Harvesting Rainwater From a Roof: What One Storm Yields Against What the Beds Draw

Two Barrels in the Shed Since Christmas, and a Downpipe That Has Not Moved

They were bought in a January sale, food-grade, 210 litres each, and they have been standing empty behind the shed ever since. The plan is the obvious one: cut the rear downpipe, put a diverter in, stand the barrels on something, and stop dragging a hose across the lawn in July. The gutter above them takes the back plane of the house and nothing else, the beds they are meant to serve are the four raised ones along the fence, and the whole job is an afternoon.

What the afternoon cannot settle is whether it is worth doing at this scale. Two numbers decide that and neither is on the barrel's label: what one ordinary storm delivers off that particular plane — not the whole roof, not the sloped surface — and what those four beds pull out of the ground in a week of the dry weather the barrels exist for. Until both are written down in the same unit, a rain barrel is garden furniture with a tap on it.

The comparison usually deflates the plan a little, which is exactly why it belongs before the downpipe is cut rather than after. Knowing that the pair holds slightly less than one week of peak demand changes the overflow, the draw-off and the decision about whether a third barrel or one larger tank is the better spend. That is the difference between a store that runs a hose for an evening and one that carries the garden through a fortnight without rain.

Tile, Gutter, Diverter, Vessel, Overflow

Five components, bought from four different places, each with its own way of losing what the one above it just delivered. The gutter has to carry the peak intensity before any of this starts — a sizing problem governed by BS EN 12056-3, Gravity drainage systems inside buildings — Roof drainage, layout and calculation — and no diverter fitted below it recovers what overshoots an undersized eaves gutter in a cloudburst. A leaf guard or outlet strainer keeps the diverter and the tank inlet from silting up, and earns its keep in the fortnight either side of leaf fall rather than across the summer.

The vessel is the one component with no obvious calculator behind it, and that is not an oversight. Its size does not come from the roof or from the beds; it comes from the interval between storms you intend to ride out, which is a climate question answered further down. Everything above and below it is a quantity somebody can put a tape to.

Two Hundred Kilograms Is a Slab Job, Not a Lawn Job

A full 210-litre barrel weighs 210 kilograms plus the vessel, and a 580-millimetre drum concentrates that into about a quarter of a square metre — call it eight kilonewtons per square metre, more than most domestic floor loadings and far more than a lawn will hold in February without tipping. A thousand-litre intermediate bulk container is a tonne on a little over a square metre and wants a bedded, level base with no rocking corner. People get this wrong through sequence rather than ignorance: the barrels go up in dry March on ground that feels firm, and the next thing that happens is a wet autumn.

Height is decided at the same moment and trades directly against stability. Raising the barrel puts head on the tap, which is what makes a can fillable and a hose worth attaching, and it also raises the centre of gravity of a quarter-tonne object. Stacked concrete blocks on a bedded slab is the sensible answer for a barrel; anything larger wants a stand rated for it. Do not improvise a tall stand out of pallets and hope.

Two barrels joined at the bottom and two joined at the top behave completely differently. Bottom-linked through bulkhead fittings and a short hose, they equalise and act as one 420-litre vessel with a single tap and a single water level. Top-linked, the second fills only from the first's overflow: simpler to fit, but the second barrel is dead weight until the first brims and cannot be drawn through the same tap. Neither gives more pressure, because the head at the tap is set by the height of the water surface above it and by nothing else.

1. Lift the turf, dig out to a firm formation and bed a paving slab or two on compacted sharp sand or a lean mix, checked level in both directions with a spirit level rather than by eye.
2. Set the stand and the empty vessel on the slab, put fifty litres in from a hose and check the level again — most of the settlement that is going to happen happens under the first load.
3. Cut the downpipe with the vessel in position, so the diverter lands at the height the inlet actually sits at.
4. Fit the diverter with the fall running towards the vessel and the pipe below it clear, so that once the store is full the flow carries on down to the drain rather than backing into the gutter.
5. Screen the inlet, fit a lid a child cannot lift, and connect the overflow before the first storm rather than after it.
6. Level a bottom-linked second stand to the first, not just to the ground: 20 mm of difference empties the pair unevenly and one tap runs dry while the other still holds water.

