How Much Water Can a Rain Barrel Collect? The Roof Runoff Math
Multiply the roof area draining to a downspout by the inches of rain and by 0.623 to get gallons. A 300 square foot section of roof sheds about 185 gallon...
Published by ThriveThyme
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The Short Answer
Far more than one barrel can hold. Nebraska Extension gives the formula:
Roof area (square feet) x rainfall (inches) x 0.623 = gallons
A typical 1,200 square foot roof sheds about 750 gallons in a 1 inch rain. If a quarter of that roof, about 300 square feet, drains to the downspout feeding your barrel, that is roughly 185 gallons, and a 55 gallon barrel will overflow (Nebraska Extension). Clemson HGIC notes that as little as 100 square feet of roof can fill a single barrel quickly.
Try your own roof in the Rain Barrel Calculator.
How to Measure Your Roof
Use the footprint, not the sloped surface. Nebraska Extension suggests measuring the length and width of the house from the ground, then adding about two feet of overhang on each side. Multiply length by width. For an odd shaped house, do each section and add them up.
Then figure out how much of that roof drains to the downspout you plan to use. Look at the roof from several spots in the yard, or check an aerial photo, since dormers and multiple roof angles split the flow.
Quick Reference for a 1 Inch Rain
| Roof area to the downspout | Gallons | 55 gallon barrels |
|---|---|---|
| 100 sq ft | 62 | 1.1 |
| 300 sq ft | 187 | 3.4 |
| 500 sq ft | 312 | 5.7 |
| 1,000 sq ft | 623 | 11.3 |
Nebraska and Virginia Tech round these a little differently. Virginia Tech puts 1,000 square feet at about 600 gallons, or about 12 barrels. Real numbers vary with roof shape and wind, so treat these as estimates.
Nebraska Extension uses 1 inch as its planning storm because about 90 percent of rain events in Nebraska are 1 inch or less.
Plan for the Overflow
Since one barrel fills so fast, the overflow is part of the design, not an afterthought. Nebraska Extension says the overflow opening should be about the same size as the downspout, and the water should go to another linked barrel or to a planted area such as a rain garden. Do not send it toward the house foundation, a neighbor, a septic system, or a paved surface that drains to the street. Clemson HGIC also calls for an emergency overflow that moves water away from the foundation.
Weight and Placement
Water is heavy. Virginia Tech gives 8.34 pounds per gallon, and Nebraska and Clemson both put a full 55 gallon barrel at over 450 pounds. Set barrels on level ground or a sturdy, level platform, and strap raised barrels to the building (Nebraska, Virginia Tech, Clemson).
Raising a barrel helps the water flow. Clemson says barrels are usually set 12 to 36 inches off the ground, and every foot of height adds about 0.4 PSI of pressure. That is still far below a spigot, so Nebraska suggests a watering can or a low pressure soaker hose.
Mosquitoes and Algae
Nebraska Extension says mosquitoes need 7 to 10 days of standing water to go from egg to adult. Cover every opening with fine mesh, keep connections tight, and use the water within about a week. Virginia Tech adds that Bacillus thuringiensis (Bt) mosquito products can treat water you cannot use within seven days.
Algae needs light and nutrients. Use a dark or opaque barrel, screen out leaves, and clean the gutters and barrel at least once a year (Nebraska, Clemson).
Can You Use It on Vegetables?
Rain barrel water is for non-potable uses only, never drinking, cooking, or bathing (Nebraska, Virginia Tech). Nebraska Extension goes further and does not recommend it for fruit and vegetable gardens, because roofs collect bird droppings and can shed chemicals from roofing materials, flashing, and treated wood. Clemson HGIC publishes separate best practices for using harvested rainwater on edibles. The safest use is ornamentals, trees, shrubs, and container flowers.
Winter Care
Where winters freeze, disconnect the barrel, drain it, and leave the spigot open or store it inside or upside down. Reattach the full downspout so melting snow does not pool at the foundation (Nebraska, Virginia Tech).
Next Steps
- Run your roof through the Rain Barrel Calculator.
- Use stored water on trees with the fall and winter watering calculator.
- Read fall and winter tree watering for how much young trees need.
Sources
- Nebraska Extension, G2220 Stormwater Management: Rainwater Harvesting with Rain Barrels
- Virginia Cooperative Extension, Stormwater Management for Homeowners Fact Sheet 2: Rain Barrels
- Clemson Home and Garden Information Center, HGIC 1900 Designing for Rain Barrel Success