How to Install a French Drain

French drains can be used for two different purposes. One is to provide specific surface drainage to wet areas, for example, a lawn, yard, or field area that has a low spot that collects water under normal conditions. You may wish to install a French drain at the top of a highly landscaped area in your yard to prevent heavy water runoff and soil erosion of decorative or bedded areas.

Another common usage is to provide a "dry well" to allow water from a surface drainage system to collect and seep or percolate slowly into the ground. French drains for this specialized application are usually large, deep, and lined with fabric cloth to prevent infiltration of soil fines into the coarse gravel and most notably are usually covered with a layer of finer gravel on top rather than being covered with sod or returned to grass.

We will cover the installation of the most common type of French drain, the purpose of which is to provide drainage for a wet area on a lawn or yard or prevent undesirable movement of surface water.

Here is how to install a French drain in a few easy steps.

1. Stake out the direction of the drain from the low wet spot to the location the discharge from the drain will occur.

2. Dig a steep-sided ditch or trench 12" or more deep and approximately 6" wide. The width and depth are not critical. A French drain can be deeper or wider as desired. Excavation can be done manually or with power equipment such as a mechanical trencher.

3. Contour the bottom of the French drain, using a slope gradient of 1% or more, which equates to 1ft. of drop for each 100 feet. of drain. If an absolute minimum is required, do not use less than 0.5% slope, or 6" per 100 feet.

4. If soil conditions are very fine or silty, do install a ground filter fabric on the sides of the drain.

5. Fill the trench with crushed rock, gravel or cobbles sized 1/2" to 1" to within about 2" or 3" of the top of the trench. Be sure that clean rock or gravel is used. Remember that fines in the gravel will be an impediment to water flow.

6. As an alternative, you can place a "weeping tile" pipe in the bottom of the trench on top of 1-2 " of gravel. Place the holes facing DOWN to prevent the pipe filling with water.

7. Install a ground fabric filter on top of the gravel. This step is optional.

8. Fill the remaining 2-3" to grade with coarse sand.

9. Plant grass seed or install sod as desired.

Be creative, dispose of the extra soil and rake the area, returning it to the desired state.

Learn more about this author, Raymond Alexander Kukkee.
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