12V or mains pump with a filter is the practical route.

Site tanks where frost is least likely to split them. Leave headroom in the tank so ice has somewhere to expand, drain exposed pipework in autumn, and keep an eye on the tap and the outlet — those are the first parts to crack.

Rules and Sensible Expectations

Collecting roof runoff on a domestic or smallholding scale is straightforward, but abstraction rules differ across the UK and if you ever plan to take water from a stream, ditch or borehole rather than a roof, check with the relevant environmental regulator before you start pumping. For most roof-fed systems, the practical constraints are storage volume and filtration, not paperwork.

The realistic target is not to become independent of the mains, but to shave the peak off it. Catch from every roof you own, screen and cover what you store, clean the system once a year, and you will have soft water on tap through the summer for the garden, the greenhouse and the troughs — which is exactly when you need it most.