It’s true that a hosepipe and summer rain have the potential to start an argument between neighbours. Both will leave you soaked if you’re within range of either and both can waste ridiculous amounts of perfectly good water if you’re not paying attention to how you use them.
So when you’re standing there in your back garden holding one while staring rather indignantly at the other with a water bill that makes Mahatma Gandhi look like a millionaire: you’re probably wondering why you pay to get drinking water dumped onto your plants when perfectly good rainwater is landing on your roof and running unhindered down the drain.
Fair question.
Outdoor water use can account for up to 30 percent of a households water consumption during summer months (Environment Agency). That heavy rain comes bundled with pollutants you don’t want on your veg and we’re certainly not desert dwellers (Met Office).
Capturing rainwater for garden and outdoor use is easy, affordable and something anyone can do.
## Calculating Potential Capture Volume
First things first: you’re not going to live off rainwater alone collecting it from your garden shed roof.
There are some fantastic systems available that turn your roof into a personal reservoir allowing you to be self-sufficient in water for irrigation and outdoor cleaning. These systems use all of your roof space, connect to your gutters, filter the water and store it away ready for use when you need it.
They’re brilliant… But tend to cost more than most people realise they want to spend.
Unless you have a realistic intention of becoming self-sufficient in water then take the distraction away. Simplify. Start small and you can capture a surprising amount of water without spending too much money or renovating your roof.
We’ll go into more detail on what collection systems are available later, but first it’s worth taking a look at how much rainwater you could potentially collect.
| UK Annual Rainfall | |
|---|---|
| Average rainfall | 885mm |
| Kings Lynn | 847 mm |
| London | 657 mm |
| Brighton | 819 mm |
| Glasgow | 1211 mm |
| Rainfall capture per year | |
|---|---|
| Average Roof Size (81.0m2) | 71,685 litres |
| Tiny London Flat Roof (10.0m2) | 8,850 litres |
| Small urban roof (50.0m2) | 44,250 litres |
We get quite a lot of rain in the UK, even in London. Your roof is capable of collecting thousands of litres of water every year depending on its size. A 100m2 roof would collect enough rainwater to supply a single person with water for almost 6 years.
Rainfall is measured in millimetres. When it rains 1mm across the UK, every square metre of your roof will collect 1 litre of water. To calculate how much water your roof could potentially collect in a year, multiply your roof’s area by annual rainfall where you live.
Roof Area (m2) x Rainfall (mm) = Potential Capture (litres)
You’ll never collect all of that rain for two very good reasons…
- You use some of it.
- The collection system you put in place won’t be 100 percent efficient.
Factor in perhaps a 10-15% loss due to evaporation, spillages and mis-connectures, but don’t be shy with your roof area. Include every roof surface connected to your guttering system. garage roofs, sheds, porches; the lot.
Don’t forget skaps or anything else your water will run off as you calculate your roof area.
Now you know you’re not planting tanks in your back garden and turning your shed roof into a supreme water collecting contraption we can get onto some simple collection systems that will save you money and water.
Here’s what you need to know to get started.
## Rainwater Harvesting Basics
The most obvious place to start rainwater collection is at your downpipes. Roof rain runs down gutters and into your downpipes before being whisked away by gravity. All we’re trying to do is intercept some of that flow and put it somewhere useful instead of the drain…
| Feature | Effective | Limitations |
|---|---|---|
| Downpipe diverters | Collect water from roofs into storage containers | Provide limited storage |
| First flush diverters | Remove debris and dirt from collected water. | Adds cost and complexity. Requires cleaning |
| Buffer Storage | Smoothing out demand on your collection system | More tanks means more things that can leak or fail |
| Bearings and Linking Kits | Allows you to link multiple water butts together. | Sourcing matching tanks in matching colours is difficult. |
| Water additives | Control algae growth and discourage mosquitoes | Mixed reputation. Tea tree oil works just as well |
Downpipe diverters aren’t cheap but a good first flush diverter is a worthwhile investment. Storage is everything when rainwater harvesting. Your collection system will collect a lot of water very quickly. Make sure you have enough tanks to capture everything or you’ll still be wasting water. Linking multiple water butts together is fairly straightforward but can look unsightly.
I’m not sold on water additives. Cleansing tablets, algae inhibitors and liquid supplements promise to extend the life of your stored water but I’ve never found them necessary. A splash of Teatree oil in your water butt works wonders for keeping mosquitoes at bay. I’ll leave you to make your own decisions on water additives. Best bet is avoid.
### Downpipe Diverter
A Downpipe diverter splits the flow in your downpipe allowing you to send some of the water to a storage container instead of letting it all run away to the drain. Diverters work by filling your storage container until it’s full then redirecting water back to the drain system. Easy!
