Excavate a trench where the french drain will be placed. The depth and width of the trench should vary, depending on location and the severity of the drainage problem. French drains can be as shallow as approximately 2 feet and can be dug much deeper if the drain is being installed next to a foundation. A standard french drain trench can be dug about 3 to 4 feet deep. A suitable width for most applications is 1 to 2 feet.
Check the slope of the trench and adjust as needed. The bottom of the trench can have a slight slope, especially if the pipe will drain to a feature outside the trench such as a ditch or pond, but the slope should be no more than 2 percent.
Lay the 4-inch perforated PVC pipe in the center of the bottom of the trench, and check to make sure that its slope reflects the slope of the trench. Adjust and rearrange the soil under the pipe if necessary.
Place at least 12 inches of washed gravel or river rocks around and on top of the PVC pipe. The gravel can be placed as high as 6 inches below ground level.
Cover the gravel with a layer of filter fabric or landscape cloth. This will keep silt from entering the gravel bed and penetrating and clogging the piping, which would shorten the functional life of the drain.
Backfill the rest of the trench, using soil that was excavated earlier, and replant the area as desired. Alternatively, the trench can be backfilled to the top with gravel or larger rocks.