The Beds Are Not Drinking the Storm; They Are Drinking Last Week's Evaporation

Demand in a garden is a physical loss rather than a habit, and it measures in exactly the same millimetres as the rain meant to replace it. Water leaves a cropped bed by evaporation from the soil surface and by transpiration through the leaves; the two together are evapotranspiration. The reference method is not folklore. FAO Irrigation and Drainage Paper No. 56, Crop evapotranspiration — Guidelines for computing crop water requirements, defines a reference rate for a standard grass surface from temperature, humidity, wind and radiation, then applies a crop coefficient to turn that into the loss from a particular planting at a particular growth stage. A local weather station publishes the reference figure; the coefficient comes from the crop.

For a working number, a well-grown vegetable bed in full leaf through a temperate dry spell loses on the order of three to four millimetres a day — twenty to twenty-eight in a week. Twenty-five is what most temperate planning converges on, and it means twenty-five litres per square metre per week, which lands in the same unit as the roof. That is the entire reason for putting demand in millimetres. The four raised beds here add up to eighteen square metres of cropped ground, so a dry week costs about 450 litres and a dry fortnight 900. The pair of barrels holds 420.

Two things move that figure and both are cheaper than more storage. Mulch cuts the evaporation half of evapotranspiration by breaking the capillary route to the surface and shading the soil, and before the canopy closes that half is the larger one. Shade from a wall or a fruit tree cuts the radiation term directly. Neither does much to transpiration once the leaves have met over the bed, which is why the saving is best in May and worst in the August fortnight the barrels were bought for.

Frequency is a separate decision from volume, settled by what is under the plants. A deep bed of good soil holds a reserve and takes its week in one or two long applications, which drives roots down and leaves the surface dry between times. A shallow bed, a container or a sandy soil holds much less against gravity and needs the same weekly depth split into smaller doses, or most of it drains past the roots. Getting this backwards is the commonest irrigation fault in a garden: little and often onto a deep bed keeps the root zone shallow and permanently thirsty, and the planting becomes dependent on the frequency that caused it.

Nine Hundred Millimetres of Head Is Not Pressure

This is where a great many barrel-and-drip plans quietly fail, and the arithmetic is short enough to do before buying anything. A metre of water is 9.81 kilopascals: 0.098 bar, or 1.42 pounds per square inch. A barrel on a 400-millimetre stand holding 500 millimetres of water puts 900 millimetres of head over an emitter lying on the ground, which is 0.088 bar; by the time it has drained to the tap that is down to the 0.039 bar the stand alone provides. Under one and a half psi at best, falling as it empties. Pressure-compensating dripline generally needs something in the region of a bar to regulate at all, and every data sheet states a minimum operating pressure. Below it the emitter is not discharging at its rated figure, it is weeping at whatever the head gives it, and the precipitation-rate calculation above stops describing anything.

Two honest ways out exist and one dishonest one. Buy dripline rated for gravity feed — products designed to run from a tank at a fraction of a bar, with their own published discharge at their own low design pressure, so the same arithmetic applies with those numbers instead. Or fit a small pump. Eighteen square metres at 6.7 emitters per square metre is roughly 120 emitters, and at 1.6 litres an hour each that is 192 litres an hour, 3.2 a minute, at whatever pressure the dripline needs: a modest duty, but it has to be a pump intended for the job, with a strainer on the suction and dry-run protection, because a 420-litre store empties in a little over two hours.

The dishonest way out is to hang mains-pressure dripline off a barrel tap and assume low pressure just means slower. It does not mean slower evenly. Near emitters run, far ones barely start, the distribution the grid was designed for collapses and the bed is watered in stripes. At gravity head with no pump the sound answers are a soaker hose on a level bed, an open pipe into a basin at the head of a furrow, or a watering can — all respectable uses of 420 litres, none of them pretending to a precipitation rate.

It Is the Gap Between Storms That Sizes the Store

Nothing above this line sizes a tank. The storm sizes the overflow, the beds size the weekly draw, and the store is sized by how long you mean to keep watering after the last rain stopped. The standards codify exactly that split: BS 8515 offered an intermediate route working from annual yield and annual demand, and a detailed route running a daily time series of rainfall against consumption to report how often the store runs dry, and BS EN 16941-1, which replaced it in 2018, keeps both, the first as its basic approach. Almost every domestic system is sized on the annual one; the detailed one exists because annual totals hide the thing that matters here, a wet year with a six-week drought in the middle of it.

