French drain slope — 1% (1/8 inch per foot) minimum fall to daylight
A French drain runs on gravity: pitch the pipe at least 1% — that's ⅛″ of drop per foot, or 1 ft per 100 ft — continuously to a lower outlet (daylight, dry well, or pop-up). Too flat and it stagnates; over ~5% and fast flow scours the gravel.
What this diagram shows
Two French-drain pipe runs compared in elevation. A French drain moves water by gravity only, so the pipe must fall continuously to a lower outlet — daylight, a dry well, or a pop-up emitter. On the left a flat or level pipe lets water stand and silt settle, which clogs the drain. On the right the pipe is sloped at least 1 percent, which equals one eighth of an inch of drop per foot, or one foot of fall per one hundred feet, so the water runs downhill and out the outlet. Target 1 to 2 percent and keep it under about 5 percent, because too steep a grade scours the gravel and strands sediment.
French Drain Calculator
Free French drain calculator estimates gravel (yd³ & tons), pipe length, and filter fabric. Get the right trench size, slope, and #57 stone amount.
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