Rainwater Collection System to Harvest the Rainwater

You first need to calculate your roofs footprint or rainwater collection area to determine how much rainwater you can expect to collect. Your roofs footprint is equal to the exterior square footage of the roof surface around the gutter line. (Remember your elementary school math? Length X Width = Area of a Rectangle.)

For some roofs it may be necessary to calculate multiple rectangular areas and add them together.

Each square foot of collection area should yield 0.6 gallons of water per inch of rainfall. Some water will be lost to evaporation and leakage, so we will use 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 chart gives monthly and yearly averages of data collected for specific cities during the past 30 years. It is alphabetized by state and then cities within that state. Just find the closest city to where you live.

The Rainwater Collection Surface

Water quality from different roof catchments is a function of the type of roof material, climatic conditions, and the surrounding environment.

Metal Roofs

The quantity of rainwater collected from a roof is in part a function of the roof texture. The smoother the better. A commonly used roofing material for rainwater harvesting is a metal roof. These are becoming more and more popular even in urban setting.

Some caution should be exercised regarding metal roof components. Roofs with copper flashing can cause discoloration of porcelain fixtures.

Clay/concrete tile

Clay and concrete tiles are both fairly porous. These materials are suitable for potable or non potable systems, but may contribute to as much as a 10% loss due to texture, inefficient flow, or evaporation.

Composite or asphalt shingle

Due to possible leaching of toxins, composite shingles are appropriate for non potable systems. They should be used for potable systems only if there is a quality filtration system to remove any possible toxins from the water.

These roofing materials are rare, and the water harvested is usually suitable only for non potable uses due to leaching of compounds. That is unless a quality filtration system is used.


Slate’s smoothness makes it ideal for a catchment surface for potable use,assuming no toxic sealant is used. slate is very expensive and cost considerations may preclude its use.

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