Calculating How Much Rainwater Your Rooftop Can Capture

An important factor in how much rainwater you can harvest is the size of your roof catchment area. This article is intended to help you know how much your rainwater your roofs can yield, which might influence your desired water tank size.

Step 1. Calculating Surface Area of Your Roof

This first step in working out how much rainwater you can harvest from your rooftops is to calculate its surface area. If your rooftop is a rectangle or square, then you can just need to multiply the length of your roof by its width.

You can estimate this by walking around your house outside. The average stride of a person is around 0.76 metres long. So, if it takes you 14 steps to walk one the side of your house, then 14 * 0.76m = 10.64m. Alternatively, you can get out a tape measure to be more precise, but if you don’t have one then a rough estimate should be fine.

Sometimes a rooftop is in more of an ‘L’ shape. If this is the case, 1) calculate the surface area of the longer rectangular portion, 2) calculate the smaller rectangular portion, and then 3) add the two results together. For example, if the main portion of your roof is 15 metres by 10 metres, and then you have another part coming out that is 3 metres by 2 metres. The total surface area is therefore (15 x 10) + (3 x 2) = 156m².

Finally, it is important to not harvest water from rooftops with lead flashings or treated timbre. Older homes may contain lead flashings, so before harvesting rainwater from such rooftop, you will need to either replace or avoid them. Understand lead is very toxic, and while many homes in Australia no longer have them, always take a safety-first approach to make sure.

Step 2. Factor in Your Downpipes

Many rooftops often slope downwards to more than one downpipe. One half of your roof might flow to one downpipe, while the other half flows to a different downpipe. If you intend to place your water tank at one downpipe, it is important to know the surface area of just that portion of your roof which will flow rainwater into it.

If calculating the whole surface area of your rooftop, then if you have a second downpipe you will need to ensure that it also flows rainwater into your tank. You might lead second piping to your tank, or possibly investigate readjusting your gutters to slant differently and/or move the second downpipe.

Step 3. Calculating Harvestable Litres of Rainwater

1 millimetre of rainwater falling on 1m² of roof will fill your tank with 1 litre of water. With this knowledge we just need to know how much rain will fall on your roof top. I have written a guide for discovering the rainfall in your area.

In many coastal and urban areas around Australia, it isn’t uncommon for a summer storm pouring down rain for a week to drop 100mm of rainwater. In eastern coastal areas, a summer month can drop 200mm of rain. If you use a roof area of 156m² with 100mm of rain, then we simply multiple the two numbers together to know how many litres of rainwater is harvestable. In this instance, 156m² x 100mm = 15,600 litres.

If you want to capture a sizeable amount of rain, then we would generally recommend a 5,000 litre tank which normally meets council requirements for new homes, a 10,000 litre tank if you want to capture a reasonable amount of rain and 22,700 litre (5000 gallon) tank to become largely water self-sufficient.

Hopefully you have found this article helpful to better understand how much rainwater can be harvested from your rooftop. If so, please share this article with others. Don’t hesitate to also leave a comment below to let us know you liked this article, or if you have any other questions that may have been left unanswered.