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Rainwater harvesting (Design-build), Ithaca, NY

Date: 2006

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Step-by-step construction method:

1. The first step is to make a diagram of the particular conditions of the house and downspouts. In this case, I had 4 downspouts to intercept at the rear of the house. Note that the number of barrels does not necessarily correspond to the number of downspouts; rather, it is a function of the drainage area of the roof, and the anticipated "design" rainstorm or storms that you intend to harvest. The following calculations determine the number of barrels needed, all connected in series:

  1. volume of water (cu.ft.) for a 1" rain = roof area (sq.ft.) / 12
    For my house, 400 sq.ft. / 12 = 33 cu.ft. required.
  2. number of cubic feet per 50-gallon barrel = 6.68
  3. number of 50-gal. barrels needed = volume of water (cu.ft.) / 6.68
    For my house, 33 cu.ft. / 6.68 = 4.9, or 5 barrels required for 400 sq.ft. of roof area, assuming a 1" rainstorm.

Alternatively, use the following online calculator to compute the number of barrels needed (first choose US or SI units; next, type in roof area, design rain storm and barrel size; then click the "update" button):

Choose American or International units (select US or SI):  
roof area design rain storm size of one barrel number of barrels needed

Create online calculators using Excel

Note that some sort of overflow strategy is needed for cases where the actual rain exceeds the "design" rainstorm assumed.

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