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

• If you have a tree that overhangs one of the roof sections, you might want to make a note to yourself that there might be a need to remove large quantities of leaves and twigs.

• 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

2.3 Roof and Gutters:  Tank Sizing

Calculating the rainwater harvesting tank size based on your roof area, rainfall, and consumption

This section is a very powerful spreadsheet where you will enter your roof area that you calculated in the last step, your household water consumption, and rainfall data that we will help you find for your area of the world.  It will graphically allow you to size your tank(s), and simultaneously depict what your tank inventory will be at the end of each month.  Use your total roof area for now.  If you’re collected rainwater turns out to be consistently greater than your consumption each month, consider using fewer roof areas, and then recalculate a tank size.  If you use too much gutter, you’ll simply overflow the tank each month.  spreadsheet   The file is an Excel spreadsheet and will require that you have this program installed locally to use it.  Your computer (depending on the browser you use) should give you the option of saving the file locally or simply opening it.  Either method is fine.  If you accidentally started changing the yellow box values, simply close the spreadsheet without saving and reopen.

Now that you have your tank(s) sized, we will move on to designing and sizing the gutters and downspouts.

2.4 Roof and Gutters:  Designing Gutters

Calculating gutter width and length between downspouts for semicircular, box, V, or trapezoid gutters made of any building material

For those of you that will design or install your own gutters, this link will open another powerful spreadsheet that will help you design a gutter, and determine how far you can go between downspouts.  spreadsheet   The file is an Excel spreadsheet and will require that you have this program installed locally to use it.  Your computer (depending on the browser you use) should give you the option of saving the file locally or simply opening it.  Either method is fine.  If you accidentally started changing the yellow box values, simply close the spreadsheet without saving and reopen.

For some of you, designing gutters will be left to an installer.  They are perfectly capable of designing a successful system.  I would suggest that you ask them to install the gutters at a ¼-degree slope for the first 2/3 of the gutter length before the downspout, and an angle of ½-degree for the last 1/3 of gutter length.  This will get the water flow fast enough to help move debris along and help prevent the pooling which can draw insects, biota, and increase corrosion.

Congratulations, you have just finished one of the most difficult tasks in rainwater harvesting; deciding on a tank, and gutter size and location.