If you have a corrugated metal or tiled roof, you can harvest a great deal of water. To give you an example, a 100m2 roof can collect 450 litres of water in 5mm of rainfall. Cape Town receives on average about 788mm of rainfall a year. When we put that number into the calculator for our little 100m2 roof, that amounts to 70,920 litres of water. The numbers are huge – why miss out on such an opportunity? There are rainwater collection calculators on the web, one of which can be found here.
The first place to start, is to buy the tank or tanks.The bigger the better, but affordability and space should influence your decision as well as how much water you consume and what you will be using the rainwater for. You can buy cylindrical tanks, both thin or wide. Some go underground and some are rectangular and would fit snugly against a wall. We came across three easily available brands, Eco tanks, Jojo, and Nel. The prices and quality differ. We picked 2 x 2500 litre beige Eco upright tanks.
The next thing to consider when installing the tank, is where the downpipe leads from off the roof gutters. It will be easier to install if the tank can stand there safely and not obstruct anything. If it is not the ideal spot, you may want investigate the option of diverting the water down a new downpipe in a better position.
All you will need is piping to the tank. When the tank fills, it will become very heavy. A 2500 litre tank will weigh 2.5 tons when it is full, so making sure you have a proper foundation is necessary. We luckily had paving in our spot so we did not need to build a foundation. However, a foundation can be built easily enough with the right tools.
To make the foundation you will need to know the size of your tank. By sketching a diagram of your tank to scale, you will be able to determine the size of the square foundation beneath it by drawing a square or rectangle with the tank’s shape inside. This will lead you to the length of four wooden planks that must be bought to hold the cement mix you will make. Start by levelling and compacting the ground where the tank will be. Once complete, assemble the boards to make a square or rectangle to contain your cement slab.You will need to get a bag of cement mix and sand, and mix it up and fill the slab container. Scrape it out evenly using a level and ensure that it is in fact level. The cement will dry and you can remove the boards or not. You can place your tank/s on the slab. Secure your tank by tying it to the ground with wire. Don’t make this too tight as the stability of your tank may be compromised. As big as tanks are, they can blow about in the wind. We have not tied ours down, we just make sure there is enough water inside to keep them from moving. Connect the downpipe to the tank. You may want to put a fitted filter at the opening or into a catchment pipe as shown in these pictures.
This picture has a fancy split which is not necessary. You could even use netted fabric and secure the fabric to the opening with elastic or rope. A filter will keep out leaves and other debris and also keep mosquitos from entering the tank and breeding in the water. Remember that installing a tank doesn’t have to be a fancy affair. If you are harvesting rainwater, you are doing it right. Look at the extras as frills.
Here is another picture to give you ideas.
The next thing you need is a tap or ball valve, some connections, plumber’s tape, and piping to divert the overflow to a place of your choice. You can divert the overflow either into the stormwater drain or even better, into another tank. We went to the hardware store and gave them the fitting size of the tank, and the store assistant helped us get all the fittings to connect to the ball valve we used and the fitting from the ball valve to the hosepipe. It’s all a bit like lego, finding what fits on what. You basically want the water outlet to also connect to a hosepipe. This picture will give you some dos and don’ts on how to assemble piping if you choose to use your water elsewhere, or put it in an irrigation system.
The system we installed, as explained above, is called the dry system, and water comes from the downpipes and straight into the tank. When the rain stops, the pipes will run dry.
There is also the wet system, which allows for the tank to be positioned further away from the roof gutters and uses the principle that water will always find a level.The pipes remain ‘charged’ so that when it rains, water will flow into the tank.
There is also the transfer system where the tank can be situated anywhere on the property and uses a pump to pump water from an underground pit into the tank.
We hope we have given you some good ideas on how to collect your own rainwater. The dry system is really easy and you will be able to do it yourself.
Here are some other tips to remember when collecting rainwater:
Check your leaf eater or filter regularly and if you have tall trees close to the house, you may need to clean the roof gutters every now and then.
Ensure that the overflow is away from the base – you don’t want your foundation compromised.
Make sure that the opening is sealed or that you have a filter in place so bugs can’t get in.
To stop anything from growing inside, the tank should not allow any light inside. Algae are little plants and with light, they will grow in your container.
Your rainwater system does not need to be expensive. You can even use old jacuzzis, water barrels, or any big container you can find. Don’t be put off by underground systems and cisterns etc. Make a start and if you really want the fancy frills, you can add them later. It is better to harvest cheaply than not to harvest rainwater at all.
Don’t choke the flow. It may be a better option to get a big ball valve rather than a small insipid tap. Trickling water can become annoying when you are trying to water the garden.