More Rain Is Coming: Could Your Garden Capture 1,000 Litres Instead of Sending It Down the Drain?

“The water you pay for next summer may be falling on your roof for free this autumn.”

Most of us have become accustomed to thinking about rain as something to get rid of.

It lands on the roof, runs into the gutter, disappears down a drain and, within a few minutes, we have forgotten all about it.

Then six months later we can find ourselves standing in the garden with a hosepipe, using carefully treated mains drinking water to keep tomatoes, hanging baskets and vegetables alive.

There is something slightly illogical about that.

Perhaps we should start treating rainwater as a resource rather than a nuisance.

And once you actually calculate how much water lands on an ordinary house, the numbers can be remarkably large.

Start With One Very Simple Calculation

Rainwater calculations are wonderfully straightforward.

1 mm of rain falling on 1 square metre = 1 litre of water.

So:

Potential water collected in litres = roof area in m² x rainfall in mm

In reality, you will not collect every drop. Some water remains on the roof, some evaporates, and filters and gutters introduce losses.

For a simple household estimate, using around 80–90% collection efficiency for a conventional pitched tiled roof is reasonable; published SuDS guidance gives an initial runoff coefficient around 0.90 for pitched tiled roofs, with further filter losses depending upon the system.

For a quick garden calculation I would therefore use:

Water collected = roof area x rainfall x 0.85

It will not be engineering-design accuracy, but it tells you whether you ought to be thinking in hundreds or thousands of litres.

What Does That Mean for an Ordinary House?

Suppose one side of your roof feeding a particular downpipe represents approximately 50 m² of horizontal roof area.

A modest 10 mm rainfall gives:

50 x 10 x 0.85 = 425 litres

A 20 mm period of rain gives:

50 x 20 x 0.85 = 850 litres

Make the collection area 60 m² and that same 20 mm rainfall becomes:

60 x 20 x 0.85 = 1,020 litres

There is our thousand litres.

Not during an exceptionally wet winter.

Potentially during one decent spell of rain.

And remember that many homes also have garages, sheds, workshops, greenhouses and extensions capable of contributing additional collection areas.

The RHS points out that even a relatively small shed roof in comparatively dry south-east England can collect several thousand litres during a year.

A Useful Ready-Reckoner

Using an estimated 85% recovery:

Roof feeding the tank	5 mm rain	10 mm rain	20 mm rain
20 m²	85 L	170 L	340 L
30 m²	128 L	255 L	510 L
50 m²	213 L	425 L	850 L
60 m²	255 L	510 L	1,020 L
100 m²	425 L	850 L	1,700 L

Suddenly a conventional 200 or 250-litre water butt begins to look rather small.

The Problem With the Traditional Water Butt

I already use a 250-litre water butt at home, and it is extremely useful.

But doing these calculations illustrates its limitation.

Imagine that it is empty before a period of heavy rain.

A 50 m² roof might deliver 400 or 500 litres surprisingly quickly.

The first 250 litres fill the butt.

Everything after that goes down the overflow.

If another substantial rainfall arrives the following day, the water butt may still be full.

Its effective storm-water storage capacity is then:

zero litres.

That doesn't mean water butts are pointless. Far from it. The RHS specifically recommends collecting roof water, using filters, keeping containers covered and linking multiple water butts where greater storage is needed.

It simply means that one water butt should perhaps be considered the beginning of a system rather than the completed system.

Option One: Link Several Water Butts Together

This is probably the easiest next step.

Instead of one 250-litre butt, imagine:

4 x 250 litres = 1,000 litres

They could be positioned behind a garage, alongside a shed or partly concealed by planting or screening.

There are two general ways of connecting them.

A high-level connection allows one container to overflow into the next.

A lower-level connection allows several tanks at the same height to fill more evenly, although installation needs to be sound because every low-level joint remains permanently under water pressure.

You don't necessarily have to install 1,000 litres immediately.

You might start with 250 litres.

Then add another.

Then another.

That is rather more in keeping with my general approach to going green: improve things gradually rather than believing everything has to become an expensive grand project.

Option Two: The 1,000-Litre IBC Tank

For someone with more room, an IBC — Intermediate Bulk Container — provides another possibility.

Many are approximately 1,000 litres.

One container can therefore provide roughly the storage capacity of four conventional 250-litre water butts.

But an IBC requires more thought.

