Part of designing a rainwater catchment system is determining how many rain barrels will be needed to catch the majority of run-off from your roof.  To do this, you first need to know the following:

*How many gallons of water are produced by one inch of rain falling over one square foot?

*What is the square footage of your roof or catchment area?

*How much rainfall is produced by a typical rainstorm in your area?

*What size are your rain barrels?


*One inch of rain falling over one square foot = .6234 gallons

The catchment system I’m designing is for a roof with a catchment area of 2,444 sq ft.

I want the system to be able to hold the water produced by a 1 inch rainfall.

I will be using rain barrels capable of holding 200 gallons each.


The first step in determining how many rain barrels you will need to catch the rainfall produced by a one inch storm, is to:

1.  Multiply the square footage of your catchment area by the number of gallons produced by one inch of rain falling over one square foot:

2,444 sq ft  x  .6234 gal per one inch of rain over one sq ft =  1523.5 gallons collected from a one inch rainstorm.

2. Multiply the number of gallons collected form a one inch rainstorm by the amount of rainfall produced by a typical rainstorm in your area:

1523.5 gal.  x  1 in of rain  = 1,523 total gallons collected from 1 inch rainfall.

3.  Divide the total number of gallons collected by the (gallon) size of rain barrel you will be using in the system:

1,523 total gallons / 55 gal per rain barrel = 27.7 rounded to 28 barrels

To place rain barrels, you will need to visually break the roof into potentially guttered areas, measure these areas individually, and use the above calculation to determine how many barrels are needed for each area.

For example the catchment system I’m designing has 5 different catchment areas:

400 sq ft, 626 sq ft, 227 sq ft, 573 sq ft, 618 sq ft.

By using the above calculation (multiplying square footage by .6234 gal per 1 inch of  rain over 1 square foot, then multiplying by amount of rain produced by a typical rainstorm, and dividing by the size of one rain barrel) I was able to determine how many 55 rain barrels were needed for each guttered area.

400 sq ft:  5 barrels

626 sq ft:  7 barrels

227 sq ft:  3 barrels

573 sq ft:  6 barrels

618 sq ft:  7 barrels

*55 gallon barrels are often used for catchments because they are relatively cheap and easy to come by, as they are often used for shipping and storage and can easily be re-purposed.  However, having 28 barrels lined up around the outside of a house isn’t exactly aesthetically pleasing.  And, although you can purchase barrels of various different sizes, if you have a large catchment area, this can become very expensive, very quickly.  So, I will recommend that the family have their gutters rerouted in order that all rain water empty’s into one 1500 gallon storage tank.

Gallery | This entry was posted in Uncategorized. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s