Farm Rainwater Harvesting Systems

Why storing rainwater can make sense for a farm

We're sure you've heard the environmental angles. We have too. We understand them: who would want dry watercourses, or an unhealthy countryside?

Even so, beyond the environmental arguments, three things have changed, and none of them is about being green.

1. Abstraction is getting tighter. If you draw from a borehole or a watercourse under a licence, you are exposed to restrictions in the summers when your farm needs the water most. Roof-collected rainwater sits outside the abstraction regime altogether: in plain terms, it is not abstraction, so a restriction on your licence does not touch the roof. The detail for England, Scotland and Wales is in what the rules actually say.

2. Mains supply is not guaranteed. Water companies prioritise household supply during an interruption. It's the right thing to do to protect the public, but it doesn't help your crops, flock, or herd. Storage is what turns an emergency delivery into something workable.

3. Dry summers are no longer unusual. Once you have been caught short, the question stops being whether storage is worthwhile and becomes how much you need. With increasingly volatile weather patterns (wetter winters, and drier summers) you are adding a water-supply buffer. Resilience improves.

How much rainwater can I collect from my roof?

Four numbers matter to understand the maximum you can capture:

roof area (m²) × rainfall (mm) × roof coefficient × filter efficiency = litres per year

Roof area is the plan catchment area. Length times width of the roof coverage, not the sloping surface. It can be measured on the ground. A 40 × 15 metre shed is 600 m² whatever its pitch.

Rainfall varies more across the UK than most people expect: around 530 mm in south Essex, 625 mm around Lincoln, 660 mm on the Shropshire plain, 1,160 mm in mid Wales, and over 1,800 mm in the central Lake District. Your postcode is what settles it, and two farms a hundred miles apart can differ by more than double.

Roof coefficient is how much of what lands on a roof actually reaches the gutter. Smooth, steep and metal is best:

Roof surface	Coefficient	Usual first use
Corrugated or profiled metal sheet	0.85	Best general catchment, including troughs after first-flush
Slate	0.80	Good catchment if the surface is clean
Concrete or clay tile	0.75	Holds moss and dirt in the joints; first-flush matters more. Fine for washdown; think carefully about putting captured run-off in troughs without additional cleaning steps
Fibre cement	0.75	Do not harvest from known asbestos-cement sheets for stock or dairy use. If you cannot tell the sheet age, or the material, pick another roof
EPDM membrane	0.70	Washdown or irrigation first; not ideal for animal watering
Bitumen / felt	0.70	Runoff only as it can leach contaminants. Avoid for livestock watering

A roof with solar panels can still be catchment. Cleaning the panels is a separate job: follow the panel maker. Do not run cleaning water with soaps, deionised water, detergents or other wash chemicals into the stock tank. Harvest the rain that falls on the array. Anything beyond fresh water used for cleaning should not be livestock water.

Filter efficiency is water lost to first-flush diversion and filtration, typically around 0.9 (or 90% capture efficiency). Worth carrying separately, because a simple leaf trap loses less than fine filtration.

Put together, a metal roof yields roughly:

Annual rainfall	Litres per m² per year	A 600 m² roof
600 mm	459	275,000 litres
700 mm	536	321,000 litres
800 mm	612	367,000 litres
1,000 mm	765	459,000 litres
1,250 mm	956	574,000 litres

The 360,000 litre figure in the introduction is water landing on a 600 m² roof at 600 mm. The 275,000 litre figure in this table is what reaches the tank after a metal-roof coefficient of 0.85 and filter efficiency of 0.9. Gutter condition, material type and pitch can move these figures either way. Treat them as a planning figure, not a guarantee.

Rainfall figures on this page are publicly available Met Office HadUK-Grid 1km gridded observations for the 1991–2020 climatology, averaged across the 1km cells containing postcodes in each district. Used under the Open Government Licence v3.

The regional figures and the three named districts are planning figures for those districts, not readings from a single Met Office station. The rainwater harvesting calculator uses the same HadUK-Grid method for a given postcode. Two farms in the same district can still differ, and past data is not a perfect predictor of the future. Climate can shift. Treat every yield on this page as a planning figure to inform decision, not a guarantee.

What counts as available roofspace?

