How Much Rainwater You Can Realistically Collect From a UK Roof

Living in a flat without a garden and paying Bristol water rates means I’ve spent far too long trying to work out precisely how much money flows down my drains every month. Researching rainwater harvesting wasn’t done with any grand visions of sustainability though; I just wanted to know if the water falling on the roof above me could save me money on bills, and if there was actually enough rain falling in the UK to make any system worthwhile.

There’s plenty of rainwater flowing off UK roofs to make harvesting work, but how practical it is depends on how much storage you have. If you want to work out whether rainwater collection makes sense for you property this post takes you through all the maths.

**TLDR: Don’t run rainwater calculations based on your roof area alone. **

## The basic maths behind rainwater collection

First up the simple physics behind rainwater harvesting are straightforward and genuinely mindblowing when you first learn them. One millimetre of rain falling on one square metre of roof will always generate one litre of water (SparkCalc). That means if you know your roof area you can work out maximum collection in litres.

**Roof Size and Runoff**

Don’t feel you need to include your entire roof though. Only roofs draining to downpipes you can connect collection tanks to count for these calculations. Extensions linked to the mains drain elsewhere? Anxious neighbours whose gardens your funnels flow across? Don’t include these areas. You’re never actually going to collect rainwater off them.

For example, we’ve installed our collection on a downspipe feeding a small family greenhouse but could have just as easily connected to the drainage stack serving our roof terrace. We own a three quarter size roof in terms of collection potential.

The simplest collection calculation is expressed in litres equal to rainfall in millimetres multiplied by roof area in square metres (Celtic Water). Using the average UK annual rainfall of 885mm(Met Office) that works out at:

Ok so we get that part sorted now what?

**How Much Rain Falls In The UK? **

885mm doesn’t sound like a lot if you live in a rainy climate. Fortunately, the Meteorological Office have kindly told us how much rain falls across the country by county (Met Office). Whilst Newton Abbot in Devon gets nearly 1200mm of rain we have escaped moving to Cornwall with an annual rainfall of about 885mm in Bristol. That makes the maths:

**Runoff coefficient is essentially how much of the water hitting your roof you can actually collect. **

133 square metres x 885 mm rainfall = 117,705 litres per year

In theory.

**Adjustments: What Actually Runs Off**

Ah yes those pesky adjustment factors. Metal or plastic roofs have a runoff coefficient of around 85-90 percent (SparkCalc). Yup, that’s right around 90 percent of the water hitting your roof you can collect. Traditional tiles are slightly worse at 75-85 percent (SparkCalc). Slate or metal roofs have coefficient values of around 0.9(Celtic Water) which means you can collect about 90 percent of rainfall. Green roofs on the other hand have a coefficient of 0.3 (Celtic Water). So only collect 30% because all that lovely vegetation drinking most of it for you.

Coefficient aside most rainwater collection systems aren’t 100 percent efficient either. The practical number used by the British Standards for water collection calculations is 0.8 (Freeflush Water Management), which works out at:

132 x 885mm x 0.8 runoff coefficient x 0.8 system efficiency = 74,707 litres per year

Just over 74,000 litres for us. Still a lot of water…

Rainfall distributed evenly across the year would give us 2063 litres of water flowing past the butts downstairs filter every single day of the year. Rainfall isn’t evenly distributed though.

**Daily Rainfall Potential**

The average rainfall in Wales works out at roughly 3mm per day from March-August(Celtic Water). That’s enough water to fill our 100sq metres of roof collection area with 300 litres of water flowing through that downstairs downpipe per day.

That doesn’t happen.

Most of the rain falls on about a quarter of the days in any year. Some days you’ll get torrential downpours where you’ll fill your storage several times. Many other days you’ll get zero.

## Real World Rainwater Harvesting Systems

Armed with your newly calculated capture potential it’s time to look at real systems. Everything below our linked water butt setup falls under this category.

### Simplest Option: Gravity divert to water butts

Install a rainwater diverter on an outdoor downpipe and connect storage tanks to it. For a small number of water butts this is gravity fed collection. Water butts are cheap and easy to fit so it’s very easy to start harvesting rainwater this way. You’ll capture the first storm or two that fills up your butts then watch every drop overflow back into the drain system as soon as it rains more.

So what does that look like in numbers?

Our setup at the bottom end of this category uses three 200 litre water butts giving us a maximum storage capacity of 600 litres. Ignore the rainfall number above.

A 200 litre water butt fills to capacity from just 4mm of rainfall on a 50 square metre section of roof (Celtic Water). That’s enough rain to fall in the space of a couple of hours here in the westcountry during Autumn and Winter. UK storms regularly generate over 10mm of rainfall in a day. Common storms produce over 20mm. For most of the year you’re harvesting 20 percent or less of the rainwater that falls on your roof with the first rainwater butt. Possibly none of it during the really heavy storms.

Water butts are easy to install. Your water butt connects straight to the downpipe feeding it. First flood event of the Autumn we’d already filled ours and watched about a third overflow back into the drain system as rain continued to fall. From then on it’s been downhill.

### Intermediate Option: Multiple tanks or larger underground tank

Still rainwater Gravity feed through barrels or tanks but increases storage capacity significantly. You can often find secondhand water butts for sale free on local adverts websites but do expect to pay something. Anything holding several hundred litres of rainwater isn’t light.

240mm (SparkCalc). Our largest rainstorms generate somewhere between 10 and 20mm of rain here in a few hours.

* 5mm of rain = 300 litres overflows a 600 litre system in total.
* 10mm of rain = Two fills and overflows again.
* 20mm of rain = Four complete fill cycles.

Rainfall totals this high are uncommon but by no means rare in the UK summer. Average monthly rainfall from June-August is often under 3mm per day which struggles to fill a single butt. Rainwater butt systems are effectively downhill collection until you start running out of storage space during rainfall events.