Most diverters work on a height restriction principle. Simply cutting the bottom out of your downpipe creates a diverter that requires you to manually redirect flow when your storage container is full. Upgrade to a purpose made downpipe diverter, attach a length of hose and you’re good to go.
[! [Rainwater Downpipe Diverter](/wp-content/uploads/2016/02/rainwater-downpipe-diverter-300×200.jpg)](/wp-content/uploads/2016/02/rainwater-downpipe-diverter.jpg) [! [Downpipe diverter kit](/wp-content/uploads/2016/02/downpipe-diverter-kit-300×200.jpg)](/wp-content/uploads/2016/02/downpipe-diverter-kit.jpg) [! [Rain Harvesting Downpipe kit complete with connection](/wp-content/uploads/2016/02/Rain-Harvesting-downpipe-kit-complete-with-connectors-300×200.jpg)](/wp-content/uploads/2016/02/Rain-Harvesting-downpipe-kit-complete-with-connectors.jpg)
Downpipe diverters are available online for £15-£30 and can be installed in less than an hour with a screwdriver.
Installing a downpipe diverter is straightforward. Position your water butt close to where you’ll use the water most, then cut your downpipe to fit.
You’ll need to cut the downpipe, fit the connector onto the downpipe, fit your storage container connector to the diverter unit, then connect both together with a length of hose.
Most diverters come with a leaf trap to stop debris from entering your storage container. When picking a position ensure the hose feeding into your container allows for gravity flow from the diverter. The water butt doesn’t need to be very high but makes filling the container easier.
The water butt will weigh approximately 1kg for every litre of capacity when full. Water Butts are heavy, make sure they’re positioned on something that won’t collapse under the weight. Purpose made water butt stands are ideal but a concrete slab will do fine.
Most containers have taps close to the bottom. Position the butt as close to where you’ll use the water as possible. Moving water around is a faff so minimise how far you’ll need to carry it.
### First Flush Diverter
When it rains, the first water that runs off your roof carries dirt, bird poop, leaves and anything else that’s fallen on your roof. A First Flush Diverter discards the first ‘flush’ of rain through your downpipe allowing only clean rainwater to enter your storage container.
[! [First flush diverters](/wp-content/uploads/2016/02/first-flush-diverters-300×208.jpg)](/wp-content/uploads/2016/02/first-flush-diverters.jpg) [! [First flush diverter kits](/wp-content/uploads/2016/02/first-flush-diverter-kits-300×208.jpg)](/wp-content/uploads/2016/02/first-flush-diverter-kits.jpg)
Add a First flush diverter to any rainfall collection system to improve the quality of water you’re storing. Systems range from £40-£80 and work by diverting the first rainfall into a chamber before allowing subsequent cleaner rain to enter you storage container.
Size the first flush diverter to the size of your roof. You should capture the first 1mm of rainfall on your roof no matter how much rain falls afterwards.
A 1mm rainfall on a 100m2 roof will produce 100 litres of runoff. Or in other words; you’ll need a 100ltr first flush chamber to filter the first millimetre of rain that falls.
The volume of the first flush storage tank is equal to your roof size in square metres:
Roof area (sq metres) x 1mm rainfall = first flush volume (litres)
Simple.
### Additional Water Butt
[! [Water Butt](/wp-content/uploads/2016/02/waterbutt-300×200.jpg)](/wp-content/uploads/2016/02/waterbutt.jpg) [! [Water Butt with Linking kit](/wp-content/uploads/2016/02/waterbutt-with-linking-kit-300×223.jpg)](/wp-content/uploads/2016/02/waterbutt-with-linking-kit.jpg)
Heavy rain fills a single water butt very quickly. Water butts are available in 100ltr, 200ltr, 350ltr, 600ltr and even 1000ltr sizes. Larger rainfalls will overflow any single butt so connect multiple containers together.
Linking kits connect two or more water butts together and allow you to fill multiple containers from the same downpipe. Kits are available for around £10 per connection. Position containers at the same height to allow water to flow between them when filling.
Each container should have it’s own tap or valve to allow you to isolate sections of your system for cleaning. Three 200ltr containers in a linked system will give you 600ltrs of storage.
Make sure your system has enough buffer capacity to capture most of the rain that falls. Aim to collect as much rainwater as your roof can generate in a year.
## Rainwater Harvesting at Scale
Systems don’t have to stop at three water butts chained to your downpipe. Buildings have more than one downpipe. Harvest rainwater from every gutter-connected roof surface you own.
[! [Water Butts connected to house](/wp-content/uploads/2016/02/water-butts-connected-to-house-300×187.jpg)](/wp-content/uploads/2016/02/water-butts-connected-to-house.jpg) [! [Rainwater Harvesting downstairs water butt](/wp-content/uploads/2016/02/Rainwater-Harvesting-downstairs-water-butt-300×224.jpg)](/wp-content/uploads/2016/02/Rainwater-Harvesting-downstairs-water-butt.jpg)
Large scale rainwater harvesting systems use underground storage and electric pumps to boost pressure. Systems connect to all roof drains feeding water into a large buffer tank before distribution.