More than people think. Every covered structure that drains to a gutter is potential catchment, though consideration must be given to how you capture and store the water:

• A main livestock shed with a clean closed roof
• Machinery and implement sheds
• The clean roof of a covered silage clamp - often the largest single area on the holding. The gutter must stay out of dirty-water and effluent drainage
• Lean-tos and open-fronted bays, where the roof sheet itself is clean catchment. The open yard under them is not roof water
• A grain store

Guttering all catchment space at once isn't typically the most cost-effective first move. Knowing the total helps you understand the maximum the holding could support later.

How much water storage does a farm need?

Advice can vary, so we'll try to keep it useful.

A tank is sized by whichever of two things binds first: what the roof can deliver, and what you draw. Greater storage than the roof can fill is overspend on a tank that stays partly empty. Storing less than the roof can usefully fill means you spill water in winter and risk running out when you need a supply.

Conventional wisdom is to use three months of average rainfall as a starting point. Treat it as a simple, broad starting point. It is the wrong size for a housed herd that drinks as the rain arrives, and it is far too small for a grazing herd that has to get through a dry summer. The worked examples below replace it with the constraint that actually binds on each farm.

The pattern that matters most is seasonal match. Where demand and rainfall fall in the same months, a modest tank is usually enough: the roof is filling the store as you draw from it. If demand comes in during a dry summer and the rain came in winter, you need a much bigger store to bridge the gap. A system sized for the first case will run dry in the second.

Cattle housed over winter: the strongest case

Stock come inside in October and go out in April. In much of lowland England a large share of the year's rain falls in that window. The roof they are standing under can be the supply they are drinking from, and the tank only has to bridge between wet days rather than across a dry season. Modest storage, high use.

Sprayer filling and irrigation: a seasonal store

Demand is concentrated between March and July; supply arrives October to March. The tank has to carry water across the late spring to mid-summer gap, so it needs to be bigger relative to daily use, but the volumes involved are usually well within what a machinery shed roof can supply.

Grazing stock through a drought: be realistic

A 120-head suckler herd at grass drinks something like 6,600 litres a day in hot weather (about 55 litres a head, a hot-weather planning figure at the top of the AHDB beef band). Three months of drinking is close to 600,000 litres, which is more than most farm roofs deliver in a full year. Rainwater can take a chunk out of the seasonal use and give you something in reserve. It will not replace the whole supply. That is a mathematical reality, not an opinion.

Three worked examples: sucklers, sprayer filling and dairy washdown

Real math, with planning rainfall figures for the postcode districts named, so you can check the method against your own farm. All three assume a metal roof at 0.85 and filtration at 0.9. Demand figures are planning assumptions so you can see how to calculate. Your stock, ration and weather will differ; that is what the enquiry is for.

120 sucklers housed over winter, Shropshire

Roofspace	1,200 m² from two sheds and a covered clamp
Rainfall	662 mm a year (district SY1), of which 355 mm falls autumn and winter
Annual yield	1,200 × 662 × 0.85 × 0.9 = 608,000 litres
Yield across the housed period	1,200 × 355 × 0.85 × 0.9 = 326,000 litres
Demand	120 head × 45 litres (mid-range housed planning figure; AHDB dry beef cows 15–40 L, lactating 40–70 L) = 5,400 litres a day, housed 180 days = 972,000 litres
What the roof covers	about 34% of the housed period's drinking water
Storage	20,000 litres. Roughly eleven days of collection, under four days of full demand

The tank is a buffer between wet days, not a drought reserve. It runs near full through the housed period and displaces around 326,000 litres of mains water over a winter.

400 hectares, sprayer filling, Lincolnshire

Roofspace	900 m² machinery shed
Rainfall	625 mm a year (district LN1)
Annual yield	900 × 625 × 0.85 × 0.9 = 430,000 litres
Demand	400 ha × 200 l/ha × 3 passes = 240,000 litres a season (illustrative spray programme, not a recommended rate)
What the roof covers	the roof yields 1.8× the season's demand
Storage	30,000–40,000 litres to carry the spring peak

Here water drawdown binds, not available supply. The roof has spare capacity, so the question becomes how much you want to carry into the season rather than how much you can collect.

