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Rainwater Collection Calculation

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The chart provided illustrates the average annual rainfall for various locations from 1991 to 2020. Please note that this data set does not encompass all cities and towns across Canada. If your area is not represented in the chart, you may refer to the Canadian Climate Normals for additional statistics. To access this data, search for your specific location, click on the “Normals Data” tab in the top row, and then scroll down to the precipitation section to view the relevant information. Use this data in the equation below to figure out how much rainwater your catchment area can provide.

 

canadian rain collection data
Data from 1991-2020. Click to enlarge.
Canadian Climate Normals

How Much Rainwater Will My Roof Area Will Provide?

A 1000 square ft roof collecting 1″ of rain will yield 600 gallons of water!

Remember different roofing matierials lose more water than others.  Most roofing types are great for rainwater harvesting.  Below is the amount of rainwater loss expected in some common roofing matierials.

  • Tin: 5% loss
  • Asphalt Shingles: 8-10% loss
  • Clay Tiles: 10-15% loss
  • Concrete: 10-15% loss
  • Wooden:  Depends on wood only used for irrigation as chemicals can leach into the water

Roof Area (ft2) X Precipitation Amount (in) X 0.6 = Amount Collected (gallons)

Roof Area (m2) X Precipitation Amount (mm) = Amount Collected (Litres)

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