Rainwater suits plants better than anything that comes out of a tap. It is soft, slightly acidic, and free of the chlorine and dissolved lime that mains supply carries. The RHS rates it the best option for garden watering and notes that hard tap water can raise soil pH, which locks nutrients away from acid-lovers such as camellias and rhododendrons. A barrel under a downspout is the cheapest way to catch it. I have run two-barrel setups on a rented plot and a linked 1,000-litre tote on my own, and almost everything that matters turned out to be plumbing and rainfall rather than the barrel itself.
What roof water is, and what it is not
Roof runoff is not drinking water. It washes dust, pollen, bird droppings and roofing material into the barrel, and no screen removes dissolved contamination. Purdue Extension warns that rainwater can carry pathogens including Salmonella, and advises using it on lawns and ornamentals rather than on crops eaten raw. The EPA makes the same point from the stormwater side: roof water picks up bacteria from birds and animals, plus chemicals from the roofing itself, and that is worth weighing before you put it on anything you eat.
I still water tomatoes with it. I apply it to the soil at the base of the plant and never as an overhead spray on leaves I plan to eat. That is my judgement call, and it is one worth making deliberately rather than by accident.
Two practical facts follow. Rainwater is acidic and low in minerals, which makes it excellent for blueberries and camellias and useless as a cure for a limy soil. It is also not sterile. Treat the barrel as a garden tool, not a larder.
The catchment maths, and why one barrel is rarely enough
Every calculation starts with the roof, not the barrel. What matters is the footprint the roof covers, not the total area of its slopes and ridges. A gable roof over a house 10 metres square covers 100 square metres of ground and presents about the same area to the rain, however steep the pitch.
The metric rule of thumb fits in your head. One square metre of roof catches roughly one litre for every millimetre of rain. That figure ignores losses. Gutters spill, screens splash, a first-flush diverter throws water away on purpose, and rough surfaces hold a film back. A realistic efficiency sits between 75 and 90 percent, depending on your roofing and how well you clean it.
Run that against a real storm. A 100 square metre roof and 25 mm of rain is 2,500 litres landing on the roof, and perhaps 2,000 litres after losses. One 200-litre butt holds less than a tenth of that. Two linked barrels hold barely more. Kentucky’s extension service works the same sum in imperial units: a 2,400 square foot roof and one inch of rain yields 1,489 gallons if every drop were captured. Purdue’s own guidance puts ordinary barrels at 40 to 80 gallons, and it makes the same point from the other direction: a 2,000 square foot roof in Indiana can shed more than 40,000 gallons in a year.
Sizing therefore runs backwards from what most people expect. You do not buy a barrel and hope. You work out how much water you can usefully hold between rains, then buy to that figure. Volume matters more than barrel count. A small butt fills in minutes from a decent roof and overflows for the rest of the storm. If you want to carry water through a fortnight of dry weather, you need volume measured in cubic metres, not in single barrels.
I learned that the hard way. My first barrel was a 220-litre recycled drum under the gutter of a small terrace. It filled during one heavy April shower and then sat there for a month doing nothing. The overflow ran into a bed and turned it to bog. When I moved house I bought a used 1,000-litre IBC tote and set it on a paved base behind the shed. That tote changed how much of the summer I could water without a tap.
First-flush diverters, and how to size one
The dirtiest water is the first water. After a dry spell the roof carries dust, pollen, bird lime and, on shingle, loose mineral granules. University of Hawaii extension material describes the drop in faecal bacteria once the roof is rinsed before water enters the tank, and explains the mechanism plainly. The first runoff fills a vertical pipe and lifts a floating ball that seals it. Cleaner water then flows on to storage. A small drain hole near the bottom empties the chamber slowly between storms, so the device resets itself.
Sizing follows a couple of published rules. The common recommendation is to divert at least 10 gallons for every 1,000 square feet of catchment served by that downspout, and roofs under trees or with heavy bird traffic want closer to double. Hawaii’s guide gives the plumbing arithmetic for a chamber you build yourself: three feet of four-inch PVC holds about two gallons, while a 78-inch length of 12-inch pipe holds roughly 38 gallons. A ready-made diverter is not expensive, and its cleanout cap has to stay reachable.
Two things break diverters in practice. The first is a blocked slow-drain hole, which leaves the chamber full so the device quietly stops working after the opening storm of the season. The second is a discharge point that drips onto a path or back against the wall. Route the dirty water to a mulch bed or a gravel strip that drains away from the foundation. Mine empties into a hedge two metres out from the house. Clear the cap each spring.
Fitting the barrel to the downspout
Downspouts come in a few standard shapes, and the diverter has to match. Rectangular sections are commonly two by three inches or three by four inches. Round ones are three or four inches across. Penn State Extension covers the common barrel designs and the diverters that go with them, which is a sensible starting point if you plan to buy a kit rather than build one. Measure it first. Check whether your downpipe is plastic or metal, because adapters differ. A diverter that intercepts the downspout and sends water through a hose is tidier than an open barrel top, and most models push the surplus back down the downspout once the barrel is full. That solves the overflow question on the spot.
