Harvesting Roof Rain: Calculating Catchment Capability

This article from Environmental Building News shares information on harvesting roof rain and calculating catchment capability.

| August/September 2003

Learn about harvesting roof rain and calculating catchment capability.

Harvesting Roof Rain: Calculating Catchment Capability

If you're using a roof to harvest rain, the size of your catchment area is the square footage of the building's footprint. Each square foot of collection area should yield 0.6 gallons of water per inch of rainfall, although some water will be lost to evaporation and leakage, which is factored in by an efficiency factor of 0.8.

To determine your roof's water-harvesting potential:

1. Multiply the square feet of collection area by 0.6 gallons (per square foot of area).

2. Take this total times 0.8 (the efficiency factor).

3. Multiply the total from Step 2 by your area's annual average rainfall (in inches). You can find the average precipitation in your area by visiting the National Oceanic and Atmospheric Administration. This data chart gives monthly and yearly averages of data collected for specific cities during the past 30 years.

Joanna Browne
1/5/2013 10:58:28 PM

We're building a concrete house in Puerto Rico. We plan on harvesting the water from the roof. Does anyone know of a concrete sealant that won't contaminate the water?

12/19/2011 5:58:55 PM

We actually design and process this in Vancouver BC where it rains most of the year at It has become something that people should really consider to collect rain water and store it.

8/4/2009 1:09:33 PM

What about bird droppings in the roofs of the buildings you are collecting rain runoff from?

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