How much rain your roof can catch

The arithmetic

One inch of rain is one-twelfth of a foot. On 1,000 square feet that is 83.3 cubic feet of water, and a cubic foot holds 7.48 gallons, which gives about 623 gallons. Use the footprint of the roof as seen from above, not its sloped area.

Losses

No roof delivers all of it. Some water splashes over the gutter, some is lost to the first flush, some evaporates, and a heavy burst overruns the downspout. Plan on collecting noticeably less than the table shows, and treat the table as a ceiling.

How many totes

Match the tote count to the biggest storms you want to catch, not the smallest. A summer afternoon storm in Miami can drop an inch or two in an hour. If you want to catch the whole event, plan for the number in the table. If you are happy to let the extra overflow, one or two totes will do.

• Link totes with a short hose near the bottom so they equalize.
• Keep the overflow at the last tote only.
• Screen each opening.
• Keep each tote on its own level base.

What to do with the water

A single tote of 275 gallons is enough to water a small yard through a dry spell. UF/IFAS gives guidance on using rainwater on landscape plants, and notes that it is not for drinking.

Worked example

A 1,200 square foot roof footprint in a one-inch rain yields about 748 gallons before losses, a bit under three 275 gallon totes. In a two-inch storm the same roof sheds about 1,495 gallons, which is more than five totes. If you can only hold two totes, plan for an overflow that sends the extra to a garden bed or a swale rather than a foundation.