A full 1,000-litre container holds approximately one tonne of water, before allowing for the tank and its frame.

It therefore requires a properly prepared, firm and level base.

I would also want:

• a known and safe previous use for any second-hand container;

• a securely closed top;

• filtration before water enters;

• protection from sunlight to discourage algal growth;

• a properly designed overflow;

• easy access to the outlet;

• screening if appearance matters;

• protection of exposed fittings where freezing might cause damage.

This is certainly not something to put on a few loosely stacked bricks at the back of the shed.

But Storage Is Only Half the Story

This is where rainwater harvesting becomes much more interesting.

The objective doesn't necessarily have to be:

“Keep every litre.”

A better objective might be:

“Stop every litre rushing immediately into the drain.”

Some can be stored.

Some can soak into the soil.

Some can support plants.

Some can temporarily occupy a rain garden.

Some can replenish a pond or other appropriately designed garden feature.

And only the excess needs to leave the property rapidly.

That is essentially the thinking behind Sustainable Drainage Systems — usually shortened to SuDS.

Current national SuDS standards in England specifically emphasise dealing with everyday rainfall close to where it falls and reducing immediate runoff into piped drainage systems.

Create a Rain Garden

One particularly attractive solution is a rain garden.

This sounds much more complicated than it necessarily is.

A rain garden is essentially a shallow planted depression that receives water from a roof, patio or other hard surface.

Instead of the downpipe disappearing directly underground, water might travel through a gravel channel, rill or pipe into a planted area.

During heavy rain the depression temporarily fills.

Then the water slowly infiltrates into the ground.

The RHS describes rain gardens as planted areas designed to accept runoff and notes that they can reduce erosion, provide habitat and attract insects and birds as well as managing rainwater.

That makes this particularly attractive for a Going Green garden.

We aren't simply conserving water.

We are potentially creating more planting and wildlife habitat at the same time.

What Could You Plant There?

The centre of a rain garden needs plants that can tolerate occasional periods of very wet soil.

The edges can contain plants that prefer somewhat drier conditions.

The precise choice depends heavily upon soil, sunlight and location, but the design principle is wonderful:

water creates the planting zones.

Instead of fighting the wettest corner of the garden, design around it.

A rain garden might contain grasses and moisture-tolerant perennials, with more drought-tolerant species towards its higher edges.

The result need not resemble a drainage engineering project.

Done properly, it can simply look like an attractive flower bed.

Don't Put It Against the House

There is an important qualification.

You do not want to deliberately direct large quantities of water towards your foundations.

Rain gardens need careful positioning, consideration of soil and knowledge of underground services.

The RHS advises obtaining appropriate professional advice where a rain garden is close to a building and recommends checking how quickly the ground drains before constructing one.

So the principle should always be:

slow the water safely away from the house — not towards it.

What About the Driveway?

There is another part of many properties that deserves attention.

The drive.

Concrete, traditional block paving and other impermeable surfaces can turn rainfall into immediate runoff.

Water hits them and heads straight towards a drain or road.

A permeable driveway works differently.

Options include properly constructed:

• gravel;

• permeable block paving;

• porous asphalt;

• other permeable paving systems.

Alternatively, runoff from an impermeable area can sometimes be deliberately directed into a planted border or rain garden rather than straight into drainage.

Government guidance specifically promotes permeable surfaces and drainage to planted areas as ways of reducing runoff, and there are planning restrictions in England relating to larger impermeable front-garden surfaces that do not drain to a permeable area.

I would therefore think very carefully before automatically concreting another part of a garden.

What appears to be “low maintenance” can simply move the water problem somewhere else.

Autumn Rain Could Become Spring Water

There is another intriguing possibility.

We tend to think about collecting summer rain to use a few days later.

But what if some of our autumn and winter rainfall could be retained much longer?

A sufficiently large, properly covered storage system could begin the spring with a substantial reserve of rainwater.

The RHS notes that as the climate changes towards generally wetter winters and drier summers, larger-scale rainwater harvesting can become increasingly useful for supporting plants through hotter periods.

The Met Office's UK climate projections similarly point towards wetter winters overall, drier summers and more intense rainfall events, although individual years will of course remain highly variable.

That creates an interesting challenge:

we may increasingly have water at the wrong time.

Storage becomes one way of moving a little of that water from the wet season towards the dry one.

Don't Forget Water Quality

Stored rainwater is not automatically drinking water.

