6 Step Rainwater Harvesting Setup with a 200-Litre Harlequin Water Butt

Why 200 litres suits a small roof run

A 200-litre Harlequin butt gives enough stored water for one deep soak of a 3-metre by 1.2-metre raised bed, with some left for containers. The Royal Horticultural Society puts established-bed watering during dry spells at about 10 to 15 litres per square metre per soak, so a full butt can cover one bed twice or keep several pots going through a week without using the mains.

The roof calculation is simple. Rainfall of 1mm on 1 square metre gives about 1 litre before losses. A 10 square metre greenhouse roof or shed run therefore needs roughly 20 to 25mm of rain to top up a 200-litre butt from empty, while a 5mm shower over the same area sends about 50 litres towards the store before splashing and gutter loss.

That scale is why the small Harlequin format fits a greenhouse, shed, or single garage. A 100 square metre house roof can overwhelm it in one storm and is better matched to a 1,000-litre tank or a pair of butts chained together.

Six fitting steps from diverter to tap

Put the butt where the watering can will actually fit under the tap. A stand or two rows of solid blocks usually gives the tap the 30cm or more of clearance it needs. Once full, a 200-litre butt weighs over 200kg, so the support belongs on a paving slab or compacted base, not soil that can settle and twist the tank.

With the butt in place, mark the downpipe about 5cm above the intended water line. Common diverters for 68mm round and 65mm square downpipes need that headroom, because the water has to back up slightly before feeding sideways into the butt.

Cut the inlet hole with the size specified by the diverter kit, often 32mm or 35mm. A Bosch or DeWalt hole saw arbor in a cordless drill goes through uPVC quickly. Trim burrs away with a knife so the rubber grommet sits flat and seals against the pipe.

Fit the diverter body into the hole, then push the connector hose into the butt lid or side inlet. Kits such as the Hozelock Rainwater Diverter usually include a flexible hose that clips into both ports without glue.

Seat the lid and sort the overflow before leaving the butt to fill. The overflow can return to the downpipe below the diverter or run to a soakaway. A self-regulating diverter stops feeding the butt once it is full and sends surplus rain down the original pipe, preventing back-pressure from cracking the butt during a heavy storm.

Tighten the brass or plastic tap against its washer, add a few litres from a hose, and check the grommet and tap thread for any weeping. A leak at this stage is far easier to fix than one found after the tank has filled.

Where stored rainwater is most useful

Rainwater stored in a butt sits close to ambient air temperature and carries no chlorine. That matters during hardening off, when trays of seedlings move from a windowsill to a cold frame. Mains water drawn from an underground pipe can be 8 to 10 degrees below the air, while butt water that has stood outside usually lands within a degree or two of the cold frame interior. The warmer watering avoids the growth check caused by cold shock.

Ericaceous and lime-sensitive plants are the other clear beneficiaries. In hard-water regions, mains water brings dissolved calcium that nudges soil pH upward over a season. Rainwater runs slightly acidic, around pH 5.5 to 6, which suits blueberries, camellias, and rhododendrons in pots where every watering otherwise adds more limescale from the supply.

Roof water has a hygiene limit. It can carry bird droppings and organic debris washed off the tiles, so it is sound for watering at soil level but unsuitable for misting edible leaves that will be eaten raw without washing. On roofs under tree cover, a first-flush diverter dumps the first dirty runoff before cleaner water starts filling the butt.

Using Strulch around thirsty beds

Stored water lasts longer under a mulch that reduces evaporation. Strulch is a mineral-treated wheat straw mulch, dyed and dosed with iron minerals, laid at about 30 litres per square metre to create a layer near 5cm deep. The maker rates one 100-litre bag at roughly 3 square metres at that depth.

Spread over damp soil in spring, it keeps moisture in the root zone, so one butt-fed soak lasts longer between refills. The iron mineral content also deters slugs and snails, which is why it often appears around hostas and young brassicas where bark chip alone gives little help. Unlike fresh bark, Strulch does not rob nitrogen from the soil surface as it breaks down, making it suitable for annual vegetable beds that are replanted each season.

Water the bed first from the butt, then cover the wet soil with the mulch. Mulching dry ground traps the dryness underneath. A deep rainwater soak followed by a 5cm Strulch cap stretches 200 litres further than watering bare soil.

A greenhouse refill example

Take a freestanding greenhouse with a roof footprint of 1.8 metres by 2.4 metres. That gives about 4.3 square metres of collecting area per side, or roughly 8.6 square metres total if both gutters feed the same butt.

A 6mm rain event over that 8.6 square metre roof yields about 51 litres before losses. Allowing the 10 to 15 percent typically lost to splash and gutter overshoot leaves about 44 litres collected. Three events like that inside a fortnight, common in a British spring, can take the butt from a quarter full to full without a single overflow.

On the draw-down side, twelve 2-litre seedling trays in a cold frame use around 6 to 8 litres per watering session every second day. That works out at roughly 28 litres a week. Through propagation season the butt stays in surplus under an average spring refill pattern, then tightens in a prolonged dry July. At that point, a second chained butt, a larger 1,000-litre tank, or heavier mulching starts to show its value.

Do not leave the overflow loose

An unconnected overflow is a common reason a butt fails in its first heavy storm. Send surplus water back to the downpipe or into a gravel soakaway at least 1.5 metres from any wall before relying on the system unattended.

Hedges, hornbeam, and trimmer clearance

A water butt at the base of a wall often occupies the same strip where a pleached hornbeam or clipped hedge line is planned. Carpinus betulus, the common hornbeam used for pleaching, is trained onto a horizontal bamboo or wire framework with a clear stem rising well above butt height. The pleached tier is usually started at 1.8 to 2 metres, leaving room for a tank and stand beneath the raised canopy.

For short pleached tiers and box near a filled butt, Stihl’s battery HSA 60 is quiet and easy to start for brief touch-ups. A petrol machine such as the Stihl HS 45, with its 600mm blade, makes more sense on a long established hornbeam run where a 200-plus metre hedge would drain several battery packs. The HSA 60 avoids fuel mixing and starts instantly for ten-minute cuts; the HS 45 will keep working for hours, while bringing exhaust, vibration, ear protection, and a 50:1 two-stroke mix into the job.

The diverter hose and overflow line often sit at the same height as a sweeping trimmer blade. If either hose is clipped or pulled loose, the damage may stay hidden until the next storm sends water down the wall instead of into the butt. Bright tape on those clips is easier to spot through new growth than black plastic against the pipe.