Above uses two 90sq metres roofs in a four person household as an example. They recommend between 1.51 and 3.16 cubic metres of storage to meet those properties rainwater demand by capturing rainwater where available.

Most of the British Standard examples don’t go above three cubic metres of storage probably because you’re becoming uneconomical at that point. They do show what looks like an upper limit around 3.6 cubic metres across most scenarios.

Professional rainwater harvesting systems aren’t gravity fed. They use an underground tank usually between 3 and 10 cubic metres in size(Plumbstore). A pump pushes water at mains pressure through your pipework. You can supply your toilets, washing machines and greenhouse from rainwater alone. It’ll even run your dishwasher if that’s your thing.

Rainwater collection and storage takes a lot of space. Top tip; don’t install your storage above ground unless you really have to.

## Rainwater Collection Maths mistakes to avoid

## How to choose storage for your rainwater collection system

This table outlines rainwater storage options but doesn’t cover underground tanks. Rainwater tanks buried in the ground offer maximum space efficiency and are genuinely unseen by visitors to your property once installed. That comes at cost of £2000-8000 installation versus above ground options.

When calculating how much storage you need think about the largest typical rain event you expect to see rather than the total annual rainfall. Looking at our Bristol rainfall averages again;

* If you can capture every last drop then rainwater harvesting will provide most of your property’s demand.
* Large rainfalls are common in the UK but nowhere near as frequent as daily demand requires.
* Stock up during winter and you’ll run out by summer.
* Add extra storage and you’ll still see water going to waste during big storms.

Rainfall even during our comparatively wet Summer is very patchy across the year. By designing storage around average daily rainfall your either always under or overflow and you’ll never quite capture enough water to meet demands.

## How reliable is this Rainwater Collection Maths?

Pretty much every rainwater harvesting calculation method I’ve found agrees with the others for a typical UK roof. Multiply your annual rainfall expressed in millimetres by your roof area in square metres and you’ll know how many litres of water you’ll theoretically collect(Celtic Water). Add runoff coefficient values and you’ll work out how much water you can expect to collect from your real roof (SparkCalc). Adjust for expected losses in a functioning system and you’ve got the rainfall runoff calculation from the British Standard methodology (Freeflush Water Management).

Every site advises coefficient values for roof type between 0.8 and 0.9. Metal roofs collect the most at 90 percent followed closely by tiles. Slates are slightly less efficient but who cares. Adding storage stops rainwater running down the drain during most rainfall events.

Yes you should use tanks bigger than 3 cubic metres. No its not because you live in a particularly dry locale. Its because those examples are based on average rainfall spread fairly evenly through the year. The Real excitement is around how much storage you can realistically fit on your property.

Roof material doesn’t matter as much as storage size. Whilst slate and metal collection systems are more efficient than average your collection potential is limited by storage first and your roof size a poor distant second.

## Does rainwater collection work on all types of UK property?

Terraced houses, semi detached homes, detached homes all have rainwater collection maths that work the same way but very different real world limitations.

**Terraced houses**

Confined to a single downpipe for collection in most cases and with limited outdoor space for above ground storage your options are almost always restricted to the simple systems above. Aim for enough storage to collect most rainfall from your roof up to about 10mm in one go. That should cover most rainy days and provide decent capture rates for garden watering.

**Semi detached properties**

Similarly constrained by downpipe count to terraced houses but usually with gardens sufficiently large to add at least two if not three water butts. Large external tanks may also work if you’re attached to your property. A 1000 litre rainwater tank is enough storage to capture all rainfall from a 50sq metre roof during single rain event. You should expect to be able to capture most of what falls on your roof across a typical year.

**Detached houses**

Larger roofs mean more rainwater. Large roofs also need more storage to capture that rainwater effectively. Still fall into the 1000-4000 litre sweet spot for useable rainwater capture across a typical year but you’ll often justify the space and cost of larger external tanks or even underground storage.

Don’t have enough roof to capture all your rainfall in a single rain event? Add storage until you do and use the maths above to work out how much you need.

**Flats and apartments**

Good luck collecting from your individual roof unless you live in a converted warehouse. Group collection systems collecting off communal roofs are pretty common in blocks of flats I know. You’ll struggle to access your downpipes without either builder access or ninja climbing skills so plan your installation before your pipework goes in.

Apartment complexes big enough to have rainwater harvesting installed can generate significant annual savings. If your system can supply your garden and/or flushing toilets you’re doing well.

**Rural**

Distance to mains supply often means higher water costs, more space for storage and greater rainwater harvesting benefits. Water butts feeding your greenhouse or vegetable patches often become essential where water restrictions are imposed during dry summers.

**Listed Buildings**

Any collection system you can install without altering the fabric of the building is generally permissible. You may need to speak to the planning office about placing above ground storage in gardens but its usually allowed.

## Will rainwater collection really make a difference?

Rainwater collection does more than just make you feel better about wasting water.

But will rainwater harvesting save you money?

That depends on your household water use, the rain where you live and how much storage you can justify adding to your property.

Does rainwater harvesting work for flats?

If everyone in your apartment block agrees its yes.

How about terraced houses?

Granted you’ll struggle to collect enough water to supply more than your garden but you also have lower water consumption overall. Rainwater butt systems will do nicely.

Do you need planning permission to collect rainwater?

Only if you propose to alter the fabric of a protected building. Water butts are classed as vegetation by the planner’s this-side-of-legally-meaningful-grade.

Completing a successful rainwater collection installation…

* Runoff runs through butt overflow back into drain system.
* Roofs don’t grow back so don’t overestimate your collection potential.
* Plan around your typical rainfall patterns.