An Introduction to Rainwater Harvesting

Posted by Urban Rain Systems

An Introduction to Rainwater Harvesting

15th January 2018


On our website (, we define rainwater harvesting as:

“.. the process of collecting rainwater that falls onto a roof surface and storing it to be used at a later time. By harvesting and utilising rainwater in our homes and in our gardens, we reduce our demand for municipal water, thus reducing our dependence on dams and water treatment plants. The use of harvested rainwater will also decrease your monthly water bill.”

First, let’s look at how rainwater is harvested. We’ll then go onto its uses and benefits.


As I’m sure you are all aware rainwater is harvested from a  roof surface. We tell all our clients to select a large (or larger) section of roof to harvest from. The larger the roof surface obviously the more water will be harvested. Try to also look for a roof surface that has as fewer over hanging trees as possible. Trees will drop leaves, twigs, bird poop and all kinds of other nasties that we’d prefer to keep out of our harvested rainwater. Selecting a cleaner section of roof will also reduce the amount of maintenance that will be required.

One of the main questions we get asked is… “Do I need gutters?” and the answer is.. “Yes you do”. Without gutters there is no way of channelling the rainwater from you roof into your RainCell™ tank. But you also do not need to install guttering around your entire house. A single stretch of gutter attached to a larger section of roof with a single downpipe into your RainCell™ Tank is all that is needed.

You will need a firm base for your RainCell™ Tank to stand on. Either level paving or a smooth concrete base is perfect.

Another commonly asked question is… “Do I need a pump?”. The answer is… “Yes you do”. You could get away with not using a pump if you are, for example, filling the swimming pool and you are happy to connect a hose pipe to your storage tank and let gravity feed the water into your pool. However for all other instances you will need a pump to generate pressure for you final application, sprinklers, irrigation systems etc.

Ok, so once the roof has been selected with the RainCell™ Tank is in place and the downpipe has been diverted in to the top of the tank, you are ready to harvest. It really is as simple as that. Once your system is all setup it’s a case of waiting for the first rainfall.


How much water can be harvested?

The amount of water you will harvest will depend on the size of your roof and the amount of rainfall. There is a general formula that you can use you work out your harvesting capacity:

1mm of rainfall                  x              1m² of roof surface         =             1lt of rainwater


Harvesting from a double garage or 36m² with an annual rainfall of 700mm per annum (the average for Gauteng according to the SA weather service) will give you:

700mm of rainfall             x              36m² roof surface            =             25 200 Litres of rainwater


Uses for Rainwater

Lastly  I’d like to cover the basic uses for rainwater. The first area is always the garden. Watering flower beds and the lawn can be quite a water intensive process, not to mention costly. The average household in South Africa uses 39% of its total water usage on the garden. Using free rainwater can offer a great water and cost saving. Below are a few other external uses for rainwater harvesting:

Rainwater for External Use

Using rainwater for outside purposes such as:

  • Watering the lawn and flower beds
  • Supply to an irrigation system
  • Filling up the pool and fish pond
  • Washing cars, motorcycles, boats and garden furniture


Rainwater for Internal Use

With the addition of a 4 Stage filtration unit, harvested rainwater can be purified and plumbed back into the house for bathrooms, toilets, the kitchen and even drinking.

We will be exploring the uses for harvested rainwater later on in this series.


Benefits of Rainwater Harvesting

There are numerous benefits to rainwater harvesting. People find certain benefits more appealing than others. However I think we can all agree that the cost saving benefit is close to, if not at the top of the pile. It’s basic economics. The more free rainwater you use the less municipal water you use. Municipal water that we would normally pay for. And remember that you always pay a sewerage fee that is determined by the number of litres of water you use. So again, the less municipal water you use, the less sewerage fee you get charged.

Many  people are concerned with water security. Imagine waking up tomorrow to find there is a water cut. That means no showering, washing, brushing your teeth and worst of all no flushing the toilet. This can be a major inconvenience and potentially a big hygiene risk. Having harvested rainwater on the premises will allow you to go about your lives unaffected.

Water shedding, planned water cuts by municipalities, is already taking place in various parts of the country. As the country’s population grows at a faster rate than the failing infrastructure, we will certainly see more of these water saving strategies.


Our next article will focus more on this as we look at domestic water storage and how this can be effectively implemented in your home.


Gary Burchell

Urban Rain Systems

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