2.2 Roof and Gutters: Calculating Area
Calculating Your Roof Area for Rainwater Harvesting
If you have anything other than a very simple roof, you will need to visually break the roof into potentially guttered areas. Figure 2.2.1 depicts a home with a valley and a hip. I am not going to distinguish between Metric or English units; the process will be the same regardless. We will use this as an example. Once you’ve got an idea of how to make the calculations, we will discuss some things you should consider when you go out and take your actual roof measurements.
The potentially guttered areas are Aa, Bb, Cc, and Dd. You would measure and calculate all of these areas individually and then add them up for the total. Remember, we are going to ignore the pitch of the roof. You will be making your measurements on the ground; as if you had a flat roof. The literature refers to this as a perimeter measurement.
Area
|
Calculation
|
Area
|
Gutter length
|
Aa
|
This is the sum of area A and a.
Area A is 9 x 23 = 207
Area a is ½ (9 x 10) = 45
|
252
|
33
|
Bb
|
Area B is 10 x 15 = 150
Area b is ½ (8 x 10) = 40
|
190
|
15
|
Cc
|
Area C is 8 x 10 = 80
Area c is ½ (8 x 10) = 40
|
120
|
10
|
Dd
|
Area D is 10 x 20 = 200
Area d is ½ (9 x 10) = 45
|
245
|
29
|
|
|
|
|
|
Total possible guttered roof area =
|
807
|
|
|
Longest single straight gutter length is along area Aa and equals
|
|
33
|
|
Maximum connected gutter length would be to combine area Dd and Aa at the corner for a maximum total length of
|
|
62
|
Things to consider when you go outside to measure your roof
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You may want to consider where to place your tank. We will calculate the longest distance between downspouts soon. This will probably dictate where you put your tank and which areas can be combined into a single gutter. You will need to refer back to this drawing several times to get an idea of which areas can be combined. If you get lucky, you will be able to combine the areas Dd and Aa for a total gutter length of 62, and put the tank at the corner that we have shown in figure 2.2.1. Otherwise, you might have to use two tanks. One as shown, and another at the corner of areas d and a. We will discuss this in more detail in the Gutter Sizing and Slope section. back to overview