Underground systems collect rain from your roof and store it away for you unseen. Tanks come in all shapes and sizes ranging from 800ltr to 2000ltr and beyond.
Electric submersible pumps raise the water to usable heights and allow you to attach sprinkler systems for garden irrigation. Larger tanks equal longer periods without emptying so justify the cost of pump and electric hookup.
Slimline and wall mounted water butts offer storage where normal sized tanks won’t fit. Holding between 50 and 80 litres they’re great for smaller gardens or collecting water for potted plants.
Gravity fed systems use elevation to improve pressure. By raising your water butt above ground level you allow gravity to push water through your hose when turned on.
Wall mounted water butt attachments offer very little storage so are aimed at top ups and supplementation of primary systems.
DONT FORGET: Calculate potential size of your rainwater collection system before you start. You roof has the capacity to supply your entire household demand depending on where you live and the size of your family. Why stop at rainwater harvesting for outdoor use?
Rainwater collection for garden irrigation is simply the beginning. Capture rainwater from your loft space to create an alternative supply for toilet flushing. Your rainwater supply has the potential to supply every moist requiring outlet in your house.
Sustainable living and water conservation tips to get you startedRainwater harvesting helps to reduce water demand. By lowering demand you’ve lowered supply needs too. The less water we use, the less we need to process and transport.
Installing water efficiency improvements across your household can lower total demand by up to 20 percent (Energy Saving Trust)
Rainwater harvesting and water butts account for a significant chunk of that 20 percent.
So how much water can you really save by harvesting rainwater?
| Item | Savings | Notes |
|---|---|---|
| Loft insulation | Up to £50/year | |
| Cold appliances | Up to £35/year | |
| Showers | £35/year | Based on family of 4 |
| Bulbs | £15/year | A person |
| Taps | £10/year | A person |
| Bathtubs | £50/year | Family of 4 |
| Lawn sprinkle system | £100/year | Average UK sprinkler system |
| Rainwater Butt toilet flush | £70/year | Flush toilets once per person per day |
| Water Butt Outdoor Cleaning | £30/year | Average british garden |
| All water saving measures | Up to £395/year |
Rainwater can only take you so far. A single person could theoretically supply all of their own water demand with rainwater collected from an loft space. A family of four sharing a 200ltr water butt will run out several times a day during summer.
Integrate rainwater butt usage with other water efficiency measures. Make a single rainwater butt flush your toilet and you can significantly reduce mains water demand.
Rainwater harvesting could save your household up to £150/year (Which?). Installing a rainwater harvesting system will pay for itself through water bill savings alone in most properties.
Bonus content: Don’t throw that water butt away!
Install a liner in your bucket and use old rainwater butts as planting pots. With a little ingenuity your rainwater butt becomes a useful item around the house that does more than just store rainwater.
## Rainwater Harvesting FAQs
Will harvesting rainwater save me money? If water butts are all you do then saving money isn’t the strongest motivator. Including rainwater harvesting as part of wider water efficiency measures reduces annual household water demand by up to 20 percent.
Whats the UK average water usage per person? According to Ofwat (Ofwat) the average UK citizen uses 142 litres of water per day. Feeding an indoor rainwater tank using loft water could supply up to two people with all of their water requirements.
Can I drink water butt water? Water butt water is great for garden watering. We don’t recommend using water butt water for drinking, cooking or similar unless you have filtered it sufficiently.
Is rainwater better for your plants? Rainwater is naturally soft with a PH around 5.6. Most plants thrive on soft rainwater rather than hard mains water supplied to your taps. Rainwater doesn’t have added chemicals like chlorine or fluoride.
Why doesn’t every house collect rainwater? Every house has the potential to harvest and reuse rainwater they just don’t know how much water they can realistically collect and use. There’s nothing worse than investing in a rainwater collection system only to find it never gets used.
How do rainwater Butts work? Water butts allow you capture rainfall from your roofs gutters and store it away until needed. Using gravity flow alone butts attach directly to your downpipe feeding off rainfall collected on your roof.
How much water can I save with a water butt? Install a water butt and you could save £30/year on watering your garden. Your butt could save your garden entirely for some months during summer. That’s a full 12 weeks of summer watering for free.
How much water does a water butt hold? Water butt sizes range from 100ltr to 1000ltr volumes. The size of your roof and rainfall will determine how many water butts you need.
What rainwater can’t be used for… DON’T use harvested rainwater for watering vegetables due to be eaten whole. River relaxation? Rainwater is ideal for filling up your paddling pool or lakes when the children get involved.
