Thinking about harvesting rainwater? Use this calculator to find out how many gallons you can collect.
Rainwater Harvesting Tips
Please bear in mind, this tool is only an estimator. It is impossible to calculate the exact amount of water a surface can collect because there are too many changing variables. However, here are some tips to help you collect as much as possible, as well as the factors that affect how much you can harvest.
Volume and Frequency of Rain
A heavy rainfall typically produces more water than a light rain, depending on the duration and how fast you can collect the water. However, sometimes a light rainfall may produce more.
For example, if you have a low-slope roof – e.g., 1:12 pitch – a heavy rain might fall too fast in too short of an amount of time for you to successfully collect much of it. On the other hand, a long, light rainfall may produce more water because you have more time to collect it since it is flowing at a slower rate.
In either case, total inches of rainfall is what matters the most.
Surface Area and Slope
The larger the area and the steeper the slope, the more water you can collect. A 1,000 sq ft collection surface with a steep pitch will collect more water than a 1,000 sq ft collection area with a low slope or no pitch.
When calculating your square footage, it is important to remember you are not calculating the total square footage of the collection surface itself, rather the total square footage of the area it covers. For most homes, this will be the square footage of the house plus the extension of overhangs – i.e. eaves.
Direction of Rainfall
It’s fairly uncommon for rain to fall straight down. Instead, it usually falls at an angle depending on the direction the wind is blowing. This can have a dramatic impact on the amount of water you’re able to collect. For this reason, if you have a roof with more than one pitch – like a standard gable roof – it is best to collect from all sides of the roof.
You basically have two methods of collecting rainwater: funneled through a gutter or dripping off the eaves. If you channel the water through a gutter and a downspout, you will collect more than if it simply drips off the eaves. However, you can still collect a good amount of water by placing open containers under the eaves. I have a method for that here.
It is best to store rainwater in a light-tight container that has screened ventilation and overflow at the top to allow vapor and excess water to escape while preventing insects from getting in. If the container is not opaque, like a translucent IBC tote, algae will eventually grow in the container.