The string-line method — how a slope becomes a total fall you can stake
A slope only becomes buildable as a total fall: fall = slope × run, so a 1% french drain over 40 ft drops 4.8″ (≈ 4 13/16″). Stake both ends, level a string with a line level, then measure down at both stakes — the outlet must read that much more than the high end.
What this diagram shows
A field-procedure diagram for setting a drainage slope with a string line, using a french drain at 1 percent over a 40-foot run as the example. Step one: drive a stake at the high end and one at the outlet, and tie a string to the high stake. Step two: level the string with a line level, so the string is a true horizontal reference. Step three: measure down from the string to grade at both stakes — the outlet reading must be the total fall more than the high-end reading. The total fall is the slope times the run: 1 percent of 40 feet is 0.4 feet, which is 4.8 inches, about 4 and 13/16 inches of drop. One percent equals one-eighth inch per foot, the IPC 704.1 pitch for 3-to-6-inch drain pipe, and a water arrow shows flow toward the outlet end.
Slope Calculator
Free slope calculator: convert rise and run to percent grade, degrees, inches per foot, and H:1V ratio — plus total fall over your run. No signup.
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