Designing your capture system

Your downpipes are the arteries. A single downpipe from a modest shed or garage can fill a 200-litre butt in one autumn afternoon. The goal is to intercept that flow before it vanishes into the drain, store what you can, and let the overflow continue harmlessly away from foundations. The simplest system is a rainwater diverter kit (£15–30) fitted into an existing downpipe, directing the first flush into your butt until full, then bypassing excess back down the pipe.

Scale up by linking multiple butts in series. When the first is full, it overflows via a connector hose into the second, then the third. Three 200-litre butts give you 600 litres — enough to coast through a dry April or a rainless June fortnight. Position butts on firm, level ground close to where you water most: beside the greenhouse, at the allotment shed, near the polytunnel door. Raising them 30–50cm on concrete blocks or a wooden stand increases tap pressure, making watering cans fill faster and hoses run more freely.

For larger gardens or allotments, consider an IBC tank (Intermediate Bulk Container) — these reclaimed industrial cubes hold 1,000 litres, cost £40–80 second-hand, and can be disguised behind trellis or cladding. Fit a tap low on one side and a finer mesh filter over the top inlet to exclude leaves and insects. Connect multiple IBCs with bulkhead fittings for 2,000–3,000 litres of reserve. At this scale you are running a micro-reservoir, capable of irrigating 30–40m² of intensive beds through a summer dry spell without touching the mains.