200-cow dairy, yard and collecting-yard washdown, Cheshire

Roofspace	2,000 m² combined: parlour, cubicle housing and collecting-yard roofs, not a single dairy shed
Rainfall	720 mm a year (district CW1)
Annual yield	2,000 × 720 × 0.85 × 0.9 = 1,102,000 litres, around 3,000 litres a day averaged out
Demand	around 3,500 litres a day, year-round (worked assumption for this example, not a dairy-industry standard)
What the roof covers	about 86%. Close, but not quite all of the required draw
Storage	25,000 litres, about a week's buffer

Washdown is the best fit on any dairy: high volume, year-round, no potable requirement. See the note on parlour hygiene in the regulations section.

What the rules actually say

There may be varying advice about this. Here is the useful information, with sources so you can check. This page is written for England, Scotland and Wales.

Searches for “rainwater harvesting regulations UK” usually want this block. The paperwork is not the same in all three nations. The practical points below are the ones that decide whether a farm system is lawful to fit.

Abstraction and your roof

Collecting rainwater is lawful and needs no abstraction licence where the store holds only harvested rainwater, but it must be kept in pipework and tanks, and isolated from rivers, drains and groundwater.

In England that is written down. The Environment Agency rainwater harvesting statement says you do not need a licence for that use, provided it does not harm the environment. A restriction on a borehole or watercourse licence does not touch the roof. If you mix roof water with river, drain or groundwater, you may need a licence. A dry spell can still empty the tank. See GOV.UK on when you need a licence to abstract water.

In Wales Natural Resources Wales (NRW) regulates abstraction. There is no equivalent of England's rainwater bulletin. The same practical test applies: roof-only water that has not entered inland water or groundwater is not being taken from the water environment. Mix it with a watercourse, drain or borehole and you are into NRW regulated territory.

In Scotland the Scottish Environment Protection Agency (SEPA) regulates water abstraction from the environment. There is no equivalent rainwater bulletin to the England bulletin. Roof-only water that has not entered a watercourse or groundwater is the same practical starting point. If you mix sources, ask SEPA before you assume it is unlicensed.

Stock drinking water

The law does not hold livestock drinking water to a potable standard. “Potable” means fit for human consumption. Trough water is not in that category.

That is not the same as “any roof water will do.” Red Tractor still requires livestock drinking water to be “fresh and clean”, troughs kept clean, and an emergency supply if the normal one fails.

Untreated roof water picks up droppings, dust and whatever sits on the sheet. First-flush diversion, an opaque tank to reduce algal growth, and drawing from mid-column (away from surface film and bottom sediment) are the basic steps that keep harvested water in that band. A leaf filter is normal; finer filtration or treatment is extra peace of mind, not a substitute for those three. If you are unsure whether a source is fit for the herd, ask your vet, or test the water. If the assurance scheme or the contract is stricter than the law, the scheme wins.

Dairy hygiene water has three different duties

Do not treat “dairy” as one rule.

Yard and collecting-yard washdown is the best dairy use of roof water: high volume, year-round, and it sits outside Red Tractor MP.19. That appendix says the milk-contact water rules do not apply to washing parlour stalls, walls and floors, and do not apply to livestock drinking water.

Water that touches teats, udders, hands in the parlour, or the inside of the plant is milk safety. The Food Standards Agency's dairy hygiene inspection guidance expects private supplies used for those jobs to be monitored. If that supply fails, the instruction is mains or treated water, not untreated roof water through the plant. That is not the standard offer on this page. It needs a different specification and a higher standard of treatment.

Red Tractor MP.19 covers only water that can reach the milk: teat and udder wash, and the inside of the plant. If that water is from the mains, the scheme treats it as safe. If it is from a borehole, spring or other private supply, you must do an annual risk assessment or an annual water test, and keep the record. That is the farm's check, not an automatic council test. It is not the drinking trough and it is not yard washdown. See the appendix on assessing the safety of private water supplies.

Mains top-up and pipe marking

If you top the store up from the mains, the two supplies must not be able to mix. Harvested rainwater is classed as fluid category 5 under the fittings rules: the public main has to be protected against backflow. That is a plumbing rule, not a judgement that roof water is unfit for troughs.

In England and Wales the Water Supply (Water Fittings) Regulations 1999 require category 5 backflow protection, typically a Type AA or Type AB air gap. A non-return valve on a submerged inlet is not enough. Ask the water company before you plumb it. Some fittings work must be notified to them before you start. Getting a direct connection wrong can be an offence.

