All you need to know about Catching rainwater

Watering can in the rain

This article will be extended over time with more information. Be sure to check back soon or subscribe as we will add more soon.

It all starts with climate

First, you need to analyze, how much precipitation (P) you can expect for each month. The website is a great resource to find out how much rain you can expect in your region. Be sure to cross check with other data sources specific to your area as well, to get a more accurate picture.

Know how much water you consume

Do you know how much water you are using each month? Take a look at your water bill. Implying you won’t change any of your habits, this is how much you will have to collect to “stay above water”.

The roof

One underestimated tool at our disposal is the roof. Most of the time, we only use them to protect us from the wind, sun, and rain. But it can do so much more for us. Often, we only divert the rainwater directly to the sewers. If you want to live self sustainably, you cannot let it go to waste.

But where to start? How big does the roof need to be? What kind of roof?

The type of roof

Not all types collect rainwater equally well. Some are more efficient than others. The efficiency can be measured as runoff coefficient (RC) and this study has gone deeper into this subject.

  • Roofs in general: 0.70 – 0.90
  • Concrete/asphalt: 0.9
  • Metal: 0.8 – 0.95
  • Aluminium: 0.70
  • Bituminous: 0.70
  • Gravel 0.8 – 0.85
  • Level cement: 0.8

As you can see, metal has a high RC and is often the desired roofing material on Earthships.

The slope

A steeper roof is able to catch water more quickly. A flatter roof might create puddles that stay on the roof and eventually evaporate.

That’s why you want to maintain an appropriate slope for your roof.

The area

The area (A) of your roof is a very important factor. It’s a no brainer that the bigger the roof, the more water you will catch.

Putting it all together

In the end, we can calculate how many liters of water you will collect in a given time period:

Potential water harvest = P x A x RC

We have created a google spreadsheet that you can simply copy to your own library. It can automatically calculate what roof area you will need and how much water you can expect to harvest.

Storing water

It’s equally important to have enough room to store your collected water. If your cisterns can’t hold enough for your need, they can run out before the next rainfall comes.

The easiest way to approach the question of how much you need to store is to ask yourself how many dry months you want to be able to endure. You should know how much you consume each month.

Let’s assume, we need 3000 liters a month and we want to be able to go for 2 months without rain – then we need at least 6000-liter cisterns.

Our google spreadsheet can assist you with those calculations as well.

Filtering your water

You may want to filter your collected rainwater. You can check out this article about filtering your water at home for now. We will be adding more information here soon regarding filtering rainwater specifically.

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