Promising myth of rain harvesting – does it worth?

Promising myth of rain harvesting – does it worth?

rain harvesting

Rain, rain, rain

United Kingdom is well-known about lots of rain. Sometimes we all have the same feeling, especially in winter, when ona rainy day follows the other and it seems, there is no chance for the water to soak in the soil or run down on the rivers. And it looks even worse if we compare the present dry summers of climate change as they are warmer and warmer and dryer and dryer.

Whilst we sit in the warm room, look through the window and watch the interminable precipitation plenty of us has the same thought: how could all these vast amount of water be used during the summer drought period? The solution is rain harvesting.

Why use of rain water is any good?

Save money and environment

Save on mains water costs. Also helps to save the environment because every drop of main water saved causing less impact on nature.

Reduce flood risk

By keeping rainwater out of the stormwater management system

Ideal for plants

Rainwater is the best natural solution to water plants. Its pH is balanced
free from chemicals such as chlorine found in mains water.

How rainwater can be used?

Promising myth of rain harvesting - does it worth? 1

Although rainwater is not
suitable for human consumption, still good
for many everyday uses.
We simply don’t need to use purified drinking water for everything.

Harvested rainwater is ideal for:

  • Plant irrigation

  • Car and terrace washing

  • Watering livestock

  • Toilet flushing

  • Washing

Promising myth of rain harvesting - does it worth? 2

How clean is the rainwater?

• A filter removes any debris as rainwater enters the tank
• Calming inlets prevent sediment from being disturbed
• Water is oxygenated in darkness to discourage algal growth

Build up a rain harvesting system


Ideally you have sloping covered surfaces (roof, terrace, etc.) where the rainwater can be collected from. Rainwater cannot be collected from the ground as it’s full of sediments.
The larger the collection area, the bigger the harvested rain amount, obviously.

Gutters, downpipes

Gutters need to be extended to deliver the rainwater to the storage tank.
Based on the location of this tank it can be a simple job, or it comes with lots of machine and hand digging around the house to carry the water to the tank on a gravity way.

Storage tank

You will need a large tank to store the harvested rain (how large, you can calculate it later).
Unless your house has strong beam slabs, the storage tank cannot be installed in the attic because of the weight of water. Every m3 of water is 1 ton.
Consequently the storage needs to be located on the ground level (over- or underground) or sometimes in the cellar.
At an overground tank you will face with an immense plastic tank, which would be better to be hidden somewhere.


We can’t forget about the leftover rainwater delivered down via the downpipes to the tank.
When more rain collected than the storage tank, the remnant needs to be led away. Depending on the local regulation the rainwater might need to be wasted either on the property or it can go in the rain sewer system.
Anyway, another pipe and equipments has to be installed.


As your rainwater is probably stored on the ground level, if you want to use beyond that, a booster pump needs to be installed.
The more sophisticated the use, the more expensive the booster set.
We have to say now, unless the system will be installed in a brand new economical-friendly house, the only use worths the cost, is irrigation.


As you would use the rainwater in the house, water needs to be delivered to the toilets, washing machine, etc. Due to the nature of rainwater (can include pathogens) it cannot be transferred on the same pipework, than main water.
And install another water pipe system in an existing house can be a nightmare from any point.


Toilet tanks, washing machines, any kind of water consumers need to be prepared to work with both rainwater and -in lack of rain- main water as well.
At this point, still the irrigation looks the best idea to use rainwater.

How can you calculate?


Measure and calculate the surfaces (roof, terrace, etc. footprint) which can be used for rain harvesting.

Count in squaremetres.


Calculate the rainfall in cubicmetres.

If you can see the average yearly rainfall in Cambridgeshire is 568mm, it means 568 litres of rain falls to every squaremeter.

The average monthly rainfall capture in Cambridge:

568 litres / 12 = 47 litres / m2

For your calculation:

Bedfordshire 680mm
Hertfordshire 681mm
London 583mm
Northamptonshire 713mm
Essex 650mm
Suffolk 620mm

Harvested rain

Now you simply have to multiply the surface m2 with the monthly litres and you get the monthly amount of rainwater harvested.

Now triple your product because collection through the wettest three winter months would be desirable.

Let’s say the useful surfaces are 250m2 on your house.

250m2 x 47 l x 3 = 35 m3

In this case I would recommend to buy a 40 m3 storage tank.

Yearly use with irrigation

Finally from here, we calculate with the irrigation use as the most cost efficient usage of rainwater.

Now there are more unknown factors in the equation. But cut to short we say the irrigation company says the average water use of the irrigation system is 1.6m3/day.

Our tank is filled up (from the previous example) and have 40m3 water. Unfortunately another two months of rainfall is lost because a storage tank for 5 months harvesting would have been a bit oversized.

But we still have a a chance to collect the rainfall of the remained 7 months. But we sadly have to say, due to the uneven rain and uneven use of irrigation water we have to cut this number back to its half.

Still, the yearly sum is an impressive 81m3 water saving with these numbers in our example.

Calculate your savings now.

Rain harvesting conclusion

Now you can see clearly what to count on when you think about rain harvesting.
If you are about to build a new house or do major refurbishment works you have a good chance to get an economical installation.
But if you want to retrofit the rain harvesting system to your existing house, there are many cisrcumstances to work out its profitability.