How a french drain works
A french drain is deceptively simple: a gravel-filled trench with a perforated pipe that gives water an easy path away from where it pools. Get the slope, fabric, and outlet right and it runs for decades; get them wrong and it silts up in a season. Here's what actually matters.
The parts
- The trench. Dug on a continuous downhill slope — usually at least 1% (about 1 inch of drop every 8–10 feet) — from the wet area to a lower outlet.
- Perforated pipe. Rigid or corrugated pipe with holes that let water in. Holes typically face down: water rises into the pipe from the gravel bed, so it drains the soil rather than only catching surface flow.
- Washed gravel. Clean, angular stone around the pipe creates the voids water travels through and keeps fine soil from packing against the holes.
- Filter fabric (geotextile). Wraps the gravel or the pipe to keep silt and roots out — the single biggest factor in how long the drain lasts.
- An outlet. Where the water goes: daylight at a lower grade, a dry well, a pop-up emitter, or a sump pit that pumps it away.
Interior vs. exterior systems
An exterior (yard) french drain intercepts surface and shallow groundwater before it reaches a foundation or low spot. An interior footing drain runs inside the basement perimeter, under the slab, to a sump pump — the fix when water is already getting into a basement. They solve different problems, and a good installer will tell you which one your situation actually needs.
Why they fail
Nearly every failed french drain traces back to one of three things: no real slope (water sits and silts), no filter fabric (fines clog the gravel and pipe), or a buried outlet with nowhere to go. A cleanout riser makes it possible to flush the pipe years later.
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