How Much Water Comes Off a Roof in Rockford?
Updated
Short answer
About 623 gallons per inch of rain per 1,000 square feet of roof, from the USGS figure of 27,154 gallons per acre-inch. At Rockford's 37.24 inches a year, a 1,500 square foot roof sheds roughly 34,800 gallons.
The arithmetic
The USGS Water Science School says a 1-inch rainstorm deposits 27,154 gallons on one acre. An acre is 43,560 square feet, so that is 0.623 gallons per square foot per inch.
| Roof area | Per inch of rain | Per Rockford year (37.24 in) |
|---|---|---|
| 1,000 sq ft | 623 gal | 23,200 gal |
| 1,500 sq ft | 935 gal | 34,800 gal |
| 2,000 sq ft | 1,247 gal | 46,400 gal |
| 2,500 sq ft | 1,558 gal | 58,000 gal |
Why it matters
All of that water goes through a handful of downspouts. If one is plugged or ends at the foundation, hundreds of gallons per storm land against the basement wall. The DOE's building guidance is to carry it to sloping ground more than 5 feet out. See where downspouts should drain.
Quick answers
How many gallons of water come off a roof in an inch of rain?
About 623 gallons per 1,000 square feet of roof, based on the USGS figure of 27,154 gallons per acre-inch.
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