French drain trench cross-section — bedding stone, perforated pipe holes-down, stone cover, and filter-fabric wrap
Inside a French-drain trench (IRC R405.1): ≥2″ of washed #57 stone bedding, a perforated pipe laid holes-DOWN, ≥6″ of stone cover, the stone column wrapped in filter fabric, and the trench run ~12″ beyond the footing. Holes face down so water enters as soon as the water table rises.
What this diagram shows
A cross-section through a French-drain trench built to IRC R405.1. From the bottom up: at least 2 inches of washed #57 stone bedding, a perforated drain pipe laid with its holes facing DOWN (the 4 and 8 o'clock positions) so rising groundwater enters the pipe immediately, then at least 6 inches of the same washed stone as cover over the pipe. The whole stone column is wrapped in an approved filter membrane to keep soil fines out, topped with backfill, and the trench is carried about 12 inches out beyond the footing. Most of the trench is stone — the pipe is the small part.
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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