In Scotland the equivalent is the Water Supply (Water Fittings) (Scotland) Byelaws 2014, enforced through Scottish Water. The same method applies: no cross-connection, mark non-potable pipework, and category 5 protection on a mains top-up. Mains-connected rainwater systems are often notifiable to Scottish Water.

Pipes and outlets carrying harvested rainwater should be clearly marked so nobody mistakes them for a drinking supply. In England and Wales that duty is in Schedule 2 of the 1999 Regulations. The Scottish Byelaws make the same demand.

Standards

The current design standard for on-site rainwater systems is BS EN 16941-1. It replaced BS 8515, which has been withdrawn. CIRIA C539 is older best-practice guidance.

This is general information about the regulatory framework, not advice on your farm. Farm assurance schemes and milk contracts can impose stricter terms than the law, so check your own scheme. Private water supplies used for dairy hygiene are a local-authority matter in England and Wales; in Scotland the private-supply rules are separate. If in doubt, ask the regulator that covers the holding.

Sources: Environment Agency rainwater harvesting statement (England, 2021); GOV.UK abstraction licensing guidance; Natural Resources Wales (abstraction in Wales); SEPA (abstraction in Scotland); Red Tractor DR.FW.3; Red Tractor private water supplies appendix; FSA dairy hygiene inspection; Water Supply (Water Fittings) Regulations 1999 (England and Wales); Water Supply (Water Fittings) (Scotland) Byelaws 2014; BS EN 16941-1; CIRIA C539.

Water quality: an important factor to check

Lawful is not the same as sensible. A roof collects bird droppings, dust and whatever blows onto it, and standing water in a warm tank is not neutral.

Three things that matter more than the tank itself:

First-flush diversion

The first few minutes off a roof carries most of the contamination. Diverting it costs very little and does more for water quality than any amount of filtration downstream.

A simple leaf trap loses less than fine filtration. Finer filters or treatment can add peace of mind for trough water. They do not replace first-flush, a dark tank, or drawing from the right depth. If the water is for the herd and you are unsure, ask your vet.

Keeping light out

Algae needs light. An opaque tank stays cleaner for far longer than a translucent one, and needs fewer clean-outs. See the water storage tanks for farms.

Drawing from the right depth

Sediment settles and a scum layer forms on top. A floating suction draws from the cleanest part of the column rather than off the bottom.

What we supply

Two offers. They are not the same job.

Supply only

Tanks and components, delivered to the farm. You or your installer fit them. Sensible if you already know the size and the layout. Browse the farm water storage tanks.

Supply and specification

Give us roof area, stock numbers or crop area, postcode, and where the water has to arrive. You get back:

• What that roof will realistically collect, using the postcode's rainfall rather than a national average
• What storage that justifies, and which constraint is setting it
• A parts list a contractor can work from: tank, filtration, first-flush, fittings

Pumps. We size the store and spec the tank side. Your pump supplier sizes the pump against our figures (head, pipe run and the flow you need). That is their trade, and they will do it properly.

Fitting. We do not install. As you may have someone you already use for groundworks and plumbing, the specification is written so they can work from it directly. If you have not got anyone, tell us where you are and we will work for you to find someone local.

Where this does not work

Worth saying plainly.

If your roofspace is small relative to your demand, the numbers will not stack up whatever tank you buy. A few hundred square metres feeding a large herd is a supplement, not a supply.

If you need potable water for milking hygiene, that is a different system: WRAS-approved tanks and treatment to drinking water standards. We can talk it through, but it is not the standard range.

If you want to replace a mains supply entirely, be careful. Rainwater is a good second source and a good buffer. As a sole supply it puts your stock's water on the weather.

If payback on your mains bill is the whole case, run the numbers before you commit. On most livestock holdings the saving should be weighed over the long-term. The argument for storage is security of supply: having water when abstraction is restricted, when the mains fails, or when the borehole drops. If that is not worth something to you, this may not be the right spend.

Get the numbers for your farm

Send us your postcode, roughly how big your roofspace is, and what the water is for. We will come back with what you can realistically collect and what size store that justifies.

It takes about two minutes, and there is no obligation. If you'd prefer to start with rainfall only, use the rainwater harvesting calculator.

Start an enquiry or call 0800 316 7355 - free, and usually quicker