Every opening still needs mesh. Standing water breeds mosquitoes, and a rain barrel is standing water. Rutgers Cooperative Extension’s fact sheet on rain barrels and mosquitoes recommends screening held tight over the barrel, an overflow hose of at least eight to ten feet to discourage insects from climbing back up, and a small screen inside the overflow adapter itself. If larvae do get in, the same sheet suggests Bacillus thuringiensis israelensis, sold as mosquito dunks, or a thin film of cooking oil that cuts the larvae off from air. Neither replaces a lid that seals.
Algae, smell and the case for an opaque barrel
Light drives algae. A translucent barrel with a little nutrient in the water goes green quickly, and green water smells once it sits. Block the light and keep organic matter out in the first place. Paint a pale drum, or choose black or dark green from the start. That single change prevented more trouble for me than any additive I tried.
Where the barrel sits matters as much. A butt in full sun grows a slick on the inner walls within a fortnight. A butt in shade stays clear for months. Mine sits on the north side of a fence and has needed nothing beyond an annual scrub. Shade does most of it. The RHS advises a close-fitting lid for exactly this reason, since it keeps out rain-borne spores of plant disease as well as light.
Smell usually means the barrel is storing debris rather than water. Cleaning is routine maintenance, not a rescue operation. Empty and scrub barrels at least once a year, and use the water often so it never goes stale. A downspout leaf screen ahead of the barrel is the most effective single step I have taken, because the coarse matter never becomes sludge on the bottom. Rinse the barrel in spring, tip out the winter grit, refill.
Gravity versus a pump
Gravity is free and predictable, and its limits are pure physics. Pressure comes from the height of the water surface above the outlet, about 0.1 bar for every metre of fall. A barrel on a 30 cm plinth delivers a trickle. Set it a metre up and you get enough flow for a soaker hose or a low-pressure drip line. Reaching three metres takes a built tower, and the weight of a full barrel argues against trying. The RHS reaches the same conclusion from the other end and simply notes that good pressure from a butt calls for a submersible or external pump.
A pump changes the arithmetic. A small submersible or inline unit takes a 1,000-litre tote from a dribble to something that will run several sprinklers. Fit a filter after the pump if you are feeding emitters, because fine sediment clogs them faster than you expect. Pumps need power at the barrel, a strainer on the inlet, and protection from frost. I run gravity on the small butts and a pump on the tote. The split works for me.
Overflow is the failure point nobody plans for
A barrel that cannot overflow properly is a flood waiting for the right storm. The overflow should be at least as large as the inlet, and it has to lead somewhere that accepts a lot of water fast. Bigger is safer. A drain, a soakaway, a swale or a planted bed away from the house will all do. The EPA’s Soak Up the Rain programme treats that diversion as the point of the exercise, since roof runoff is water the drain network no longer has to carry. RHS guidance on rain gardens sizes a planted soakage area at about 20 percent of the roof area feeding it. That is a useful number when the surplus has nowhere else to go, and it is also where rain barrels overlap with proper stormwater management.
The older failure is a barrel that seals completely when full. The next storm then forces water back up the downspout, over the gutter, and down the wall. Keep the overflow clear, and check it before the wet season rather than during it.
Where you live decides how much you may store
Rules vary more than most gardeners expect, and they change. Laws move faster than guides. In the UK, collecting roof water is legal and encouraged by councils and water companies. In parts of the American West, water is a right held downstream, and rooftop capture counts as intercepting somebody else’s supply. Colorado’s Division of Water Resources allows most single-family homes two rain barrels with a combined capacity of 110 gallons, with no permit, rooftop collection only, outdoor use on the same property, and a sealable lid. The water cannot be used indoors or for drinking. A property with its own well can follow a separate route that collects far more, but that one does require a permit.
Other states attach their own caps and registration requirements, and a few go the other way entirely by protecting the right to harvest. Local building codes can also bite, usually on cisterns above a few hundred gallons. Homeowner associations frequently have their own rules about placement and appearance, so read the covenants before you buy. If you are on a shared private supply, ask about cross-connection control, because harvested water must never be able to reach a drinking-water line. Check your own state or local authority before spending money.
What I would do differently
If I started again, I would size the storage before buying anything. I would put the barrel where it stays shaded and sits high. I would fit a first-flush diverter and a leaf screen on day one, and plumb a proper overflow to a planted area instead of letting it splash the wall. Then I would leave the system alone and use the water.
Worth saying plainly: a rain barrel saves less mains water than people assume. One 200-litre butt emptied and refilled a dozen times over a summer is 2,400 litres, a rounding error on a household bill. For scale, the RHS reckons a small shed roof in dry south-east England collects 2,500 to 3,500 litres a year, which fills ten to fifteen standard 240-litre butts. The value is in the plants rather than the meter. The RHS water science team puts water butts first on its list of ways to cut a garden’s water use. Rainwater is the right chemistry for a garden, it is there when a hose ban arrives, and it costs nothing to collect. That is reason enough.