How Much Rainwater Can I Collect From My Roof

by Nov 16, 2023Environment, Help & Advice

Discovering how much rainwater your roof can capture is a pivotal step in optimizing your rainwater harvesting system. This guide is designed to assist you in determining the rainwater yield from your roof, a crucial consideration when choosing the right-sized Water Tank for your needs.

Step 1: Unravelling Roof Surface Area

Initiate the process by calculating the surface area of your roof. If your roof boasts a rectangular or square structure, a straightforward multiplication of its length by width provides the surface area.

For a quick estimation, take a stroll around your home exterior. With the average stride being approximately 0.76 metres, count your steps on one side. For instance, if it takes 14 steps, the estimated length is 10.64 metres. While a tape measure offers precision, a rough estimate is sufficient for a quick calculation.

In scenarios where the roof takes on an ‘L’ shape, compute the surface area of each rectangular segment and amalgamate the results. For example, if the primary section measures 15 metres by 10 metres, and there’s an additional segment measuring 3 metres by 2 metres, the total surface area becomes (15 x 10) + (3 x 2) = 156m².

Remember to steer clear of harvesting water from roofs with lead flashings or treated timber, as lead is toxic. For older homes with potential lead flashings, precautionary measures such as replacement or avoidance are imperative.

Step 2: Navigating Downpipes

Given that many rooftops slope towards multiple downpipes, it’s essential to factor in the distribution of downpipes. If your plan involves situating your water tank near a specific downpipe, calculate the surface area channeling rainwater into it.

For a comprehensive assessment where multiple downpipes are in play, ensure each one contributes rainwater to your tank. This might entail tweaking gutter positions, introducing additional piping, or adjusting slopes to optimize water flow.

Step 3: Quantifying Harvestable Rainwater in Litres

Understanding that 1 millimetre of rain on 1m² of roof equals 1 litre of water in your tank, the final step involves gauging the amount of rainfall your roof can capture. Localised rainfall data is crucial, and a guide on finding this information can be referenced.

In various regions of Australia, especially coastal and urban areas, a week-long summer storm can deliver around 100mm of rain, while a month might yield 200mm in specific coastal zones. Applying this knowledge to a roof area of 156m² with 100mm of rainfall results in 15,600 litres of harvestable rainwater (156m² x 100mm).

For substantial rainwater capture, we recommend a 5,000 litre water tank, aligning with common council requirements for new homes. If your goal is a more substantial harvest, a 10,000 litre water tank is advisable, while a 22,700-litre (5000-gallon) tank provides substantial water self-sufficiency.

Check Out Our Rain Catchment Calculator

To help you get an accurate figure on how much water you can harvest from your rooftop we have a handy rain catchment calcuator

Ready to Harvest Rainwater with Select Water Tanks?

We trust this guide proves valuable in comprehending the potential rainwater harvest from your rooftop. If you found this information helpful, please share it with others. Feel free to leave a comment below to share your thoughts or pose any lingering questions. And when you’re ready to make the most of your rainwater harvesting system, contact Select Water Tanks for expert guidance and quality solutions.