Rainwater Catchment Systems

Chuuk and Yap States, FSM

Sizing New Tanks

Here are the STEPS to use if you want to know how BIG to make a NEW tank:

  • STEP 1.
    Measure the LENGTH and WIDTH of your roof.
  • STEP 2.
    Find your ROOF SIZE.
    Multiply Length times Width.
    Roof Size = Length x Width
  • STEP 3.
    Select a ROOF FACTOR.

    If the entire roof has gutters, use 1.0
    If 3/4 of the roof has gutters, use 0.75
    If 1/2 of the roof has gutters, use 0.50
    If 1/4 of the roof has gutters, use 0.25
  • STEP 4.
    Select a GUTTER FACTOR.

    If your gutter has only a few leaks, use 0.90
    If your gutter has a number of leaks, use 0.75
    If your gutter has a lot of leaks, use 0.60
  • STEP 5.
    Find your USEABLE ROOF SIZE.
    Multiply Roof Size times Roof Factor times Gutter Factor.
    Usable Roof Size = Roof Size x Roof Factor x Gutter Factor
  • STEP 6.
    Write down the NUMBER OF PEOPLE that live in your home.
  • STEP 7.
    Use the CHARTS at the back of this booklet to get the size of your NEW tank. Click here to view these charts.

EXAMPLE ONE

Your roof is 34 feet in length by 25 feet wide. There are gutters around 3/4 of your roof and they have very few leaks. There are 5 people in your family. How BIG should your NEW tank be? By following the steps on the previous page, we can fill in the blank spaces below.

  • Step 1.
    Length 34 feet
    Width 25 feet
  • Step 2.
    34 x 25 = 850 square feet
    Length Width Roof Size
  • Step 3.
    Roof Factor = 0.75 s
  • Step 4.
    Gutter Factor = 0.90 s
  • Step 5.
    850 x 0.75 x 0.90 = 574 square feet
    Roof Size Roof Factor Gutter Factor Useable Roof Size
  • Step 6.
    5 people

    3/4 of the roof has gutters, and the gutters have few leaks
  • Step 7.
    At this point you would go to the tank sizing chart summary and choose the chart for your useable roof size. Since your roof size is 574 square feet, you should use the chart labeled "For useable roof size of 400 to 600 square feet". The same chart is shown below for easy reference. Locate the row labeled 5 people in the left-hand column.

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For Usable Roof Size of 400 to 600 Square Feet

People Full 3/4 Full 1/2 Full 1/4 Full
14        
13        
12        
11        
10        
9        
8        
7        
6        
5        
4        
3        
3        
3        
Roof too small 500 Gallons 1 BAR
1400 Gallons
11/2 BAR
3100 Gallons
2 BAR
5700 Gallons

There are FOUR answers to the question of how BIG your NEW tank should be. First look again at the graph on the previous page. You will see that these answers are represented by four colored boxes to the right of the box that shows the number of people. These colored boxes tell us what size tank you need depending on WHEN you stop using the water in your tank for bathing and washing dishes. For example:

If you stop using your tank for bathing and washing clothes when the tank level decreases below 1/4 full, you should look in the last column to the right that is labeled 1/4 full. Notice this column contains a (blue box) in the five people row. This corresponds to a 2 Bar tank which has a capacity of approximately 5,700 gallon. However, if you stop using your tank for bathing and washing clothes when the tank level is below 1/2 full, the tank size recommended in the 1/2 full column is a 3,100 gallon tank (green box).

If you stop using your tank for bathing and washing clothes when the tank is below 3/4 full, you only need a 1,400 gallon tank (red box). So, it is easy to see that IF YOU CONSERVE WATER, YOU NEED A MUCH SMALLER TANK! For this example, if you NEVER use your tank for bathing and washing clothes when the tank is less than 3/4 full , you will only need a 1,400 gallon tank (red box). If you want to start conserving water only when the tank is less than 1/2 full then a 3,100 gallon tank (green box) will be required. If you want to start conserving water only when the tank is less than 1/4 full then a 5,700 gallon tank (blue box) will be required.

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EXAMPLE TWO

Now suppose that your USABLE roof size is 1,620 square feet and you have 20 people in your home. How big should your tank be now?

Since we already know the usable roof size and the number of people, we can go right to the charts to get an answer. Your usable roof size is between 1,600 and 1,800 square feet. At this point you would go to the tank sizing chart summary and choose the chart for your useable roof size. Since your roof size is 1620 square feet, you should use the chart labeled "For useable roof size of 1600 to 1800 square feet". A copy of a part of this chart is shown below. The chart tells us that if you NEVER use water in your tank for bathing or washing a 5,700 gallon tank (blue box) is the right size. If you would like to use your tank for bathing and washing clothes, you will need a bigger tank than what is covered in the design charts of this bulletin.

For Usable Roof Size of 1600 to 1800 Square ft.

People Full 3/4 Full 1/2 Full 1/4 Full
23        
22        
21        
20        
19        
18        
17        
16        
15        
14        
13        
12        
11        
10        
9        
8        
7        
6        
5        
4        
3        
2        
1        
Roof too small 500 Gallons 1 BAR
1400 Gallons
11/2 BAR
3100 Gallons
2 BAR
5700 Gallons

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