ECOsmarte blog

One of the first steps in any roof-reliant landscaping project is estimating the appropriate size of your water storage tank. This estimate will be required as you begin to design your landscape, estimate its cost, create your water budget and schedule the installation of your project. Knowing the exact size of your system will, of course, be of critical importance when you actually design your rainwater system, using a cistern approach.

You can determine the appropriate size of your cistern by taking the following simple steps:
• Calculate the catchment area of your roof
• Estimate your “normal” rainwater harvest
• Apply the One-Third Rule

Calculate the Catchment Area of Your Roof The amount of water that can be harvested is determined by the size of the catchment area and the amount of rain that falls on that catchment area. Start by determining the size of your roof in square feet. Figure 3-1 shows that the square footage of a rectilinear roof can be easily calculated by multiplying the length of the roof by its width.

Length (in feet) x width (in feet) = square feet

However, it is not uncommon for a roof to be affected by other factors that can slightly complicate this simple calculation. The most common of these factors occurs when two roof surfaces need to be added together, as in Figure 3-2. The house shown below has a garage, which should be included in the total roof square footage. Buildings such as portals, sheds, shade structures and other roof surfaces that can serve as collection areas also need to be included in your calculations.

Roof-Reliant Landscaping Note that the increased angle of a pitched roof does not increase your catchment area. While it is true that more materials are needed to cover a house with a pitched roof than a flat roof, a pitched roof still covers the same amount of ground surface as a flat roof (of the same length and width measured at the given buildings’ ceilings).

One advantage that pitched-roof structures do have over flat-roof structures is that pitched roofs often have large overhangs. Given the same building footprint, a pitched-roof house will typically have larger roof dimensions than a flat-roof house. In Figure 3-3, we see how a two-foot overhang can significantly increase a roof’s catchment area.

Other minor mathematical complications occur when roof lines are not rectilinear. Typically, such roofs can be reduced to either triangular or curvilinear shapes. In the case of triangular shapes in which one of the angles is 90 degrees, simply multiply the length of the roof by the width of the roof, then divide this product by two:

Length x width / 2 = area of a triangle

Curvilinear shapes are rare, but most of them can be reduced to circular shapes, the areas of which are determined by multiplying the square of the radius of the circle by pi (3.14).

Radius x radius x 3.14 = area of a circle

It is imperative that your square-footage calculation is accurate. The proper sizing of your cistern, the total cost of your project and perhaps even the success of your project will depend on your precision here.

Estimate Your “Normal” Harvest Now that you have determined the square footage of your collection area, the next step in sizing your cistern is to estimate the amount of precipitation that you might collect in a given time period. It is important to note that there is a significant distinction between “average” and “normal” when discussing the amount of precipitation your location receives in a year.

Although average annual precipitation data is easy to find for most municipalities and counties throughout the state5, the concept of average precipitation is misleading in New Mexico. It is actually normal for a location to get 20 percent less precipitation than the average annual precipitation figure. This is because occasional wet years skew the average.

Take the example of Albuquerque, NM from 1996 through 2005. Albuquerque received an average of 9.09 inches of precipitation during this 10-year period (which is 0.43 inches more than its historic average of 8.66 inches). Albuquerque received less than the average annual rainfall during five years of this period, and during three of those years it received only about 70 percent of the 10-year average.

(ECOSmarte acknowledges text from the State of New Mexico)

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