How Deep Should You Install French Drain Pipes?

Water standing in your yard is not only annoying, it’s also unsanitary; stagnant water is a breeding ground for insects. Drainage problems can be corrected by installing a French drain, an underground drainage system that uses pipes to direct water to a designated drainage point. The depth of the pipes is subjective to the topography of the property.

Scott Lunt, owner of The Garden Center in San Antonio, Texas, says "The actual topography will dictate how deep you need to lay the pipes. Obviously, water doesn’t flow uphill, so you have to evaluate the grade of the slope before you make that determination."

In a French drain system, pipes are perforated and placed onto gravel beds in trenches. The pipes must then be covered with another bed of gravel to help filter sediment and carry the excess water to a designated turn-out. According to Lunt, covering the pipe with a weed mat to keep root systems and sediment out of the pipes is as important as digging the trench deep enough.

"The purpose of a French drain is to effectively drain water, while creating an aesthetically pleasing landscape," says Lunt. "You don’t want the pipes to show, so in a typical 12- by 12- by 4-inch system, you’d be digging a trench 12 inches wide by 12 inches deep and using a 4-inch pipe. If you place a 2-inch bed of gravel in the bottom of the trench, add the pipe, and cover it with another two inches of gravel, you’ll have adequate depth above the pipe and gravel bed to add topsoil and grass."

Lunt emphasizes that your drain should be at least 12 inches below the ground at the ground’s lowest point; depending upon the topography, you may be digging a trench of varying depth.

Check your state legal requirements for installing a French drain. Some states require a license and a permit. If installation of any drainage system leads to the flooding of another property, you may be held financially responsible for making corrections to the system and for providing an alternative water turn-out. Additionally, you may be held accountable for any damage resulting from the drainage.

At least a week before you begin digging, call all utility companies, including phone and cable, and ask for verification of underground cables. They will mark the path of the cables with spray paint to help ensure that you won’t dig into any lines. Ask for a letter stating the depth of the cables and a print-out of the exact location as a back-up reference.