French Drains The Right Way
By Scott Cohen

Most everyone assumes that French drains have their name because engineers in France were way ahead of the curve in figuring out how to deal with the hydrostatic pressure that can damage walls and other types of structures. The truth, however, is that the systems were actually an innovation by an American lawyer and gentleman farmer named Henry French (1813-1885).

Although French used roofing tiles arranged so that gaps between them admitted water, these days the systems are usually made by placing sections of perforated pipe in areas subject to such pressure. When placed behind retaining walls at the bases of slopes, for example, water flows into the pipes and moves safely away from areas where it could otherwise do great harm.

Through the years, French drains have proved remarkably effective in relieving hydrostatic pressure in a variety of applications. If set up properly, they allow water to follow its natural tendency to pursue the path of least resistance. But if — as happens all too often — these systems are installed incorrectly, the consequences can be extreme and unfortunate. And embarrassing as well, because the problems are so easily avoided.

Scenario: The perforated pipe used to create French drains includes a number of holes, typically in a “Y” pattern, that run down one side of the drain. The mistake many people make is installing the pipe with the holes facing up, under the incorrect assumption that water will flown down into the pipe and then be transported away by gravity.

In reality, that’s not how it works. Instead, the lines should be installed with the holes oriented downward. That way, as water (from irrigation, rainfall or groundwater movement) travels into a critical area, it will well up, follow the path of least resistance into the pipe and be removed.

If the holes face upward, water will have a hard time finding its way into the pipe. In fact, it will need to rise through the gravel and soil surrounding the top of the drain, at which point you will already wind up with saturated soil right next to whatever structure the system is meant to protect.

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Scott Cohen is president and supervising designer of The Green Scene, an outdoor design and construction firm based in Northridge, CA. He provides consultation for clients nationwide and gives seminars on designing landscapes, swimming pools and outdoor kitchens.