For the sort of straightforward garden system discussed here, I would treat it as non-potable water.

Keep gutters reasonably clean.

Fit a debris filter.

Keep tanks covered.

Clean water butts periodically.

The RHS also advises using stored water regularly and avoiding stagnant or foul-smelling water. It recommends caution particularly around seedlings and young plants where contaminated stored water can potentially introduce plant diseases.

For established garden plants, however, rainwater is excellent — and many plants actually prefer it to hard mains water.

Could You Actually Save Money?

Yes, but this needs to be kept in perspective.

1,000 litres = 1 cubic metre of water.

If you have a water meter, every cubic metre of mains water you replace with collected rainwater reduces the amount you buy.

But I would not buy an elaborate £2,000 rainwater system on the basis that collecting one tankful will transform the household finances.

It won't.

The economics become more interesting when:

• equipment is inexpensive;

• storage is filled and emptied repeatedly;

• you already use significant quantities of garden water;

• mains water is metered;

• the system has a long working life.

If you do not have a water meter, the immediate household bill saving may be nonexistent.

But there are other benefits.

You are reducing demand for treated drinking water.

You have water available during dry periods.

You reduce some of the peak runoff from your property.

And your garden becomes more resilient.

Going green doesn't always have to produce an immediate financial return measured to the nearest penny.

But wherever possible, I still prefer environmental changes that give us something useful in return.

The Crucial Point: Make Space Before the Storm

There is one slightly counter-intuitive idea worth understanding.

A full water butt doesn't protect the drainage system from the next storm.

If your 1,000-litre system is already holding 1,000 litres when heavy rain begins, almost everything arriving next will overflow.

That is why a really effective garden water strategy includes both storage and somewhere safe for overflow to go.

For example:

Roof → filter → storage tank → overflow → rain garden → soil

rather than simply:

Roof → drain

Now the system has several opportunities to slow the water down.

Try a Rainwater Audit This Weekend

You don't have to purchase anything initially.

Walk around the property with a tape measure and notebook.

1. Estimate your collection area

Measure the horizontal footprint of the section of roof feeding each downpipe.

Don't worry about measuring along the slope of the tiles. We are interested in the approximate projected area receiving rainfall.

2. Calculate a 10 mm rainfall

Use:

Roof area x 10 x 0.85

A 40 m² roof would give:

40 x 10 x 0.85 = 340 litres

3. Calculate a 20 mm rainfall

The same roof gives:

40 x 20 x 0.85 = 680 litres

4. Compare that with your storage

If you have one 200-litre water butt, you immediately know how much water you are losing once it fills.

5. Follow every overflow

Where does the water actually go?

Into a surface-water drain?

Across paving?

Onto the lawn?

Towards the house?

Into a flower bed?

6. Look for additional storage

Could you add another butt behind the shed?

Could two be linked?

Could a garage roof supply a separate tank?

Would an IBC be practical?

7. Look for somewhere to slow the overflow

Could a border be lowered slightly?

Could you build a rain garden?

Could an impermeable strip of paving eventually become permeable?

That turns an ordinary garden walk into a miniature water-management survey.

Think Beyond the 1,000-Litre Tank

The most interesting conclusion is that the answer isn't necessarily to install enormous tanks everywhere.

The better garden might combine several small interventions.

Perhaps:

500 litres stored in linked water butts

plus

a small rain garden

plus

permeable paving

plus

better soil containing more organic matter

plus

mulching that reduces evaporation next summer.

Suddenly the garden has become part of the solution.

It captures water when there is too much of it and holds onto moisture for longer when there isn't enough.

Conclusion: Don't Just Watch the Rain Disappear

When the autumn rain begins, go outside and watch your downpipes.

That water has fallen onto your property.

Within seconds, much of it may be disappearing underground.

Perhaps some of it should stay.

Not flooding your lawn.

Not sitting against your foundations.

Not filling an unsuitable open container.

But safely captured, slowed, stored or allowed to infiltrate.

A 60 m² roof receiving 20 mm of rain can have more than 1,000 litres available for collection before realistic losses.

That is more than four conventional water butts' worth.

Perhaps the next stage of going green is therefore not simply asking:

“Have I got a water butt?”

Ask instead:

“What happens to every thousand litres of rain that lands on my home?”

Because the water you pay for next summer really could be falling on your roof for free this autumn.

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