For a garden the useful currency is days of supply, and it is one division. The 420-litre pair against a 450-litre week is six and a half days. A 1,000-litre intermediate bulk container is 2.2 weeks. A 1,500-litre slimline tank against a wall is 3.3 weeks, long enough for most temperate dry spells and roughly where harvesting stops being a gesture and becomes the garden's water supply. The barrels, by contrast, need a seventeen-millimetre refill every six days through the driest part of the year — precisely when such events do not happen.

Size it against the yield as well, though, because a vessel much larger than the catchment can refill in a reasonable time spends most of the year part empty. Check the store against the demand it has to survive and against the rainfall that has to fill it, and use published frequency data for the second rather than memory — NOAA Atlas 14, Precipitation-Frequency Atlas of the United States, in North America, and the national meteorological service's long-term averages elsewhere.

Overflow, Insects and February

The overflow runs longest and gets the least thought. It has to be at least the bore of the inlet, because once the vessel is full everything the roof delivers arrives there at the rate the roof delivers it — the 40-millimetre event in the table above sends 568 litres out of that connection in a couple of hours. Where it discharges is the whole question, and onto the ground beside the wall is how a dry basement stops being dry. The original gully, a soakaway well clear of the building, or a rain garden that can take it: those are the sound destinations, chosen when the barrel is plumbed rather than when the water is already running.

Insects and light are the two maintenance problems and both come down to closing the vessel properly. Standing water mosquitoes can reach is a breeding site, and the rainwater standards — ARCSA/ASPE 63, Rainwater Catchment Systems, addresses it directly — require every opening screened: inlet, overflow and vent, not just the lid. A hole cut for a hose and left open defeats the vessel. Light is the other: an opaque tank grows no algae, a translucent one grows a bloom that then blocks emitters, and a barrel left open does both.

Winter, in a freezing climate, is a design item rather than a maintenance one. Water expands as it freezes and a rigid full vessel with no room for it splits, usually at a seam or a bulkhead fitting and usually discovered in April. The autumn routine is to divert the downpipe back to the drain, open the tap and let the vessel stand empty and lidded through the frosts — which is also the one annual chance to deal with the sediment. Left connected, it takes the heaviest leaf load of the year straight into the store.

1. Before the first storm: run a hose into the gutter and watch the diverter, the inlet screen and the overflow work together at a real flow rate rather than a trickle.
2. Monthly in the growing season: lift the lid, check the screen is not blinded with pollen or moss grit, and clear the first-flush chamber's drain orifice.
3. After leaf fall: clear the gutter and outlet strainer, and empty the flush chamber by hand if it has stopped draining.
4. At the first hard frost: divert the downpipe back to the drain, empty the vessel, leave the tap open and the lid on.
5. Before reconnecting in late winter: sluice the sediment out through the base, flush the draw-off, and check every fitting and seam for a split.

What the Water Is Fit For, and What You Are Allowed to Keep

Harvested rainwater is a non-potable supply and every standard covering it treats it as one. It is entirely suitable for irrigation, for washing tools and paving, and for topping a pond; it is not drinking water without a treatment train designed for the purpose, and the runoff carries whatever the roof is made of. Three coverings rule it out for edible crops without further argument: asbestos cement, anything with lead flashing draining across it, and any roof treated with a moss killer or biocide in the same season. Preservative-treated timber shingles are in the same conversation. That is a question about the covering, not the storage, and no downpipe filter changes the answer.

Where a store is topped up from the mains in dry weather, the backflow protection is neither optional nor a check valve. In the United Kingdom the Water Supply (Water Fittings) Regulations 1999 put a rainwater store in the highest fluid category, calling for an unrestricted air gap of the kind specified in BS EN 13076: the mains discharges into open air above the maximum water level, with no continuous path back under any failure. Non-potable pipework has to be identified and any tap off it labelled, which matters most to the next occupant. The North American equivalents sit in the non-potable water systems provisions of the International Plumbing Code and in ARCSA/ASPE 63.

Whether you may collect it at all varies more than people expect, and is worth five minutes rather than an assumption. Rooftop collection is unrestricted across most of the world, but in parts of the western United States it sits inside a prior appropriation water rights system: Colorado permits residential rooftop collection under House Bill 16-1005 of 2016, in no more than two barrels of 110 gallons combined. Elsewhere it is size that triggers involvement — tanks above a threshold, any below-ground vessel, any connection into the building's plumbing. None of that argues against the job; it argues for finding out before the tank is delivered.