Flow and grade logic
Peak flow uses roof area x rainfall intensity x 0.01039 x capture factor. Target fall uses extension length x target slope x 12.
RainYard Tools
Roof discharge path
Downspout Drainage Calculator
Estimate peak downspout flow, short-burst volume, and target fall for an extension or drain line.
Peak flow14.8 gpmEstimated flow from one roof section in the selected intensity.
15-minute burst222 galQuick screen for a short hard rain.
Target fall4.8 inGrade drop needed over the extension length.
Example
A 600 sq ft roof section in a 2.5 in/hr burst produces about 14.8 gpm at 95% capture. Over 20 ft, a 2% target slope needs about 4.8 inches of fall.
This page is about one roof outlet
Use the roof area that feeds one downspout, not the whole house. That keeps the flow and grade check close to the problem corner you are trying to fix.
A steep line can still fail
Slope helps, but the line still needs cleanouts, discharge space, and enough downstream capacity. If the yard stays wet, pair this with a French drain or storage page.
Questions
Common checks.
Why use rainfall intensity here instead of storm depth?
Downspout lines care about how fast water arrives during the burst, not only the total inches over a whole storm.
Can I bury a corrugated extension with no fall?
That is a common failure path. A flat or reverse-pitched line can hold water, clog, or freeze.
What if the target fall does not fit the yard?
Shorten the run, change the outlet, switch to surface conveyance, or combine the line with storage before the outlet.
Helpful referencesEPA downspout disconnection and rain harvesting