How do I Install a French Drain?

Paul Woods

A French drain is gravel-lined trench designed to channel water away from a low point on a piece of property. To install a French drain, start by determining where and how long the trench should be, and then gather necessary tools and materials. The next steps include digging and lining the trench and, finally, recovering it.

Gravel may be used to line a french drain.
Gravel may be used to line a french drain.

Begin the process to install a French drain by determining where water is gathering on the property. A French drain typically is necessary when an adjoining property has a higher elevation and runoff water collects down the slope from it. The trench for the French drain will run parallel to the sloping hillside toward a place the water can be harmlessly channeled. On residential property, for example, the trench often slopes toward the street where the channeled water can find the gutter. As an alternative, install a French drain so the trench slopes toward a patch of sandy soil where the water can easily be dissipated and is less likely to pool.

Plan on a trench about 6 inches (15 cm) wide and as deep as the water tends to penetrate into the soil. Assemble the tools and materials needed, which include enough landscape fabric to cover twice the length of the trench, enough gravel to line the trench, and sand to cover the gravel up to the level of surrounding soil. Tools required to install a French drain include two wooden or metal stakes, string, a level, a tape measure, and a spade or shovel.

Placing the trench begins with driving stakes in the ground at the beginning point and end point. Tie string between the stakes and tighten. Next, level the string using the level. Begin digging out the trench along this string line. The trench should slope downward at a 1 percent grade; this means if the trench is 100 feet (30.5 meters) long, it should be 1 foot (30.5 cm) deeper at the end than the beginning. Measure the depth of the trench from the leveled string to ensure the slope is correct.

When the trench is dug, line it with landscape fabric. Then add a layer of gravel along the length of the trench and cover that with landscape fabric. This will keep debris out of the gravel and allow runoff water to percolate through the gravel and down the trench. Cover the gravel and fabric with sand to the level of the surrounding soil. The final step to install a French drain is covering the sand with a layer of compatible sod and tamping it down.

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